polymers Review Recent Progress of Imprinted Polymer Photonic Waveguide Devices and Applications Xiu-You Han 1, * ID , Zhen-Lin Wu 1 , Si-Cheng Yang 1 , Fang-Fang Shen 1 , Yu-Xin Liang 1,2 , Ling-Hua Wang 3 , Jin-Yan Wang 4 , Jun Ren 5 , Ling-Yun Jia 5 , Hua Zhang 6 , Shu-Hui Bo 6 , Geert Morthier 2 and Ming-Shan Zhao 1, * ID 1 2 3 4 5 6 * School of Optoelectronic Engineering and Instrumentation Science, Dalian University of Technology, Dalian 116024, China; zhenlinwu@dlut.edu.cn (Z.-L.W.); ysc_@mail.dlut.edu.cn (S.-C.Y.); maili@mail.dlut.edu.cn (F.-F.S.); liangyuxin0318@mail.dlut.edu.cn (Y.-X.L.) Photonics Research Group, Department of Information Technology (INTEC), Ghent University-IMEC, 9000 Ghent, Belgium; geert.morthier@intec.ugent.be College of Physics and Information Engineering, Fuzhou University, Fuzhou 350116, China; linghua.wang@fzu.edu.cn School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China; wangjinyan@dlut.edu.cn School of Life Science and Biotechnology, Dalian University of Technology, Dalian 116024, China; renjun@mail.dlut.edu.cn (J.R.); lyj81@dlut.edu.cn (L.-Y.J.) Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China; zhanghua@mail.ipc.ac.cn (H.Z.); boshuhui@mail.ipc.ac.cn (S.-H.B.) Correspondence: xyhan@dlut.edu.cn (X.-Y.H.); mszhao@dlut.edu.cn (M.-S.Z.); Tel.: +86-411-8470-6492 (ext. 602) (X.-Y.H.); +86-411-8470-6491 (M.-S.Z.) Received: 8 April 2018; Accepted: 22 May 2018; Published: 31 May 2018 Abstract: Polymers are promising materials for fabricating photonic integrated waveguide devices. Versatile functional devices can be manufactured using a simple process, with low cost and potential mass-manufacturing. This paper reviews the recent progress of polymer photonic integrated devices fabricated using the UV imprinting technique. The passive polymer waveguide devices for wavelength filtering, power splitting, and light collecting, and the active polymer waveguide devices based on the thermal-optic tuning effect, are introduced. Then, the electro-optic (EO) modulators, by virtue of the high EO coefficient of polymers, are described. Finally, the photonic biosensors, which are based on low-cost and biocompatible polymer platforms, are presented. Keywords: polymer photonics; imprinting technique; passive photonic integrated waveguide devices; active photonic integrated waveguide devices; photonic biosensors 1. Introduction Polymers, which have the advantages of low cost, flexile tuning of their refractive index, and a designable function with synthetization at the molecular level, have promising applications for fabricating photonic integrated waveguide devices [1–5]. Showing the high thermal-optic (TO) and electro-optic (EO) coefficients, they are excellent materials for low-power TO functional devices and low half-wave voltage, high-bandwidth EO modulators [6–8]. With the direct physical adsorption of proteins on the waveguide surface or their immobilization in a covalent way, they are also desirable materials for label-free optical biosensors [9–12]. In addition, due to their good compatibility on diverse substrates, they provide good platforms for the hybrid integration of photonic chips [13–15]. Polymers 2018, 10, 603; doi:10.3390/polym10060603 www.mdpi.com/journal/polymers Polymers 2018, 10, 603 2 of 28 Besides the conventional photolithography and etching technique, the imprinting technique can be utilized to fabricate polymer photonic waveguide devices, which can fully exert the plastic property of polymer materials. The imprinting technique was initiated by S.Y. Chou and co-workers in 1990s to fabricate nanometer structures to overcome the diffraction limitation of photolithography [16,17]. It involved a modification of the manufacturing process and was similar to the nanofabricating process developed at NTT Laboratories in Japan in 1970s [18] and by Wulff’s and the Mosbach’s groups in 1980s [19,20]. The technique has been being widely investigated, not only in academic institutes but also in the industrial community [21,22]. Several papers have been presented to review the progress of the imprinting technique comprehensively, including the imprinting craft, the material requirement, and the fabricated function components [23–33]. Since the nanofabricating process was developed at NTT Laboratories, which is considered as the rudiment of the imprinting technique, it has been investigated for over 40 years. Several commercial polymer materials are available for imprinting craft. For thermal imprinting, there are Poly (methyl methacrylate) PMMA, Polystyrene (PS), Polycarbonate (PC), and Cyclic olefin copolymer (COC). For UV imprinting, there is a UV15 series from Masterbond Inc., Ormostamp and Ormocore from Micro Resist Technology Inc., and an NOA series from Norland Inc. Also, for the combined thermal and UV imprinting, there is a SU8 series from MicroChem Corp. and an MR series from Micro Resist Technology. Certainly, there are still some critical issues to be solved to obtain good performance imprinted structures, elements, and devices. First, the imprinting technique is a contact process. The pattern distortions can easily happen during the mold separation process. The flexible/soft molds can be applied, or the mold surface can be coated with a thin anti-adhesive film to facilitate the demolding process. Second, there is usually a residual layer left on the substrate after imprinting, which needs to be removed before subsequent processing. It can be removed by using the etching process, or it can be decreased by using improved techniques, such as the selective imprint technique. Photonic integrated waveguide devices have broad applications in optical fiber communication systems, the photonic on-chip interconnection network, optical biosensors, and the microwave photonic signal processing system. In this paper, the recent progress of polymer photonic waveguide devices fabricated using the imprinting technique, and their applications are reviewed. In order to understand the development of imprinted polymer waveguide devices, some works conducted at the beginning of the imprinting technique are also cited. The rest of the paper is organized as follows. Section 2 introduces briefly the imprinting techniques and focuses on the UV imprinting technique. Section 3 presents the passive photonic integrated devices based on the imprinted polymer waveguide, including microring resonators, optical waveguide splitters, arrayed waveguide gratings, long-period waveguide gratings, and microlenses. Section 4 reviews the active photonic integrated devices using imprinting techniques, for example, TO tuning filters, optical switches, and variable optical attenuators. Section 6 presents the polymeric EO materials and the imprinted EO modulators (EOMs). In Section 5, the optical biosensors based on imprinted polymer waveguides, such as microring resonators and Young’s interferometers, are introduced. Finally, the conclusions and outlook are given in Section 7. 2. Imprinting Techniques To overcome the diffraction limitation of photolithography, the imprinting technique was initiated by S.Y. Chou and co-workers in 1990s [16,17]. Ever since the nanometer scale structures were patterned using the imprinting technique, it has been considered as a promising alternative to expensive UV, deep-UV, and extreme-UV optical lithography for semiconductor integrated circuit industry [21]. Certainly, not only the nanometer scale structures, but also various dimensions, ranging from the nanometer to the micrometer or even millimeter scale, can be formed using the imprinting technique. Therefore, besides the integrated circuits, the photonic and optoelectronic devices can be fabricated using the highly efficient and inexpensive imprinting technique [27]. By using the master mold (hard mold or soft mold) fabricated by photolithography (for large-scale features) or electron-beam lithography (for small-scale features), a reverse replica of the mold pattern can be formed on the Polymers 2018, 10, x FOR PEER REVIEW 3 of 27 using the master mold (hard mold or soft mold) fabricated by photolithography (for large-scale features) or 10, electron-beam lithography (for small-scale features), a reverse replica of the mold Polymers 2018, 603 3 of 28 pattern can be formed on the polymer layer through heating and applied pressure for solid plastic polymers or pressing and following UV curation for liquid polymers, as shown in Figure 1a,b, polymer through applied pressure solid polymers or pressing[24], and which is layer known as the heating thermaland imprinting techniqueforand theplastic UV imprinting technique following UV curation for liquid polymers, as shown in Figure 1a,b, which is known as the thermal respectively. imprinting technique and the UV imprinting technique [24], respectively. Mold UV transparent mold Plastic polymer UV curable polymer Substrate Substrate UV radiation Heat & Press Press @ room temperature Cool & Demold Demold Cross-linked polymer (a) (b) Figure Figure1.1.Schematic Schematicof offabrication fabricationprocess: process:(a) (a)thermal thermalimprinting imprintingand and(b) (b)UV UVimprinting. imprinting. The The UV UV imprinting imprinting technique, technique, compared compared to to the the thermal thermal imprinting imprinting method, method, has has several several advantages. (I) The UV imprinting process works at room temperature, in most cases advantages. (I) The UV imprinting process works at room temperature, in most cases under under low low pressure vacuum contact. This is favorable for molding polymer films onto delicate or curved pressure vacuum contact. This is favorable for molding polymer films onto delicate or curved substrates. substrates. The Themechanical mechanicalproperties propertiesofofthe themold moldare arealso alsoless lessstringent, stringent,and andsuch suchmolds molds can can be be obtained economically by replicating the master mold; (II) The UV curing process is rapid, which obtained economically by replicating the master mold; (II) The UV curing process is rapid, which can can as short of seconds. high throughput can with be achieved with simple be asbeshort as tensas of tens seconds. Therefore,Therefore, high throughput can be achieved simple equipment and equipment and easy processing; (III) The final properties of the imprinted structures can bethe finely easy processing; (III) The final properties of the imprinted structures can be finely tuned by mix tuned bycomponents the mix ratioinofpolymer components in polymer materials or thetime. UV irradiation time. For example, ratio of materials or the UV irradiation For example, high aspect ratio high aspect ratio structures can be obtained from polymeric precursors with lower viscosity. structures can be obtained from polymeric precursors with lower viscosity. For Forthe theUV UV imprinting imprintingtechnique, technique,the themold moldshould shouldbe be transparent transparent for for UV UV irradiation. irradiation. There Thereare are two kinds of UV transparent molds. One is the hard mold made of the dielectrics, such as silicon two kinds of UV transparent molds. One is the hard mold made of the dielectrics, such as silicon oxide oxide and and silicon silicon nitride nitride [34], [34], and and the the other other isis the the soft soft mold mold made made of of polymer polymer materials, materials, such such as as polydimethylsiloxane (PDMS) [35–36], perfluoropolyether (PFPE) [38,39], and UV-curable resins polydimethylsiloxane (PDMS) [35–37], perfluoropolyether (PFPE) [38,39], and UV-curable resins [40]. [40]. mold is relatively cheap simple fabricationprocess. process.In In addition, addition, it The The soft soft mold is relatively cheap duedue to to thethe simple fabrication it is is flexible flexible in terms of the geometry and curvature of the surface, which provides better contact in terms of the geometry and curvature of the surface, which provides better contact with with the the substrate. substrate.Furthermore, Furthermore,the thedemolding demoldingisisusually usuallyeasy easyusing usingthe thesoft softmold, mold,especially especiallyfor forlarge-area large-area imprinting imprinting [41,42]. [41,42]. Therefore, Therefore, the the UV UV imprinting imprinting technique technique with with soft soft mold, mold, named named the the UV UV soft soft imprinting technique, is being widely investigated and applied to fabricate photonic integrated imprinting technique, is being widely investigated and applied to fabricate photonic integrated devices. devices. 