Корреляционные методики измерениия сверхкоротких

Корреляционные методики измерения
коротких импульсов терагерцового излучения
Alexej Semenov
German Aerospace Center
N. Novgorod, IFM, 20.09.2011
Outline
 Коррелляция и автокорреляция
 Нелинейность и интерференция в
автокорреляционных измерениях
электромагнитных полей
- электрическое поле
- интенсивность
- Crosstalk
 Получение коротких терагерцовых
импульсов
 Результаты измерений
Folie 2
Fluorescent Correlation Spectroscopy
Magde, D., Elson, E., and Webb, W.W. (1972) Phys. Rev. Lett. 29, 705
Fluorescent Correlation Spectroscopy
Autocorrelation function
Wz
Wx,y
G( ) 
I (t ) T (t   )
I (t )
2
1
1
1
N 

1  8 D   1  4 D 
2 
2

w
w
x
,
y
z


N – average number of the molecules in the focal volume
D – diffusion coefficient
Fluorescent Correlation Spectroscopy
Diffusion coefficient
Different light - time correlation of photons
Thermal sources, gas discharge (natural light) - bunched photons (Bose statistics,
strong fluctuation)
Lasers (coherent light)
- random photons
(Poisson distribution, low fluctuation)
Single photon sources (fluorescence, quantum dot)
- anti-bunched photons
Folie 6
Time correlation of photons
Correlation function with a single photon detector
Tkoh is the measure for the
degree of coherence in
thermal light sources
C. Zinoni et al., APL 2007
Folie 7
Time correlation of photons
Hanbury-Brown/Twiss-Experiment
Finite response time and/or dead time of a single photon detector brought up
the HBT method
Folie 8
Outline
 Коррелляция и автокорреляция
 Нелинейность и интерференция в
автокорреляционных измерениях
электромагнитных полей
- электрическое поле
- интенсивность
- Crosstalk
 Получение коротких терагерцовых
импульсов
 Результаты измерений
Folie 9
Femtosecond pulse lasers
How to measure the pulse duration?
Autocorrelator
Interferometric autocorrelation
SHD – second harmonic generator
(non-linear optical crystal)
D – any slow detector
Folie 10
Interferometric autocorrelation
Interferometric autocorrelation
Two ultra-short pulses (a) and (b) with their respective interferometric
autocorrelation (c) and (d). Because of the phase present in pulse (b) due to
an instantaneous frequency sweep (chirp), the fringes of the autocorrelation
trace (d) wash out in the wings. Note the ratio 8:1 (peak to the wings),
characteristic of interferometric autocorrelation traces.
Folie 11
Fast optical detectors
How to measure the response time of the detector?
Use the nonlinearity V(P) of the detector
response and do not forget to eliminate
interference
Interferometer
P
V
L – femtosecond pulse laser
P – polarizer
V – slow voltmeter
D –detector under study
A. Semenov et al., JLTP 1996
Folie 12
Intensity autocorrelation
YBCO superconducting detector and Ti-Sapphire laser
Interferometer
1.3
1.25
Correlation signal (r.u.)
P
R1fl
R2fl
GAUSS 7 ps
V
1.2
1.15
1.1
1.05
1
0.95
0.9
-30
-20
-10
0
10
20
30
Delay (ps)
P. Probst et al., PRB <2012>
Folie 13
Intensity autocorrelation
Intensity
autocorrelation
V(I)  a I 2
V(t)  a ( E1(t)  E2 (t) )2
2
2
Two ultra-short pulses (a) and (b) with their respective intensity
autocorrelation (c) and (d). Because the intensity autocorrelation ignores the
temporal phase of pulse (b) that is due to the instantaneous frequency
sweep (chirp), both pulses yield the same intensity autocorrelation. Here,
identical Gaussian temporal profiles have been used, resulting in an intensity
autocorrelation width twice as long as the original intensities. Note that an
intensity autocorrelation has a background that is ideally half as big as the
actual signal. The zero in this figure has been shifted to omit this background
Folie 14
Fast optical detectors
How to measure the linear response time?
Use the mutual current
drain of two identical
detectors
L. Shi et al., APL 1992
Crosstalk correlation
Folie 15
Crosstalk correlation
I0 = const
Crosstalk correlation
I2
I1
R2
(t)
R1
(t+ )
R( t )    (I( t )  IC ) I( t )n  InC (1   P( t ))
R1, 2 ( t )  R0  R( t, I)
I1R1  I2R 2  V ( t )
I1  I2  i0
Folie 16
Outline
 Коррелляция и автокорреляция
 Нелинейность и интерференция в
автокорреляционных измерениях
электромагнитных полей
- электрическое поле
- интенсивность
- Crosstalk
 Получение коротких терагерцовых
импульсов
 Результаты измерений
Folie 17
THz Synchrotron Radiation
Synchrotron
radiation
Signal appearance
Bending magnet
J. Feikes et al., PR ST AB 2011
Folie 19
Synchrotron radiation
Typical values
e  25 ps
t e  0.035 fs
Folie 20
Coherent synchrotron radiation
c
v
bunch length (x)
 10 ps
re(x)
orbit
acceptance portion
 25 ps
Coherence condition  x < l
Folie 21
Coherent THz Radiation from a Synchrotron
reference orbit: L = 240 m
longitudinal bunch length
intensity vs. number of electrons
hn
normal user optics
sz > 5 mm
t > 35 ps
sz > l
L
=
vc
bunch, p
hn
7·10-3
low alpha optics
sz  1 mm
momentum compaction factor:
p/p  = L/L
 fs2
t < 7 ps

