polymers-10-00603 photonic polymer resonators

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; [email protected] (Z.-L.W.); [email protected] (S.-C.Y.);
[email protected] (F.-F.S.); [email protected] (Y.-X.L.)
Photonics Research Group, Department of Information Technology (INTEC), Ghent University-IMEC,
9000 Ghent, Belgium; [email protected]
College of Physics and Information Engineering, Fuzhou University, Fuzhou 350116, China;
[email protected]
School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China;
[email protected]
School of Life Science and Biotechnology, Dalian University of Technology, Dalian 116024, China;
[email protected] (J.R.); [email protected] (L.-Y.J.)
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics
and Chemistry, Chinese Academy of Sciences, Beijing 100190, China; [email protected] (H.Z.);
[email protected] (S.-H.B.)
Correspondence: [email protected] (X.-Y.H.); [email protected] (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].
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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
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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
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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.
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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,
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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].
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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
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2018, 10,
10, xx FOR
FOR PEER
PEER REVIEW
REVIEW
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27
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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.
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