3. Passive Photonic Integrated Waveguide Devices 3. Passive Photonic Integrated Waveguide Devices The passive photonic integrated waveguide devices, such as microring resonators, waveguide The passive integrated waveguide devices, suchand as microring resonators, waveguide splitters, arrayedphotonic waveguide gratings, long-period gratings, microlenses, are key components splitters, arrayed waveguide gratings, long-period gratings, and microlenses, are key components consisting of large scale integration photonic circuits. This section presents recent work on passive consisting of large scale integration photonic This section presents recent work on passive photonic integrated waveguide devices using circuits. the UV imprinting technique. photonic integrated waveguide devices using the UV imprinting technique. Polymers 2018, 10, 603 Polymers 2018, 10, x FOR PEER REVIEW 4 of 28 4 of 27 3.1.3.1. Microring MicroringResonators Resonators The basic of aa microring microringwaveguide waveguidecoupled coupled with a single The basicmicroring microringresonator resonatorusually usually consists consists of with a single straight waveguide as shown shownininFigure Figure2a,b. 2a,b.They They can utilized straight waveguideorordouble doublestraight straight waveguides, waveguides, as can bebe utilized as as thethe key structures switches,and andsensors sensors photonic integrated circuit key structuresofoffilters, filters,modulators, modulators, switches, inin thethe photonic integrated circuit [43]. [43]. Figure Schematicstructures structuresof of microring microring resonators. resonators. (a) “add-drop” type. Figure 2. 2.Schematic (a)“All-pass” “All-pass”type; type;(b) (b) “add-drop” type. For the imprinted polymer waveguide, the residual layer of waveguide core should be thin For the imprinted polymer waveguide, the residual layer of waveguide core should be thin enough to confine the optical mode field in the core section. Otherwise, it results in a large radiation enough to confine optical mode in the coreinto section. Otherwise, it results in a for large radiation loss due to the the expanding of thefield optical field the residual layer, especially the bent loss due to the expanding of the optical field into the residual layer, especially for the bent waveguide. waveguide. The etching process is generally needed to reduce the thickness of the residual layer The etching process is generally needed to reduce the thickness of theTwo residual layer of [44]. However, [44]. However, it increases the complexity of waveguide fabrication. new kinds imprinted it increases the complexity of waveguide fabrication. Two new kinds of imprinted polymer waveguide polymer waveguide structures have been developed. One is the inverted ridge waveguide structures have been developed. One is and the inverted generated imprinting generated by imprinting the cladding, the otherridge is thewaveguide ridge waveguide withby a thin residualthe cladding, and the using other is ridge waveguide a thin residual layer generated using the selective layer generated thethe selective imprinting with technique. imprinting technique. 3.1.1. Microring Resonators with Inverted Ridge Waveguide 3.1.1. Microring Resonators with Inverted Ridge Waveguide In [35], the microring resonator with inverted ridge waveguide using the UV soft imprinting In [35], is thepresented. microringThe resonator inverted ridge (PSQ-L) waveguide the UV imprinting technique polymerwith polysiloxane-liquid withusing two forms of soft a high index polymer is PSQ-LH and aThe lowpolymer index polymer PSQ-LL are utilized as the core andforms cladding technique presented. polysiloxane-liquid (PSQ-L) with two of a materials high index for thePSQ-LH invertedand ridge waveguide. PSQ-LPSQ-LL is a kind of silicate-based hybridfor polymer a low index polymer are utilized as the coreinorganic–organic and cladding materials properties of purely (solvent free) and UV curable, which is compatible with thepolymer invertedwith ridge waveguide. PSQ-Lliquid is a kind of silicate-based inorganic–organic hybrid polymer with soft-lithography [45]. Figure 3 shows the fabrication process of the inverted ridge waveguides. The properties of purely liquid (solvent free) and UV curable, which is compatible with soft-lithography [45]. cladding layerthe rather than the core layer imprinted to waveguides. form the groove, and then layer the core Figure 3 shows fabrication process of theisinverted ridge The cladding rather waveguide defined using spin-coating step to filland the then groove. smartly avoids the than the core is layer is imprinted to form the groove, theThis coreprocess waveguide is defined using difficulty of controlling the thickness of residual layer in the conventional imprinting process with spin-coating step to fill the groove. This process smartly avoids the difficulty of controlling the ridge structure. The thickness of the slab layer can be made thin enough by increasing the thickness of residual layer in the conventional imprinting process with ridge structure. The thickness spin-coating speed or using diluted core polymer material. In addition, the structure imprinted in of the slab layer can be made thin enough by increasing the spin-coating speed or using diluted core the cladding layer is helpful for forming the core waveguide using other techniques, such as ink-jet polymer material. In addition, the structure imprinted in the cladding layer is helpful for forming the imprinting [46]. core waveguide using other techniques, such as ink-jet imprinting [46]. Figure 4a,b shows the scanning electron microscope (SEM) pictures of the imprinted low index Figure 4a,b shows the scanning electron microscope (SEM) pictures of the imprinted low index layer and the waveguide cross section after spin-coating the high index core layer. Due to the high layer and the waveguide crosslayer section aftera spin-coating the high index core layer. Due Ittohas the no high spin-coating speed, the core forms shallow concave on the waveguide surface. spin-coating speed, the core layer forms a shallow concave on the waveguide surface. It has influence on the performance of the device, since the most light is confined in the ridge of the core no influence onracetrack the performance the device, since most light is confined in the ridge of the core layer. The microringofresonator based on the the inverted ridge waveguide is fabricated using layer. The racetrack microring resonator based on the inverted ridge waveguide is fabricated using the UV soft imprinting technique, with a radius of 400 μm, a gap between parallel straightthe UVwaveguides soft imprinting a radius 400 µm,length a gap of between straight waveguides of thetechnique, coupler ofwith 1.2 μm, and aof coupling 150 μm.parallel The transmission spectra of of thethe coupler µm, andusing a coupling length 150a µm. Thelaser transmission spectraoperating of the device device of are1.2 measured coupling light of from tunable to the waveguide at TEare measured usingand coupling light from arespectively. tunable laserThe to the waveguideresults operating TE polarization polarization TM polarization, measurement are at shown in Figure 5.and Since the TM mode shows a similar response to the TE mode, the following therefore, TM polarization, respectively. The measurement results are shown in Figureanalysis 5. Sinceis, the TM mode doneafor TE polarization Themode, free spectrum range analysis (FSR) of is, thetherefore, microringdone is about 0.57polarization nm. The shows similar response toonly. the TE the following for TE full-width half-maximum (FWHM) is about 0.037isnm. By 0.57 taking ratio betweenatthe resonance only. The freeatspectrum range (FSR) of the microring about nm.the The full-width half-maximum (FWHM) is about 0.037 nm. By taking the ratio between the resonance wavelength and FWHM, Polymers 2018, 10, 603 Polymers2018, 2018,10, 10,xxFOR FORPEER PEERREVIEW REVIEW Polymers Polymers 2018, 10, x FOR PEER REVIEW 5 of 28 of27 27 55of 5 of 27 4 is calculated. It is expected a quality (Q) factor of aboutaa4.2 × 10(Q) the polarization-independent wavelength and FWHM, FWHM, quality (Q) factor factor of of about about 4.2 4.2 ×× 10 1044 isis that calculated. expected that that the the wavelength and quality calculated. ItIt isis expected wavelength and FWHM, a quality (Q) factor of about 4.2 × 104 is calculated. It is expected that the waveguides can be fabricated by optimizing the waveguides dimension. polarization-independent waveguides can be fabricated by optimizing the waveguides dimension. polarization-independent waveguides can be fabricated by optimizing the waveguides dimension. polarization-independent waveguides can be fabricated by optimizing the waveguides dimension. Figure 3.Fabrication Fabrication processofof ofthe theinverted invertedridge ridgewaveguide waveguideusing using UV soft imprinting technique [35]. Figure process the inverted ridge technique [35]. Figure 3. 3. Fabrication process waveguide usingUV UVsoft softimprinting imprinting technique [35]. Figure 3. Fabrication process of the inverted ridge waveguide using UV soft imprinting technique [35]. Figure 4.4. The The SEM SEMpictures pictures of of(a) (a)the theimprinted imprintedlow lowindex indexlayer layerand and(b) (b)the thewaveguide waveguidecross crosssection section Figure Figure 4. The SEM pictures layer and and(b) (b)the thewaveguide waveguide cross section Figure 4. The SEM picturesofof(a) (a)the theimprinted imprinted low low index index layer cross section afterspin-coating spin-coatingthe thehigh highindex indexlayer layer[35]. [35]. after after spin-coating the high after spin-coating the highindex indexlayer layer[35]. [35]. Figure 5.5. Measured Measured transmission transmission spectra spectra of of the the inverted inverted ridge ridge waveguide waveguide microring microring resonator resonator Figure Figure 5. 5.Measured ridge waveguide waveguidemicroring microringresonator resonator Figure Measuredtransmission transmission spectra spectra of of the the inverted inverted ridge operating at (a) TE polarization and (b) TM polarization [35]. operating at (a) TE polarization and(b) (b) TMpolarization polarization [35]. operating at at (a)(a) TETE polarization operating polarizationand and (b)TM TM polarization [35]. [35]. Polymers 2018, 10, 603 Polymers 2018, 10, x FOR PEER REVIEW 6 of 28 6 of 27 3.1.2. Microring Microring Resonators Resonators with with Ridge Ridge Waveguide Waveguide 3.1.2. Photonic some applications, applications, Photonicwaveguide waveguidedevices deviceswith withridge ridge structure structure are are more more desirable desirable for for some especially for for biosensors. biosensors. There There will will be be much much interaction interaction between between the the optical optical evanescent evanescent wave wave and and especially the tested biological sample, resulting in a high sensitivity. However, the residual slab layer of the the tested biological sample, resulting in a high sensitivity. However, the residual slab layer of the imprinted ridge waveguide is usually very thick, which limits the minimum radius of microring, as imprinted ridge waveguide is usually very thick, which limits the minimum radius of microring, well as as thethe FSR. Some researchers proposed as well FSR. Some researchershave haveexpended expendedaalot lotofofeffort effortto tosolve solve it, it, but but usually usually the the proposed solutions require expensive imprinting equipment or a specific setup, for example, the solutions require expensive imprinting equipment or a specific setup, for example, the vacuum-assisted vacuum-assisted microfluidic technique [47]. The selective imprinting technique was developed in microfluidic technique [47]. The selective imprinting technique was developed in our group to reduce ourthickness group to of reduce the thickness of the residual slab layer [36]. the the residual slab layer [36]. The schematic processes of the conventional selective imprinting imprinting The schematic processes of the conventionalimprinting imprintingtechnique technique and and the the selective techniqueare areillustrated illustratedin in Figure 6. Instead of using the single to directly technique Figure 6. Instead of using the single trenchtrench patternpattern mold tomold directly imprint imprint the waveguide into the core layer as shown in Figure 6a, the ridge shape waveguide is the waveguide into the core layer as shown in Figure 6a, the ridge shape waveguide is formed by formed by imprinting two trenches on both sides of the waveguide (see Figure 6b). Only a small imprinting two trenches on both sides of the waveguide (see Figure 6b). Only a small amount of amount of the polymer to bebysqueezed byTherefore, the mold.the Therefore, theofresistance of the polymer the polymer needs to beneeds squeezed the mold. resistance the polymer against the against the mold is low, and the requirement for the strength of the force exerted on the mold can mold is low, and the requirement for the strength of the force exerted on the mold can be