sz  l
10-4
10 ps
Single electron
1 ps window
THz -pulse
Synchrotron
MLS data sheet
Folie 23
Outline
 Коррелляция и автокорреляция
 Нелинейность и интерференция в
автокорреляционных измерениях
электромагнитных полей
- электрическое поле
- интенсивность
- Crosstalk
 Получение коротких терагерцовых
импульсов
 Результаты измерений
Folie 24
Problems
 Radiation pulses in the range 0.1 – 1 THz
 Pulse duration 10 – 20 ps
 Available detectors
Slow – semiconductor bolometers (linear)
Fast – superconducting electron bolometers (linear)
Fast – superlattice detector (non-linear)
 Beam size a few millimeters & detector size a few
micrometers
Folie 25
Antennensimulation
Au-Antenne (100nm) auf Saphir
S11=-18 dB bei f = 0,95 THz
Antennen + Filter Layout
Gesamtstruktur
Antennen + Filter S-Parameter im THz-Bereich
• S11 = S22 = -43 dB bei 0,95 THz
• S21 = S12 = -32 dB sowie S31 = S32 = -24 dB bei 0,95 THz
Signal wird gut in
Antenne eingekoppelt
und nur wenig reflektiert
Martin-Puplett Interferometer
Input 1
Input 2
Output
Folie 30
Typical autocorrelation signal
Combination from crosstalk correlation
and field correlation
Normalized Autocorrelation signal
Beam parameter: 629 MeV, 480 kV,
7.05 kHz, 100mA beam current
Streak camera: FWHM) = 26ps
Normalize to [0, 1] of B
GaussAmpFit von C
1.0
0.5
0.0
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
Time (ps)
Detector signals seem to overlap over the whole scan length
Negative autocorrelation signal
Neither the peak at 0 nor the whole response corresponds with the streak
camera measurements
Period of about 20 ps
Peak at zero shorter than the other peaks
Field detector
Dielectric mirror
Metallic mirror
THz response (mV)
10
5
I0 = const
0
-5
0.0
0.2
0.4
0.6
0.8
1.0
I2
I1
R2
Time (ns)
(t)
R1
(t+ )
Folie 32
Field autocorrelation
Two ultra-short pulses (a) and (b) with their respective field autocorrelation (c) and
(d). Note that the autocorrelations are symmetric and peak at zero delay. Note
also that unlike pulse (a), pulse (b) exhibits an instantaneous frequency sweep,
called chirp, and therefore contains more bandwidth than pulse (a). Therefore, the
field autocorrelation (d) is shorter than (c), because the spectrum is the Fourier
transform of the field autocorrelation (Wiener-Khinchin theorem).
Folie 33
Autocorrelation with superlattice detector
S. Winnerl et al., APL 1998
Combination from field and intensity correlation
Folie 34
Thank you
Folie 35