minimized be minimized Duringimprinting the selective imprinting process, waveguide the polymeriswaveguide patterned greatly. Duringgreatly. the selective process, the polymer patterned is only by the only by the capillary forces themold PDMS softthe mold and the substrate, anyimprinting additional capillary forces between thebetween PDMS soft and substrate, without anywithout additional imprinting tools. A polymer waveguide with good rectangular shape, as well as residual negligible residual tools. A polymer waveguide with good rectangular shape, as well as negligible slab layer slab layer thickness (below 100 nm), fabricated with this technique is shown in Figure 7. thickness (below 100 nm), fabricated with this technique is shown in Figure 7. The ridge waveguide waveguide racetrack racetrack microring microring resonator resonator with with negligible negligible residual residual layer layer is is fabricated fabricated The ridge using the selective imprinting technique. Figure 8 shows the SEM images of the entire structure and using the selective imprinting technique. Figure 8 shows the SEM images of the entire structure and the detail detailofofthe thecoupling coupling region. Because consisted waveguides are with air cladding, the region. Because all all the the consisted waveguides are with air cladding, a smalla small transformation of the ring structure into a racetrack is made as well in order to increase the transformation of the ring structure into a racetrack is made as well in order to increase the coupling. coupling. Figure 9 gives the measurement results for the racetrack microring resonator with Figure 9 gives the measurement results for the racetrack microring resonator with a radius of 150 µma radius of 150as μm andasQ-factor high × 104. ThetoFSR is nm increased to 1.252 with and Q-factor high 5 × 104 .as The FSRasis5increased 1.252 compared withnm thecompared inverted ridge the inverted ridge waveguide-based microring, while no degradation of the performances is waveguide-based microring, while no degradation of the performances is observed. From this result, observed. From this result, it can be concluded that no significant bending loss can arise from the it can be concluded that no significant bending loss can arise from the residual layer, which has been residual to layer, which been reduced to a very thin level during the imprinting process. reduced a very thinhas level during the imprinting process. Figure Figure 6. 6. The The schematic schematic process process of of (a) (a) the the conventional conventional imprinting imprinting technique technique and (b) (b) the the selective selective imprinting imprinting technique technique [36]. [36]. Polymers 2018, 2018, 10, 10, 603 x FOR PEER REVIEW Polymers 2018, 2018, 10, 10, xx FOR FOR PEER PEER REVIEW REVIEW Polymers 7 of 28 27 of 27 27 77 of Figure 7. The SEM picture of the fabricated ridge shaped waveguide with negligible residual layer Figure7. 7. The The SEM SEM picture picture of ofthe thefabricated fabricatedridge ridgeshaped shapedwaveguide waveguide with withnegligible negligible residual residual layer layer Figure Figure 7. The SEM picture of the fabricated ridge shaped waveguide with negligible residual layer thickness [36]. thickness [36]. thickness [36]. thickness [36]. Figure 8. The SEM pictures of the racetrack microring resonator fabricated by the selective Figure8.8. 8.The The SEM pictures ofracetrack the racetrack racetrack microring resonator fabricated by the the selective Figure SEM pictures of the microring resonator fabricatedfabricated by the selective imprinting Figure The SEM pictures of the microring resonator by selective imprinting technique. (a) Image of the entire structure; (b) detail of the coupling region [36]. imprinting technique. (a) Image of the entire structure; (b) detail of the coupling region [36]. technique. Image of(a) theImage entireof structure; detail of (b) thedetail coupling region [36]. region [36]. imprinting(a) technique. the entire(b) structure; of the coupling Measurementresults results of polymer the polymer microring resonator withwaveguide. ridge waveguide. (a) Figure 9.9.Measurement of the microring resonator with ridge (a) Through Figure 9. 9. Measurement Measurement results results of the the polymer polymer microring resonator with ridge ridge waveguide. waveguide. (a) Figure of microring resonator with (a) Through port;port. (b) drop port. port; (b) drop Through port; (b) drop port. Through port; (b) drop port. 3.2. Optical Waveguide Splitters Splitters 3.2. 3.2. Optical Optical Waveguide Waveguide Splitters Splitters 3.2. Optical Waveguide Optical splitters, also alsoknown knownasas splitters, of most the most important passive in Optical splitters, areare oneone of the important passive devicesdevices in optical Optical splitters, splitters, also also known known as as splitters, are one of the the most most important passive devices in Optical splitters, splitters, are one of important passive devices in optical fiber communication system. With the rapid development of telecommunication system, fiber communication system. With the rapid development of telecommunication system, especially the optical fiber communication system. With the rapid development of telecommunication system, optical fiber communication system. With the rapid development of telecommunication system, especially the fiber-to-the-home the need for high-performance, low-cost has fiber-to-the-home (FTTH), the need(FTTH), for high-performance, low-cost splitters has becomesplitters increasingly especially the the fiber-to-the-home fiber-to-the-home (FTTH), the need need for for high-performance, high-performance, low-cost splitters has especially (FTTH), the low-cost splitters has become The increasingly urgent. Thesplitters fiber-based splitters with oneoutputs input and outputs urgent. fiber-based optical withoptical one input and multiple aremultiple usually utilized. become increasingly urgent. The fiber-based optical splitters with one input and multiple outputs become increasingly urgent. The fiber-based optical splitters with one input and multiple outputs are usuallythe utilized. uniform thethe divided power insertionwhen loss will However, outputHowever, uniform ofthe theoutput divided powerof and insertion lossand willthe deteriorate the are usually usually utilized. utilized. However, the output uniform of the divided divided power and the insertion loss loss will will are However, the output uniform of the power and the insertion deteriorate when the number of outputs increases. The photonic integration technique provides a deteriorate when when the the number number of of outputs outputs increases. increases. The The photonic photonic integration integration technique technique provides provides aa deteriorate Polymers 2018, 10, 603 Polymers 2018, 10, x FOR PEER REVIEW 8 of 28 8 of 27 number outputs increases. photonic integration techniquesplitters provideswith a potential potentialof way with which The to fabricate optical waveguide small way sizewith andwhich good to fabricate optical waveguide splitters with small size and good performance [48,49]. With the help of performance [48,49]. With the help of imprinting technique, polymer-based optical waveguide imprinting technique, polymer-based optical splitters splitters can be fabricated with low cost and waveguide low insertion loss. can be fabricated with low cost and low insertion loss. Y-branch-based optical waveguide splitter with preliminary demonstration of 1 × 4 structure is Y-branch-based with[50]. preliminary × 4experiment structure fabricated using the optical UV softwaveguide imprintingsplitter technique Polymer demonstration materials used of in 1the is fabricated usingacrylate the UV resin, soft imprinting technique [50]. Polymer materials the experiment are fluorinated LFR (ChemOptics, Daejeon, Korea), withused the in refractive indices are fluorinated acrylate resin, LFR (ChemOptics, Daejeon, Korea), with the refractive indices ranging ranging from 1.375 to 1.395 at 1550 nm wavelength. PDMS mold is replicated from a silicon mold from 1.375 to 1.395molding at 1550 nm wavelength. PDMScured moldafter is replicated a silicon through a casting method and properly baking atfrom 60 °C for 4 h.mold LFR through with the alower casting moldingindex method and properly after baking at 60 4 h. LFR with layer. the lower refractive is spin-coated oncured the silicon substrate as◦ C theforunder cladding The refractive index is spin-coated on the silicon substrate as the under cladding layer. The grooves on the grooves on the cladding layer fabricated from PDMS mold imprinting are filled with the LFR with cladding layer fabricated moldThe imprinting are filledlayer with the LFRspun with higher higher refractive index asfrom the PDMS core layer. upper cladding is then on top refractive using the index as the core This layer. The upper cladding layer is then the spun on top of using the samelayer, procedure. same procedure. inverted waveguide helps to reduce thickness the residual which This inverted waveguide helps to reduce the thickness of the residual layer, which can be controlled can be controlled by the spinning speed during the core layer preparation. Figure 10a shows the by the spinning during theafter corebeing layer filled preparation. Figure 10a shows the LFR. cross-section the cross-section of speed the waveguide with higher refractive index It can beofseen waveguide after being withrectangular, higher refractive index It can beon seen the core shape is that the core shape is filled perfectly and no layerLFR. is residual thethat waveguide top. The perfectly rectangular, and noarea layer residual on the10b, waveguide top. The image of is Y-junction optical image of Y-junction is is given in Figure which shows thatoptical the waveguide split into area given in Figure which shows that the waveguide is split into two paths with a good form. two is paths with a good10b, form. Figure 10. Cross-section view showing rectangular core shape and no residual layer on the waveguide Figure 10. (a) Cross-section view showing rectangular core shape and no residual layer on the top; (b) the optical microscopic image of Y-junction area [50]. waveguide top; (b) the optical microscopic image of Y-junction area [50]. During During the the imprinting imprinting process, process, the the deformation deformation of of the the waveguide waveguide structure structure and and the the Y-junction Y-junction area forfor the the splitter performance [51]. It[51]. is found sometimes the Y-junction is demolded areaisiscrucial crucial splitter performance It is that found that sometimes the Y-junction is with defects after demolding due to the ultra-narrow gap at the splitting section. In order to solve demolded with defects after demolding due to the ultra-narrow gap at the splitting section. In order this problem, multimode interference interference (MMI)-based(MMI)-based optical waveguide is designed and to solve this the problem, the multimode opticalsplitter waveguide splitter is fabricated using the similar UV soft imprinting technique [52]. The MMI based splitter, shown designed and fabricated using the similar UV soft imprinting technique [52]. The MMI based in Figureshown 11a, which avoids narrow structure as Y-junction splitter, isbased tolerant of the splitter, in Figure 11a,the which avoids the narrow structurebased as Y-junction splitter, is structure by the fabrication process. A taper is introduced the between multimode tolerant ofdeviations the structure deviations by the fabrication process. A taper isbetween introduced the waveguide single mode waveguide lower coupling loss.coupling According the self-imaging multimode and waveguide and single modefor waveguide for lower loss.to According to the theory of multimode waveguide, the MMI-based waveguide splitter is optimized with the structure self-imaging theory of multimode waveguide, the MMI-based waveguide splitter is optimized with parameters L =parameters 850 µm, D =L 22 µm, μm, W =D 40 =µm, µm. The simulated beam propagation taper the structure = 850 22 and μm, LW = =40350 μm, and Ltaper = 350 μm. The simulated in 1 × propagation 4 waveguidein splitter is illustrated in Figure 11b. As in theFigure splitter is expected to connect with beam 1 × 4 waveguide splitter is illustrated 11b. As the splitter is expected fiber array, S-bend waveguide is applied to enlarge the distance between output ports (to be 250 µm). to connect with fiber array, S-bend waveguide is applied to enlarge the distance between output Figure 11cbe shows the microscopic image the of the connectionimage section the multimode waveguide ports (to 250 μm). Figure 11c shows microscopic of between the connection section between the and the two single mode waveguides. The average insertion loss of the 1 × 4 waveguide multimode waveguide and the two single mode waveguides. The average insertion loss ofsplitter the 1 ×is4 12.98 dB withsplitter uniformity 1.08 dB dB. In addition, the average polarization-dependent is waveguide is of 12.98 with uniformity of 1.08 dB. In addition, loss the (PDL) average measured as small as 0.05 dB due to the low birefringence of the polymer material and the square polarization-dependent loss (PDL) is measured as small as 0.05 dB due to the low birefringence of cross-section of the waveguide. the polymer material and the square cross-section of the waveguide. Polymers 2018, 10, 603 Polymers 2018, 10, x FOR PEER REVIEW 9 of 28 9 of 27 Figure Figure 11. 11. (a) (a) Schematic Schematic of of the the MMI-based MMI-based waveguide waveguide splitter; splitter; (b) (b) the the simulated simulated beam beam propagation propagation in 1 × 4 waveguide splitter; and (c) the microscopic image of the connection section in 1 × 4 waveguide splitter; and (c) the microscopic image of the connection section between between the the multimode multimode waveguide waveguide and and the the two two single single mode mode waveguides waveguides [52]. [52]. 3.3. Arrayed Waveguide Gratings 3.3. Arrayed Waveguide Gratings The arrayed waveguide grating (AWG) with the similar function of diffraction grating can be used as a wavelength multiplexer or demultiplexer. AWGs play an important role for the dense wavelength division multiplexing (DWDM) optical fiber communication system. In [53], the soft PDMS mold with AWG patterns is prepared from a fine quartz mold, which is then used to stamp on the polymer core layer with precise contact, as well as easy lift-off process. Figure 12a shows the top view of the fabricated AWG, in which ZPU12-450 with refractive index of 1.45 at wavelength of 1.55 µm and ZPU12-430 with refractive index of 1.43 at wavelength of 1.55 µm from ChemOptics are utilized as core and cladding, respectively. It can be seen from Figure 12a that there are mainly five parts in the AWG, including a single input waveguide, two slab waveguide, a group of arrayed waveguides, and multiple output waveguides. Except the slab waveguide, the waveguide is basically a single mode one. In order to improve the lightwave coupling efficiency between the slab waveguide and the single mode waveguide, a tapered structure has been incorporated in between. A reverse imprint procedure is applied, as shown in Figure 12b, incorporating a PDMS stamp, in which the waveguide patterns are formed in the core instead of the lower cladding without resorting to spin-coating process. In this way, the height of the pattern can be determined using the precisely prepared PDMS stamp. In addition, the(a)thickness ofdiagram the residual layer also be controlled by pressing PDMS (b) against Figure 12. Schematic showing thecan plan view of the imprinted AWG device; the the lowerreverse cladding withprocedure various strengths, as well as varying the viscosity amount of the core polymer. imprint for core layer patterning [53]. Copyright 2007and Elsevier. An 8-channel AWG is fabricated showing a 0.8 nm spacing with the center wavelength ranging from 1543.7 to arrayed 1548.3 nm. The 3 dB grating bandwidth is about and the channel crosstalk is grating approximately The waveguide (AWG) with0.8 thenm, similar function of diffraction can be 10 dB.asItaiswavelength found that the existenceor of demultiplexer. the undesired residual layerancaused bandwidth used multiplexer AWGs play important role forbroadening, the dense which is attributed the mismatch betweenoptical the output and the focal length of the wavelength divisiontomultiplexing (DWDM) fiber channel communication system. In [53], thebeam. soft This unwanted residual mayisbeprepared improved using sol-gel polymer materials or used by introducing PDMS mold with AWG layer patterns from a fine quartz mold, which is[54] then to stamp further post-etching on the polymer core processes. layer with precise contact, as well as easy lift-off process. Figure 12a shows the top view of the fabricated AWG, in which ZPU12-450 with refractive index of 1.45 at wavelength of 1.55 μm and ZPU12-430 with refractive index of 1.43 at wavelength of 1.55 μm from ChemOptics are utilized as core and cladding, respectively. It can be seen from Figure 12a that there are mainly five parts in the AWG, including a single input waveguide, two slab waveguide, a group of arrayed waveguides, and multiple output waveguides. Except the slab waveguide, the waveguide is basically a single mode one. In order to improve the lightwave coupling efficiency between the slab waveguide and the single mode waveguide, a tapered structure has been incorporated in between. A reverse imprint procedure is applied, as shown in Figure 12b, incorporating a PDMS stamp, in which the waveguide patterns are formed in the core instead of the lower cladding without resorting to spin-coating process. In this way, the height of the pattern can be determined using the Figure 11. (a) Schematic of the MMI-based waveguide splitter; (b) the simulated beam propagation in 1 × 4 waveguide splitter; and (c) the microscopic image of the connection section between the multimode waveguide and the two single mode waveguides [52]. Polymers 2018, 10, 603 3.3. Arrayed Waveguide Gratings 10 of 28 Figure 12. 12. (a) (a)Schematic Schematicdiagram diagram showing the imprinted AWG device; (b) the Figure showing thethe planplan viewview of theofimprinted AWG device; (b) the reverse reverse imprint procedure for core layer patterning [53]. Copyright 2007 Elsevier. imprint procedure for core layer patterning [53]. Copyright 2007 Elsevier. The arrayed waveguide grating (AWG) with the similar function of diffraction grating can be 3.4. Long-Period Waveguide Gratings used as a wavelength multiplexer or demultiplexer. AWGs play an important role for the dense Long-period gratings have various as sensors,system. band-rejection filters, wavelength division multiplexing (DWDM)applications, optical fiber such communication In [53], the soft and gain flatteners for erbium-doped fiber amplifier (EDFA). During the manufacturing process, PDMS mold with AWG patterns is prepared from a fine quartz mold, which is then used to stamp the conventional long-period fiber gratings are limited byas theeasy geometrical size of the fiber12a andshows materials. on the polymer core layer with precise contact, as well lift-off process. Figure the To thisfabricated issue, long-period gratingswith (LPWGs) haveindex been demonstrated with large topovercome view of the AWG, inwaveguide which ZPU12-450 refractive of 1.45 at wavelength of variety of materials and different fabrication methods to select from [55]. In [56], the UV soft imprinting 1.55 μm and ZPU12-430 with refractive index of 1.43 at wavelength of 1.55 μm from ChemOptics technique utilized to fabricate LPWG. Figure 13abe shows schematic structure of the are utilizedisas core and cladding,arespectively. It can seen the from Figure 12a that there are LPWG, mainly in which the rib waveguide is raised by the periodic grating on the cladding-core interface. There are five parts in the AWG, including a single input waveguide, two slab waveguide, a group of arrayed two processes for fabricating the LPWG with UV soft imprinting technique, as illustrated in Figure 13b. waveguides, and multiple output waveguides. Except the slab waveguide, the waveguide is The first process to imprint the grating waveguides as the steps shown from (i) to between (v) in Figure 13b. basically a singleismode one. In order to improve the lightwave coupling efficiency the slab The cladding layer is using polymer material UV15. A PDMS soft mold with 80 µm period grating waveguide and the single mode waveguide, a tapered structure has been incorporated in between. patterns is imprint used to stamp on the layerindue to its12b, goodincorporating flexibility foraitsPDMS betterstamp, accuracy A reverse procedure is uncured applied, UV15 as shown Figure in for structure replication.patterns The second to imprint rib waveguide as the cladding steps shown from which the waveguide areprocess formedis in the corethe instead of the lower without (vi) to (x) to in spin-coating Figure 13b. Another with waveguide feature can is then applied onto the top resorting process. PDMS In this mold way, the height of the pattern be determined using the cladding layer to form the rib waveguide. Polymer waveguides are diced with a high-speed polishing saw, which can form nice end-faces for butt-coupling tests afterwards. It is found that the resonance of the grating occurs at 1585 nm with a 3 dB bandwidth of 12 nm and rejection level of 10 dB. These results show that the UV soft imprinting technique has the capability to fabricate three-dimensional functional photonic devices [36]. waveguide as the steps shown from (vi) to (x) in Figure 13b. Another PDMS mold with waveguide feature is then applied onto the top cladding layer to form the rib waveguide. Polymer waveguides are diced with a high-speed polishing saw, which can form nice end-faces for butt-coupling tests afterwards. It is found that the resonance of the grating occurs at 1585 nm with a 3 dB bandwidth of 12 nm and level of 10 dB. These results show that the UV soft imprinting technique has11the Polymers 2018, 10,rejection 603 of 28 capability to fabricate three-dimensional functional photonic devices [36]. (a) i) ii) vi) iii) vii) iv) viii) v) x) (b) Figure 13. 13.(a)(a)Schematic two processes processes for for imprinting imprintingthe thegrating grating Figure Schematic structure structure of of the the LPWG; LPWG; (b) two waveguides (i)–(v) and forfor ribrib waveguide IEEE. waveguides (i)–(v) and waveguide(vi)–(x) (vi)–(x)[56]. [56].Copyright Copyright 2005 IEEE. Polymers 2018, 10, x FOR PEER REVIEW 11 of 27 3.5. Microlenses Microlenses and microlens arrays (MLAs) have shown promising ability for light collection and collimation, of of applications including imaging sensors, interconnects, and collimation, with witha alarge largerange range applications including imaging sensors, interconnects, photodetectors. Various fabrication methods for MLAs havehave beenbeen investigated. However, the and photodetectors. Various fabrication methods for MLAs investigated. However, fabrication costcost is still too too highhigh for large-scale production duedue to the complex steps involved [57].[57]. In the fabrication is still for large-scale production to the complex steps involved [42], thethe polymer 4040 × 40 microlens array with a pitch ofof250 In [42], polymer × 40 microlens array with a pitch 250μm, µm,aadiameter diameterofof240 240μm, µm, and and a sag height of of24.5 24.5µm μm have been prepared. Microlenses are replicated into UV polymer curing polymer have been prepared. Microlenses are replicated into the UVthe curing material material PAK-01 by stamp step and UV imprinting on silicon substrates with aofdiameter PAK-01 by step and UVstamp imprinting technique technique on silicon substrates with a diameter 150 mm. of mm. The resulting substrates are used masters to cast With PDMS the The150 resulting substrates are used as masters to castasPDMS templates. thetemplates. optimized With dispense optimized dispense strategy, including the adjustment of the dispense height, pressure, and strategy, including the adjustment of the dispense height, pressure, and amplitude, air bubbles can be amplitude, airdispensed bubbles can avoided in14a theshows dispensed resist layer. Figure 14a shows the SEM avoided in the resistbelayer. Figure the SEM image of the replicated microlenses into image of PAK-01. the replicated microlenses into microlens the resist profile PAK-01. The surface of imprinted microlens the resist The surface of imprinted measured by a white light profilometer is profile by a white light profilometer is shown in Figure 14b. shown measured in Figure 14b. Figure 14. (a) (a) SEM SEM image image of ofthe thereplicated replicatedmicrolenses microlensesinto intothe theresist resistPAK-01; PAK-01; surface profile of Figure 14. (b)(b) surface profile of an an imprinted microlens [42]. Copyright 2010 Elsevier. imprinted microlens [42]. Copyright 2010 Elsevier. 4. Active Photonic Integrated Waveguide Devices Compared with the inorganic optical materials, such as silica, glass, and SiN4, organic polymers have large TO coefficient, which has the potential to decrease the power consumption of TO integrated waveguide devices [58,59]. This section gives some imprinted active polymer waveguide Polymers 2018, 10, 603 12 of 28 4. Active Photonic Integrated Waveguide Devices Compared with the inorganic optical materials, such as silica, glass, and SiN4 , organic polymers have large TO coefficient, which has the potential to decrease the power consumption of TO integrated waveguide devices [58,59]. This section gives some imprinted active polymer waveguide devices based on TO tuning effect. 4.1. Tunable Microring Resonator Filters Polymers 2018, 10, x FOR PEER REVIEW 12 of 27 As mentioned in Section 3.1, the microring resonators are the key unit in the photonic integrated tuningItduring thisthat process generates an additional phase shift to the microring resonator, which circuit. is desired the coupling coefficient or the resonant wavelength of the microring resonator results in a small resonant wavelength shift. can be compensated for by15b tuning the the phase shift on can be tuned according to the requirement ofItapplications [60,61]. Figure shows schematic the microring waveguide. The resonant wavelength of the fabricated polymer microring resonator structure of the tunable microring resonator, in which the Mach-Zehnder interferometer (MZI) with a can also be on tuned. This is realized by applying the the power to the micro-heater on the microring micro-heater one of its arms is introduced to replace conventional directional coupler, as shown waveguide, as the measured results show in Figure 16c. It can be seen that the resonant wavelength in Figure 15a, while another micro-heater is placed on the microring waveguide. Consequentially, may be tuned freely within the resonant FSR around 0.13 nm.can be tuned with micro-heaters. the coupling coefficient and the waveguide Figure15. 15.Schematic Schematicstructures structuresofof microring resonators consisting ofa(a) a directional coupler Figure thethe microring resonators consisting of (a) directional coupler and and (b) a MZI based tunable coupler. (b) a MZI based tunable coupler. The MZI-based polymer microring resonator is fabricated using the UV soft imprinting technique, and then the micro-heaters are patterned using the lift-off process [62]. The optical microscopic image of the tunable microring resonator device is shown in Figure 16a. The tunable properties of the fabricated polymer microring resonator are characterized. Different coupling states [60], including “over-coupling”, “critical coupling”, and “under-coupling”, are realized by applying the power only to the micro-heater on the MZI arm, as shown in Figure 16b. The controllable equivalent coupling coefficient is very useful for the application of microring resonator-based tunable time delay lines [63]. It is noted that such equivalent coupling coefficient tuning during this process generates an additional phase shift to the microring resonator, which results in a small resonant wavelength shift. It can be compensated for by tuning the phase shift on the microring waveguide. The resonant wavelength of the fabricated polymer microring resonator can also be tuned. This is realized by applying the power to the micro-heater on the microring waveguide, as the measured results show in Figure 16c. It can be seen that the resonant wavelength may be tuned freely within the FSR around 0.13 nm. The fabricated tunable microring resonator is utilized as a notch filter to suppress one sideband of the radio frequency (RF) modulated lightwave signal with the output of single sideband (SSB) signal. The SSB signal can overcome the chromatic dispersion-induced power fading effect for high frequency RF signal transmission over a long fiber [64]. The experimental setup is shown in Figure 17, in which Figure (a)(b) The optical image of the fabricated polymer waveguide filter; (b) filter the insets (a)16. and show themicroscopic measured optical spectra of RF (7.2 GHz) modulated lightwave before response spectra of the microring resonator by tuning the MZI based coupler; and (c) resonant and after the microring resonator. It can be seen that about 18 dB of the left sideband is suppressed. wavelengththe tuning response [62].filtering response of the microring resonator, Quadrature Phase To demonstrate validity of SSB Shift Keying (QPSK) signal of 1 Msps carried by 7.2 GHz microwave is transmitted over 70 km single The fabricated tunable microring resonator is utilized as a notch filter to suppress one sideband mode fiber (SMF). For comparison, the double sideband (DSB) signal transmission is also tested, of the radio frequency (RF) modulated lightwave signal with the output of single sideband (SSB) in which the equal insertion loss of the integrated waveguide chip is provided by a tunable optical signal. The SSB signal can overcome the chromatic dispersion-induced power fading effect for high frequency RF signal transmission over a long fiber [64]. The experimental setup is shown in Figure 17, in which the insets (a) and (b) show the measured optical spectra of RF (7.2 GHz) modulated lightwave before and after the microring resonator. It can be seen that about 18 dB of the left sideband is suppressed. To demonstrate the validity of SSB filtering response of the microring Polymers 2018, 10, 603 13 of 28 attenuator. After 70 km SMF transmission, the error vector magnitude (EVM) of SSB carrying signal is structures of the microring resonators offeasibility (a) a directional 22.4%,Figure while15. theSchematic one of DDB carrying signal is 61.3%, which consisting proves the of thecoupler microring and (b) a MZI based tunable coupler. resonator as a notch filter. Figure16. 16. (a) (a) The The optical optical microscopic microscopic image image of of the the fabricated fabricated polymer polymerwaveguide waveguide filter; filter;(b) (b)filter filter Figure responsespectra spectraofofthe themicroring microringresonator resonatorby bytuning tuningthe theMZI MZIbased basedcoupler; coupler; and and(c) (c)resonant resonant response wavelengthtuning tuningresponse response[62]. [62]. wavelength Polymers 2018, 10, x FOR PEER REVIEW 13 of 27 The fabricated tunable microring resonator is utilized as a notch filter to suppress one sideband RF signal Tunable signal ring of the radio frequency (RF) modulated lightwave with the output of single sideband (SSB) generator resonator Optical fiber signal. The SSB signal can overcome the chromatic dispersion-induced power fading effect for high frequency RF signal transmission over a long fiber [64]. The experimental setup is shown in Figure RF spectrum TLS 17, in which the insetsEOM (a) and EDFA (b) show the measured optical spectraPDof RF (7.2 GHz) modulated analyzer PC PC lightwave before and after the microring resonator. It can be seen that about 18 dB of the left sideband is suppressed. To demonstrate the validity of SSB filtering response of the microring resonator, Quadrature Phase Shift Keying (QPSK) signal of 1 Msps carried by 7.2 GHz microwave is transmitted over 70 km single mode fiber (SMF). For comparison, the double sideband (DSB) signal transmission is also tested, in which the equal insertion loss of the integrated waveguide chip is provided by a tunable optical attenuator. After 70 km SMF transmission, the error vector magnitude (EVM) of SSB carrying signal is 22.4%, while the one of DDB carrying signal is 61.3%, which proves the feasibility of the microring resonator as a notch filter. Figure 17. Experiment setup for RF signal transmission over fiber by using the tunable microring Figure 17. Experiment setup for RF signal transmission over fiber by using the tunable microring resonator as a notch filter, in which the insets (a) and (b) show the measured optical spectra of RF resonator as a notch filter, in which the insets (a) and (b) show the measured optical spectra of RF modulated lightwave before and after the microring resonator [62]. modulated lightwave before and after the microring resonator [62]. 4.2. 4.2. Tunable Tunable Waveguide Waveguide Bragg Bragg Grating Grating Filters Filters Optical waveguide a basic waveguide filtering element, can be utilized to select Optical waveguideBragg Bragggrating, grating,asas a basic waveguide filtering element, can be utilized to aselect certain wavelength for external cavity lasers [65] or the channel selective receiver in the optical a certain wavelength for external cavity lasers [65] or the channel selective receiver infiber the network [66]. network A tunable[66]. TO polymer waveguide Braggwaveguide grating (TOPWBG), with a(TOPWBG), structure as shown optical fiber A tunable TO polymer Bragg grating with a in Figure 18a, is fabricated using the imprinting technique [67]. The TOPWBG is with is a structure as shown in Figure 18a, is fabricated using the imprinting technique [67].measured The TOPWBG reflectivity of 25 a and 3 dB bandwidth of 0.8 It is used asof a wavelength selective in the ring measured with reflectivity of 25 and 3 nm. dB bandwidth 0.8 nm. It is used asfilter a wavelength laser system, as shown in Figure 18b. The established tunable laser exhibits an output of 4.1 dBm, selective filter in the ring laser system, as shown in Figure 18b. The established tunable laser side-mode suppression ratio of 55, and 3 dB bandwidth of less than 16 pm. By driving the heater exhibits an output of 4.1 dBm, side-mode suppression ratio of 55, and 3 dB bandwidth of less with than different powers, wavelength can bepowers, tuned due thewavelength TO tuning of thebeBragg 16 pm. By drivingthe thelaser heater with different the to laser can tunedwavelength due to the TO tuning of the Bragg wavelength of the TOPWBG. The measured laser output spectra with different heating powers are shown in Figure 18c with the wavelength tuning range of 8.4 nm. According to the relationship between the heating power and the laser wavelength, a good linearity of about 32 pm/mW is obtained. structure as shown in Figure 18a, is fabricated using the imprinting technique [67]. The TOPWBG is measured with a reflectivity of 25 and 3 dB bandwidth of 0.8 nm. It is used as a wavelength selective filter in the ring laser system, as shown in Figure 18b. The established tunable laser exhibits an output of 4.1 dBm, side-mode suppression ratio of 55, and 3 dB bandwidth of less than Polymers 10, 603 the heater with different powers, the laser wavelength can be tuned due to 14 of 28 16 pm. 2018, By driving the TO tuning of the Bragg wavelength of the TOPWBG. The measured laser output spectra with different heating powers are shown in Figure 18c with the wavelength tuning range of 8.4 nm. of the TOPWBG. The measured laser output spectra with different heating powers are shown in According to the relationship between the heating power and the laser wavelength, a good linearity Figure 18c with the wavelength tuning range of 8.4 nm. According to the relationship between the of about 32 pm/mW is obtained. heating power and the laser wavelength, a good linearity of about 32 pm/mW is obtained. Figure Figure 18. 18. (a) (a)The Thestructure structureof ofthe theTOPWG; TOPWG;(b) (b) the theschematic schematic of of the the proposed proposed tunable tunable fiber fiber ring ring laser laser with with TOPWBG TOPWBG as as the the wavelength wavelengthselective selectivefilter; filter;and and(c) (c)the the output outputspectra spectra of of the the tunable tunablefiber fiber ring ring laser laser with with different different heating heating power power [67]. [67]. Copyright Copyright 2015 2015 Elsevier. Elsevier. 4.3. Optical Switches Optical switches and related arrays play important roles in many applications, such as optical routing [68], optical add–drop multiplexing [69], and optical true time delay [70] in the DWDM optical fiber communication system and the optical signal processing system. A printable TO switch is demonstrated by utilizing imprinting and ink-jet printing techniques [46]. A polymer material named UV15LV with refractive index of 1.501 at wavelength of 1.55 µm from MasterBond is selected as bottom cladding layer, SU8-2000.5 with refractive index of 1.575 at wavelength of 1.55 µm from MicroChem as core layer, and UFC-170A with refractive index of 1.496 at wavelength of 1.55 µm from URAY Co. Ltd. (Austin, TX, USA) as the top cladding layer. Figure 19 illustrates the process flow for fabricating a 2 × 2 TO polymer total-internal-reflection (TIR) switch. The UV imprinting process is utilized for transferring the pattern from the soft mold to the bottom cladding polymer. Then, the core layer is ink-jet printed on top; following this step, another layer (top cladding layer) is ink-jet printed on the top of the core layer. The last step involves depositing a gold heating electrode on the top. Figure 20a shows the schematic of the 2 × 2 TO polymer switch with a horn structure [71] at the junction to construct the TIR structure. The parameters of the TIR switch are optimized for cross-talk minimization and low switching power. The X junction is 4◦ , and the width of the heating element at the center is 8 µm. The SEM cross section image of the fully printed device is shown in Figure 20b. The performance of fabricated TO polymer switch at different applied voltages is characterized. The relationship between the normalized optical output power from two output channels and the electrical power consumption is shown in Figure 20c. It can be seen that the power for the cross port to reach its maximum output is about 100 mW, and the cross-talk is over 32 dB. The response speed of the fabricated TIR switch is measured with a rising/falling time of less than 0.5 ms and an operation frequency of up to 1 kHz. For the true time delay (TTD) application, utilizing large-area imprinting (with large-area flexible PDMS mold) and ink-jet printing process, a 4-bit TO switch-based characterized. The relationship between the normalized optical output power from two output channels and the electrical power consumption is shown in Figure 20c. It can be seen that the power for the cross port to reach its maximum output is about 100 mW, and the cross-talk is over 32 dB. The response speed of the fabricated TIR switch is measured with a rising/falling time of less than 0.5 ms and an operation frequency of up to 1 kHz. For the true time delay (TTD) application, Polymers 2018, 10, 603 15 of 28 utilizing large-area imprinting (with large-area flexible PDMS mold) and ink-jet printing process, a 4-bit TO switch-based TTD module has been successfully fabricated [41], which is capable of providing sufficient time delay for achieving in an X-band phased array antenna TTD module has been successfully fabricated ±60° [41], steering which isangle capable of providing sufficient time delay system. for achieving ±60◦ steering angle in an X-band phased array antenna system. Figure × 22 thermo-optic Figure 19. Process Process flow flow for for fabricating fabricating a 2 × thermo-optic polymer polymer switch switch using using imprinting imprinting and and ink-jet method. (a) (a)Silicon Siliconand andflexible flexible mold; imprinting bottom cladding; (c) layer core ink-jet printing method. mold; (b)(b) imprinting the the bottom cladding; (c) core layer deposition; (d) top cladding deposition; and (e) metal electrode [46]. Copyright deposition; topPEER cladding deposition; and (e) metal electrode depositiondeposition [46]. Copyright 2013 OSA.15 of 27 Polymers 2018, 10, x(d) FOR REVIEW 2013 OSA. Figure Schematic showing the top the 2of× the 2 TO2TIR layout; (b) SEM cross-section Figure20. 20.(a)(a) Schematic showing theview top of view × 2switch TO TIR switch layout; (b) SEM view of printed layers in the TO polymer switch; (c) normalized optical output power versus electrical cross-section view of printed layers in the TO polymer switch; (c) normalized optical output power power both bar portfrom (Channel port (Channel B) [46]. Copyright OSA. versusfrom electrical power both A) barand portcross (Channel A) and cross port (Channel2013 B) [46]. Copyright 2013 OSA. 4.4. Variable Optical Attenuators 4.4. Variable Optical Attenuators Variable optical attenuators (VOAs), which enable dynamically flexible optical power regulation, optical attenuators (VOAs), which enable dynamically flexible optical regulation, have Variable a lot of applications in the photonic integrated circuits and reconfigurable optical power networks [72,73]. Ahave polymer TOapplications VOA is designed and fabricated via the UV imprinting technique in [74]. The silicon is a lot of in the photonic integrated circuits and reconfigurable optical networks [72,73]. utilized as the The and quartz is applied the substrate material due toinits[74]. transparency A polymer TOmold VOAmaterial. is designed fabricated viaas the UV imprinting technique The silicontois UV irradiation. Figure 21 illustrates the UV imprinting steps for polymer waveguide utilized as the mold material. The quartz is applied as the substrate material due to itsVOA. transparency to UV irradiation. Figure 21 illustrates the UV imprinting steps for polymer waveguide VOA. 4.4. Variable Optical Attenuators Variable optical attenuators (VOAs), which enable dynamically flexible optical power regulation, have a lot of applications in the photonic integrated circuits and reconfigurable optical networks [72,73]. A polymer TO VOA is designed and fabricated via the UV imprinting technique in [74]. The silicon is utilized as the toof 28 Polymers 2018, 10, 603 mold material. The quartz is applied as the substrate material due to its transparency16 UV irradiation. Figure 21 illustrates the UV imprinting steps for polymer waveguide VOA. Figure UVimprinting imprinting steps VOA-based on polymer materials. (a) Figure 21. 21.UV steps for forintegrated integratedwaveguide waveguide VOA-based on polymer materials. Spin-coating and curing for under-cladding; (b) core material dispensing; (c) imprinting under (a) Spin-coating and curing for under-cladding; (b) core material dispensing; (c) imprinting pressure with UVUV irradiation; (d) (d) detachment of the mold; (e) (e) spin-coating andand curing for for under pressure with irradiation; detachment of the mold; spin-coating curing cover-cladding; and (f) TO electrode patterning [74]. Copyright 2006 Elsevier. Polymers 2018, 10, x FORand PEER 16 of 27 cover-cladding; (f)REVIEW TO electrode patterning [74]. Copyright 2006 Elsevier. The polymer waveguide VOA, as shown in Figure 22a, comprises a single-mode waveguide The polymer waveguide VOA, as shown in Figure 22a, comprises a single-mode waveguide channels, taper structures, and a multi-mode waveguide channel. Figure 22b shows the fabricated channels, taper structures, and a multi-mode waveguide channel. Figure 22b shows the fabricated waveguides: (i) SEM picture of UV-imprinted single-mode waveguide core and (ii) cross-sectional waveguides: (i) SEM picture of UV-imprinted single-mode waveguide core and (ii) cross-sectional view view of the fabricated polymer waveguide. The propagation loss of the straight single mode is 0.35 of the fabricated polymer waveguide. The propagation loss of the straight single mode is 0.35 dB/cm at dB/cm at a wavelength of 1.55 μm, and the average insertion loss is less than 2 dB. The attenuation a wavelength of 1.55 µm, and the average insertion loss is less than 2 dB. The attenuation characteristics characteristics according to the applied electrical power are measured with 30 dB attenuation with according to the applied electrical power are measured with 30 dB attenuation with electrical power of electrical power of 80 mW and a rising/falling time of less than 5 ms. 80 mW and a rising/falling time of less than 5 ms. Figure 22. (a) TOVOA VOAand and (b) (b) characterization characterization of UV-imprinted (a) Schematic Schematicof ofpolymer polymerwaveguide waveguide TO single mode waveguide [74]. Copyright 2006 Elsevier. 5. 5. Electro-Optic Electro-Optic Modulators Modulators Compared most widely used LiNbO 3, the polymeric EO Compared with withinorganic inorganicmaterials materialssuch suchasasthethe most widely used LiNbO 3 , the polymeric materials have many advantages including lower cost, ease of processing, and EO materials have many advantages including lower cost, ease of processing, and larger larger EO EO coefficients. coefficients. Especially, Especially, the the polymeric polymeric materials materials have have the the potential potential for for achiving achiving high high EO EO coefficients coefficients by by virtue virtue of of synthetization synthetization at at molecular molecular level level [75,76]. [75,76]. For For example, example, the the long long flexible flexible chain-modified, chain-modified, julolidinyl-based chromophores linked by thiophene bridge and TCF acceptor julolidinyl-based chromophores linked by thiophene bridge and TCF acceptor (chromophore (chromophore WJ5) WJ5) afford EO coefficient coefficient (r (r33 33)) value value of of 266 266 pm/V pm/V [77]. thiophene-modified afford the the EO [77]. The The chromophore chromophore with with thiophene-modified π-conjugation enhances the the EO EO coefficient coefficient to to be be 337 337 pm/V pm/V [78]. π-conjugation enhances [78]. Incorporating the highly nonlinear and stable chromophore doped in amorphous polycarbonate, a MZI based EO intensity modulator is designed and fabricated using the UV imprinting technique [79]. In order to reduce the radiation loss of the Y junction, two S-shaped bends with radii of 1 mm are adopted. Figure 23a shows the process steps for the fabrication of PDMS stamp (i–iii) and the molding of EO polymer layer (iv–vi). The waveguide MZI structure is Polymers 2018, 10, 603 17 of 28 Incorporating the highly nonlinear and stable chromophore doped in amorphous polycarbonate, a MZI based EO intensity modulator is designed and fabricated using the UV imprinting technique [79]. In order to reduce the radiation loss of the Y junction, two S-shaped bends with radii of 1 mm are adopted. Figure 23a shows the process steps for the fabrication of PDMS stamp (i–iii) and the molding of EO polymer layer (iv–vi). The waveguide MZI structure is formed by the molding of the core Polymers 2018, 10, xthe FORsoft PEERPDMS REVIEW 17 of 27 polymer using stamp. The polymer materitals with low refractive index for under cladding and upper cladding should be chosen carefully, because the solution solvent of core material waveguide using the imprinting technique, the core material and the electrode material should must not dissolve the under cladding polymer, and the solution solvent of upper clading material have suitable viscosity to be ink-jet printed. Figure 25a,b shows the microscope image of printed EO must not dissolve the core polymer. The cross-sectional view of the electro-optic waveguide structure polymer modulator and ink-jet printed top electrode, respectively. Figure 25c gives the SEM picture is illustrated in Figure 23b. The fabricated EO modulater is measured with the halfwave voltage (V π ) of the modulator cross section. It can be seen that the imprinted trench is filled with EO polymer, of 8.4 V at 1600 nm, and the on/off extinction ratio is better than 19 dB. As the first imprinted EO and the top and ground electrodes are uniformly deposited with the top electrode located right on polymer modulator, the halfwave voltage is relatively high. It could be improved by thinning the top of the rib waveguide. cladding layers in a push–pull fashion and optimizing the poling efficiency. 23. (a) (a)Fabrication Fabricationofofthethe PDMS mold (i–iii) replication thestructure MZI structure A Figure 23. PDMS mold (i–iii) and and replication of theofMZI (iv–vi).(iv–vi). A master master MZI(i) device (i) is patterned using photolithography of SU-8. PDMS is on the master MZI device is patterned using photolithography of SU-8. PDMS is poured onpoured the master device (ii), device peeled, (ii), cured, and A diced A drop of electro-optic core polymer solutiononisthe placed on cured, andpeeled, diced (iii). drop(iii). of electro-optic core polymer solution is placed silicon the silicon(iv). substrate (iv).stamp The PDMS stamp (v) is depressed (v) polymer until theiscore polymer is cured. The substrate The PDMS is depressed until the core cured. The stamp is peeled stamp is the peeled to reveal the(vi); replicated device (vi);view (b) cross-sectional viewwaveguide of the electro-optic to reveal replicated device (b) cross-sectional of the electro-optic structure with electrode [79]. Copyright 2004 AIP. waveguide structure with electrode [79]. Copyright 2004 AIP. Despite utilizing imprinting to pattern one layer, other material layers (metal electrode) still need to be prepared via spin-coating, evaporation, etching, or lift-off methods, which increase the process complexity and cost. Chen’s group worked toward achieving an all-printable EO modulator [80]. They demonstrated an electro-optic polymer-based MZ modulator fabricated by utilizing advanced UV imprinting and aligned ink-jet printing techniques for patterning and layer deposition. The main process flow for fabricating the MZ modulator is illustrated in Figure 24. The bottom electrode layer is directly patterned on the substrate by ink-jet printing. By using the UV imprinting method with a transparent flexible mold, the waveguide structure is transfered into the under cladding polymer. All other layers are ink-jet printed step by step. The top electrode is printed over the MZI arm with alignment. Besides the requirement of materials to form a optical waveguide using the imprinting technique, the core material and the electrode material should have suitable viscosity to be ink-jet printed. Figure 25a,b shows the microscope image of printed EO polymer modulator and ink-jet printed top electrode, respectively. Figure 25c gives the SEM picture of the modulator cross section. It can be seen that the imprinted trench is filled with EO polymer, and the top and ground electrodes are uniformly deposited with the top electrode located right on top of the rib waveguide. Figure 23. (a) Fabrication of the PDMS mold (i–iii) and replication of the MZI structure (iv–vi). A master MZI device (i) is patterned using photolithography of SU-8. PDMS is poured on the master device (ii), cured, peeled, and diced (iii). A drop of electro-optic core polymer solution is placed on the silicon substrate (iv). The PDMS stamp is depressed (v) until the core polymer is cured. The stamp is peeled to reveal the replicated device (vi); (b) cross-sectional view of the electro-optic Polymers 2018, 10, 603 waveguide structure with electrode [79]. Copyright 2004 AIP. 18 of 28 Figure 24. Main process flow forfabricating fabricating an an EO EO polymer using imprinting and ink-jet Figure 24. Main process flow for polymermodulator modulator using imprinting and ink-jet printing methods. (a) Silicon and SSQ mold; (b) bottom electrode deposition; (c) bottom cladding printing methods. (a) Silicon and SSQ mold; (b) bottom electrode deposition; (c) bottom cladding deposition; (d) imprinting; (e) core and cladding top cladding deposition; (f) electrode top layerlayer deposition; (d) imprinting; (e) core layerlayer and top layerlayer deposition; and and (f) top electrode deposition [80]. Copyright 2013 OSA. deposition 2013 OSA. Polymers 2018,[80]. 10, x Copyright FOR PEER REVIEW 18 of 27 Figure 25. Microscope images of (a) printed EO polymer modulator; (b) ink-jet printed top electrode; Figure 25. Microscope images of (a) printed EO polymer modulator; (b) ink-jet printed top electrode; and (c) SEM picture of the modulator cross section [80]. Copyright 2013 OSA. and (c) SEM picture of the modulator cross section [80]. Copyright 2013 OSA. 6. Photonic Biosensors 6. Photonic Biosensors Biosensors have been attracting more and more attention due to their wide applications in the fields of clinical environmental monitoring, new drugdue development, and food safety. in Biosensors havediagnosis, been attracting more and more attention to their wide applications Photonic biosensors offer distinct advantages for sensing applications compared with their the fields of clinical diagnosis, environmental monitoring, new drug development, and food safety. electronic counterparts, including electrical passiveness and insensitivity to electromagnetic Photonic biosensors offer distinct advantages for sensing applications compared with their electronic interference. For the requirements of label-free and real-time monitoring of the dynamics of counterparts, including electrical passiveness and insensitivity to electromagnetic interference. For the biomolecular reactions, the photonic integrated waveguide biosensors can avoid the labor and time requirements of label-free and real-time monitoring of the dynamics of biomolecular reactions, consuming process of the conventional methods, such as the enzyme-linked immunosorbent assay the photonic integrated waveguide biosensors can avoid the labor and time consuming process (ELISA). Furthermore, these planar waveguide photonic biosensors could be integrated with other of theelectronic, conventional methods, such as the enzyme-linked (ELISA). Furthermore, optical, and microfluidic components, which immunosorbent is helpful for the assay realization of lab-on-chip thesedevices planar[81]. waveguide photonic biosensors could bebeen integrated other electronic, optical, Among different materials, polymers have proven towith be ideal material platforms and microfluidic which is helpful for the realization of lab-on-chip devices Among for photoniccomponents, integrated waveguide biosensors. Besides the advantages of low cost, [81]. tunable properties, and easy fabrication, compared with inorganic materials, biocompatibility is an especially important and unique feature of polymers. Accordingly, the biochemical surface treatment of the polymer waveguide devices can be greatly simplified. This section presents the recent work about polymer waveguide photonic biosensors using the UV imprinting technique. 6.1. Microring Based Biosensors Polymers 2018, 10, 603 19 of 28 different materials, polymers have been proven to be ideal material platforms for photonic integrated waveguide biosensors. Besides the advantages of low cost, tunable properties, and easy fabrication, compared with inorganic materials, biocompatibility is an especially important and unique feature of polymers. Accordingly, the biochemical surface treatment of the polymer waveguide devices can be greatly simplified. This section presents the recent work about polymer waveguide photonic biosensors using the UV imprinting technique. 6.1. Microring Based Biosensors Among the various structures for integrated waveguide photonic biosensors, waveguide microrings are key components due to their high Q-factor, compactness, and potential high-density array for sensing multiple targets [82–84]. The high Q-factor enables the lightwave to travel round in the waveguide ring, resulting in the enhanced interaction between the optical evanescent field and the analyte. Compared with other sensor structures [85], it can provide a higher sensitivity with the same area. The polymer microring-based biosensor with a ridge shape waveguide structure is fabricated using the selective imprinting technique [36]. The fabricated polymer microring is a racetrack with a radius of 350 µm. The gap and coupling length between the straight waveguide and the microring waveguide are 1 and 70 µm, respectively. Human-IgG (Human Immunoglobulin G) and PA (Staphylococcal protein A) are chosen as an affinity model for specific antigen-antibody sensing, as shown schematically in Figure 26 [9]. The physical immobilization via direct incubation is applied for surface functionalization, which is much simpler for polymer-based optical chips than the covalent immobilization methods. With the “stamp-and-stick” method [86], the PDMS fluidics channel is mounted on top of the microring resonator chip, as shown in Figure 27a. The biosensor assembled with fluidic channel and connected to a syringe is shown in Figure 27b. Polymers 2018, 10, x FOR PEER REVIEW 19 of 27 Polymers 2018, 10, x FOR PEER REVIEW 19 of 27 Figure 26. Schematic of the biosensing process using PA/IgG affinity couple [9]. Figure Schematicofofthe thebiosensing biosensing process process using [9]. Figure 26.26.Schematic using PA/IgG PA/IgGaffinity affinitycouple couple [9]. Figure 27. (a) The fluidic channel aligned with the ring resonator on the chip and (b) a biosensor Figure Thefluidic fluidicchannel channel aligned aligned with with the Figure 27.27.(a)(a)The the ring ring resonator resonatoron onthe thechip chipand and(b)(b)a abiosensor biosensor assembled with fluidic channel and connected to a syringe. assembled with fluidicchannel channeland andconnected connectedto to aa syringe. syringe. assembled with fluidic Surface binding sensing is measured with the following steps. With a flow speed of 10 μL/min, Surface binding sensing is measured with the following steps. With a flow speed of 10 μL/min, Surface (DI) binding sensing is measured with the solution following steps. With a flow speed of 10 µL/min, deionized water, and 0.01× phosphate buffer (PBS), Human-IgG with a concentration deionized (DI) water, and 0.01× phosphate buffer solution (PBS), Human-IgG with a concentration deionized (DI) water, and 0.01 × phosphate buffer solution (PBS), Human-IgG with a concentration of 50 μg/mL and 0.01× PBS are pumped into the fluidics channel consecutively. During the process, of 50 μg/mL and 0.01× PBS are pumped into the fluidics channel consecutively. During the process, output and resonant is measured recorded as consecutively. a function of the time, the with the of the 50 µg/mL 0.01×peak PBSposition are pumped into the and fluidics channel During process, the output resonant peak position is measured and recorded as a function of the time, with the results shown in Figure 28a. It can be seen from Figure 28a that a wavelength change of about 230 results shown in Figure 28a. It can be seen from Figure 28a that a wavelength change of about 230 pm is caused by the affinity of Human-IgG for the protein A molecules immobilized on the chip pm is caused by the affinity of Human-IgG for the protein A molecules immobilized on the chip surface as bio-receptors. The strong binding strength between the protein A and Human-IgG and surface as bio-receptors. The strong binding strength between the protein A and Human-IgG and the coating of protein A to the chip surface is confirmed, because no obvious reverse wavelength the coating of protein A to the chip surface is confirmed, because no obvious reverse wavelength shift is observed in the last phase of PBS rinsing. Then, the biosensors operating at different shift is observed in the last phase of PBS rinsing. Then, the biosensors operating at different Figure 27. (a) The fluidic channel aligned with the ring resonator on the chip and (b) a biosensor assembled with fluidic channel and connected to a syringe. Polymers 2018, 10, 603 20 of 28 Surface binding sensing is measured with the following steps. With a flow speed of 10 μL/min, deionized (DI) water, and 0.01× phosphate buffer solution (PBS), Human-IgG with a concentration the output peak measured recorded as a function of theDuring time, with the results of 50 resonant μg/mL and 0.01×position PBS are is pumped into and the fluidics channel consecutively. the process, peakbeposition is measured and that recorded as a function of the with shownthe in output Figureresonant 28a. It can seen from Figure 28a a wavelength change oftime, about 230the pm is results shown in Figure 28a. It can be seen from Figure 28a that a wavelength change of about 230 caused by the affinity of Human-IgG for the protein A molecules immobilized on the chip surface as pm is caused by the binding affinity ofstrength Human-IgG for the AA molecules immobilized chip of bio-receptors. The strong between theprotein protein and Human-IgG andon thethe coating surface as bio-receptors. The strong binding strength between the protein A and Human-IgG and protein A to the chip surface is confirmed, because no obvious reverse wavelength shift is observed in the coating of protein A to the chip surface is confirmed, because no obvious reverse wavelength the last phase of PBS rinsing. Then, the biosensors operating at different concentrations of Human-IgG shift is observed in the last phase of PBS rinsing. Then, the biosensors operating at different solutions are investigated, with the results shown in Figure 28b. It indicates that the photonic biosensor concentrations of Human-IgG solutions are investigated, with the results shown in Figure 28b. It exhibits different dynamic responsivities the antibody concentrations, with a detected indicates that the photonic biosensortoexhibits different dynamic responsivities to theconcentration antibody of Human-IgG as low as a5 detected µg/mL.concentration of Human-IgG as low as 5 μg/mL. concentrations, with 28.The (a) The monitoring processofofthe thepolymer-based polymer-based microring forfor thethe surface FigureFigure 28. (a) monitoring process microringbiosensor biosensor surface binding sensing and (b) the responsivities of the photonic biosensor with different concentrations of binding sensing and (b) the responsivities of the photonic biosensor with different concentrations of Human-IgG [36]. Human-IgG [36]. Polymers 2018, 10, x FOR PEER REVIEW 20 of 27 The nonspecific binding is another important aspect of label-free photonic biosensors. To test Theof nonspecific bindingphotonic is another important aspectnon-targeted of label-free photonic biosensors. To Serum test the response polymer-based biosensor toward biomolecules, Bovine the response of polymer-based photonic biosensor toward non-targeted biomolecules, Bovine Albumin (BSA) is chosen as a control protein and brought into contact with the PA-coated microring Serum Albumin (BSA) is chosen as a control protein and brought into contact with the PA-coated surface [9]. The concentration of the BSA for the test is 50 µg/mL (in 0.01 Mol PBS, pH 7.4). From the microring surface [9]. The concentration of the BSA for the test is 50 μg/mL (in 0.01 Mol PBS, pH measured results as shown in Figure 29, it can be seen that a resonant wavelength shift of about 40 pm 7.4). From the measured results as shown in Figure 29, it can be seen that a resonant wavelength is caused by the nonspecific binding of the BSA molecules, and the saturation is obtained after 40 min shift of about 40 pm is caused by the nonspecific binding of the BSA molecules, and the saturation with the BSA solution contrast, a resonant shift of 90 pmwavelength using the specific is obtained after 40injection. min withAs thea BSA solution injection.wavelength As a contrast, a resonant shift binding of the Human-IgG with a concentration of 5 µg/mL is observed. Therefore, recognition of 90 pm using the specific binding of the Human-IgG with a concentration of 5 μg/mLthe is observed. abilityTherefore, of the PA-coated microring biosensors is validated. the recognition ability of the PA-coated microring biosensors is validated. Figure 29. The responsivities surfacefunctionalized functionalized photonic photonic biosensor and Figure 29. The responsivities of of thethe PAPA surface biosensortotospecific specific and nonspecific interactions. Resonant wavelength shift induced by the interaction with human IgG (50 nonspecific interactions. Resonant wavelength shift induced by the interaction with human IgG (50 and and 5 μg/mL) and with BSA (50 μg/mL) [9]. 5 µg/mL) and with BSA (50 µg/mL) [9]. 6.2. Young’s Interferometer Based Biosensors Interferometers, such as MZIs and Young’s interferometers (YIs), are considered as one of the most sensitive devices [87] for optical sensing due to the background compensation between the sensing and reference arm against environmental disturbances, such as temperature fluctuations. In [88], a YI-based biosensor utilizing layered polymeric-inorganic composite waveguide Figure 29. The responsivities of the PA surface functionalized photonic biosensor to specific and nonspecific interactions. Resonant wavelength shift induced by the interaction with human IgG (50 and2018, 5 μg/mL) Polymers 10, 603 and with BSA (50 μg/mL) [9]. 21 of 28 6.2. Young’s Interferometer Based Biosensors 6.2. Young’s Interferometer Based Biosensors Interferometers, such as MZIs and Young’s interferometers (YIs), are considered as one of the such as for MZIs and Young’s (YIs), are considered as one of the most mostInterferometers, sensitive devices [87] optical sensinginterferometers due to the background compensation between the sensitiveand devices [87] for optical sensing due to the background compensation between the sensing and sensing reference arm against environmental disturbances, such as temperature fluctuations. In [88], a arm YI-based utilizing layered composite reference against biosensor environmental disturbances, suchpolymeric-inorganic as temperature fluctuations. In [88],waveguide a YI-based biosensor utilizing layered polymeric-inorganic composite configuration developed. configuration is developed. The schematic structure of waveguide the YI-based photonic is biosensor is The schematic structure ofThe the YI-based photonic is illustrated in Figure 30a. arm The lightwave illustrated in Figure 30a. lightwave is split biosensor into a reference arm and a sensing via the Y is split into a reference and a sensing theas Y in shaped splitter. of recombining shaped splitter. Insteadarm of recombining thearm twovia arms the MZI [85], Instead the optical output from the two arms as transmits to a[85], detector screenoutput to form interference fringes. The shift the interference in the MZI the optical from the two arms transmits to aindetector screen to fringes is inducedfringes. by the The phase change the sensing arm, is which can by bethe utilized monitor form interference shift in theon interference fringes induced phasetochange on the sensing arm, which be utilized to monitor the concentration of the analytes. concentration of thecan analytes. Figure 30. (a) based onon inverted-ridge waveguide; (b) (a)Schematic Schematicimage imageofofthe theintegrated integratedYIYIbiosensor biosensor based inverted-ridge waveguide; cross-section SEM image of the inverted ridge waveguide [88]. Copyright 2012 OSA. (b) cross-section SEM image of the inverted ridge waveguide [88]. Copyright 2012 OSA. The materials used for sensor fabrication consist of the UV-curable Ormocer series of hybrid polymers (Micro resist technology GmbH). Ormocomp, Ormocore, and Ormoclad, having refractive indexes of 1.520, 1.553, and 1.536 at wavelength of 633 nm, are used for the under cladding, core, and upper cladding, respectively. The polymer-inverted ridge waveguide is fabricated using the imprinting technique with two main steps: patterning the waveguide under-cladding and filling the patterned grooves with the core material [89]. A thin layer of Ta2 O5 with a high index (n = 2.1) is deposited on top of polymer layers, as shown in Figure 30b, which pushes the guided mode field up towards the surface, resulting in the enhanced interaction between the optical field and biomolecules under test [90]. Figure 31a shows the top view microscope image of fabricated, integrated YI with a distance between the sensing and referencing waveguide of 50 µm, in which the simulated intensity profiles of the propagating TM modes are also displayed. It can be seen that by coating Ta2 O5 on top of polymer layer, the guided mode field is pushed up towards the surface. The generated fringe pattern from the referencing waveguide and sensing waveguide is shown in Figure 31b. By using two-dimensional fast Fourier transform, the fringe pattern phase shifts can be analyzed with good resolution by virtue of the clear visibility of the interferogram. The specific molecular binding between C-reactive protein (CRP antigens) and monoclonal anti-human C-reactive protein (CRP antibodies) is chosen to characterize the performance of the developed YI photonic biosensor. The monitored phase responses are plotted in Figure 31c. Considering the noise level of 0.003 rad, the detection limit for surface sensing can be estimated to be about 100 fg/mm2 . In order to improve the sensitivity of YI-based photonic biosensors, the slot waveguide can be utilized to replace the channel waveguide in the sensing arm [91]. To manufacture disposable photonic biosensors with low cost and batch-based fabrication, roll-to-roll (R2R) processing can be utilized. In R2R processing, different structures and materials are patterned onto a continuously moving carrier. As shown in Figure 32, the polymeric single-modal photonic integrated circuits for disposable sensors based on YI have been manufactured on rolls with the length of hundreds of meters [92]. two-dimensional fast Fourier transform, the fringe pattern phase shifts can be analyzed with good resolution by virtue of the clear visibility of the interferogram. The specific molecular binding between C-reactive protein (CRP antigens) and monoclonal anti-human C-reactive protein (CRP antibodies) is chosen to characterize the performance of the developed YI photonic biosensor. The monitored the Polymers 2018,phase 10, 603 responses are plotted in Figure 31c. Considering the noise level of 0.003 rad, 22 of 28 detection limit for surface sensing can be estimated to be about 100 fg/mm2. Polymers 2018, 10, x FOR PEER REVIEW 22 of 27 In order to improve the sensitivity of YI-based photonic biosensors, the slot waveguide can be utilized to replace the channel waveguide in the sensing arm [91]. To manufacture disposable Figure 31. image the YIYI biosensor with aroll-to-roll high-index Ta2 O coating; (b) the Figure 31. (a) (a) Microscope Microscope image thefabricated fabricated biosensor with a high-index Ta52O 5 coating; (b) photonic biosensors with low costofofand batch-based fabrication, (R2R) processing can be generated interference pattern on the detector screen at a distance of 2.5 mm; and (c) the measured the In generated interference pattern on the detector screen are at apatterned distance onto of 2.5a continuously mm; and (c) the utilized. R2R processing, different structures and materials moving phase the molecular events between surface attached CRP surface antibodies andfor different measured phase responses of binding the molecular binding events between attached CRP carrier. Asresponses shown inofFigure 32, the polymeric single-modal photonic integrated circuits disposable concentrations CRPbeen antigens [88]. Copyright 2012with OSA. antibodies and different concentrations of CRP antigens [88]. 2012 OSA.of meters [92]. sensors based on YIof have manufactured on rolls the Copyright length of hundreds Figure Figure 32. 32. (a) (a) Roll Roll of of the the manufactured manufactured sensor sensor by by roll-to-roll roll-to-roll processing processing and and (b) (b) sensor sensor chip chip cut cut from from the roll [92]. Copyright 2016 OSA. the roll [92]. Copyright 2016 OSA. 7. Conclusions and Outlook recent progress of the polymer photonic waveguide devices We have havereviewed reviewedthethe recent progress of UV theimprinted UV imprinted polymer photonic waveguide and their applications. By using the liquid, UV curable and spin-coating properties of polymer devices and their applications. By using the liquid, UV curable and spin-coating properties of materials, various passive photonic integrated devices, such as microring polymer materials, variousfunctional passive functional photonic integrated devices, such as resonators, microring resonators, waveguide splitters, arrayed waveguide gratings, long-period gratings, and microlenses, can be fabricated using the UV imprinting technique with a simple and low-cost process. With the high TO coefficient, TO-tunable active devices including the microring resonator filters, waveguide Bragg grating filters, optical switches, and variable optical attenuators can be realized with low power consumption. By virtue of the ultrahigh EO coefficient, polymer-based EO modulators can Polymers 2018, 10, 603 23 of 28 waveguide splitters, arrayed waveguide gratings, long-period gratings, and microlenses, can be fabricated using the UV imprinting technique with a simple and low-cost process. With the high TO coefficient, TO-tunable active devices including the microring resonator filters, waveguide Bragg grating filters, optical switches, and variable optical attenuators can be realized with low power consumption. By virtue of the ultrahigh EO coefficient, polymer-based EO modulators can be prepared with high performance. Photonic biosensors based on imprinted polymer waveguide devices can achieve the label-free detection of biomolecules due to the biocompatibility of polymer film. This manuscript mainly focuses on the polymer photonic integrated devices using the UV imprinting technique, which shows the easy process, low cost, and mass production of polymer waveguides. Certainly, other imprinting-based techniques exerting the advantages of polymer materials, such as thermal imprinting for big dimension structures [93,94], injection molding for arrayed elements [95,96], and ink-jet printing for complex and 3D structures [97,98], can also be utilized to fabricate polymer waveguides. Furthermore, by utilizing the compatibility of polymer materials with diverse materials, such as the III-V semiconductor, the silicon-on-insulator (SOI), silica, and the silicon nitride, hybrid photonic integration can be implemented on polymer platforms. In the common motherboard, individual components with high performance from the respective best-suited material are assembled, yielding great optimization and high cost-efficiency. The hybrid photonic integrated modules can be applied not only in data centers, metro-area optical networks, and DWDM optical networks [99], but also in the lab-on-chip biosensing systems [100,101]. Author Contributions: X.-Y.H. and M.-S.Z. suggested this review and organized all sections. X.-Y.H. wrote Section 1 Introduction and Section 2 Imprinting Technique. Z.-L.W., Y.-X.L., L.-H.W., and J.-Y.W. wrote the Section 3 Passive Photonic Integrated Waveguide Devices. X.-Y.H., F.-F.S., S.-C.Y., L.-H.W., and G.M. wrote the Section 4 Active Photonic Integrated Waveguide Devices. H.Z. and S.-H.B. wrote Section 5 Electro-Optic Modulators. L.-H.W., J.R., L.-Y.J., and G.M. wrote Section 6 Photonic Biosensors. X.-Y.H. and Z.-L.W. carefully proofread the manuscript. Funding: This research was funded by National Natural Science Foundation of China (60577014, 60807015, 1077015, 61704017), National High-Tech R&D Program (2012AA040406), International Science & Technology Cooperation Program of China (2014DFG32590), and National Research Foundation of China (614045001035); Natural Science Foundation of Liaoning Province (20102020, 2014020002), Petro China Innovation Foundation (2016D-5007-0603), and Fundamental Research Funds for the Central Universities (DUT13JB01, DUT2014TD05, DUT18ZD106, DUT18GF102, and DUT18LAB20). Conflicts of Interest: The authors declare no conflict of interest. References 1. 2. 3. 4. 5. 6. 7. Chiang, K.S. Development of optical polymer waveguide devices. Proc. SPIE 2010, 7605. [CrossRef] Zhang, Z.Y.; Mettbach, N.; Zawadzki, C.; Wang, J.; Schmidt, D.; Brinker, W.; Grote, N.; Schell, M.; Keil, N. Polymer-based photonic toolbox: Passive components, hybrid integration and polarization control. IET Optoelectron. 2011, 5, 226–232. 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