N. Novgorod, IFM, 20.09.2011 Корреляционные методики измерения коротких импульсов терагерцового излучения Alexej

N. Novgorod, IFM, 20.09.2011

Корреляционные методики измерения коротких импульсов терагерцового излучения

Alexej SemenovGerman Aerospace Center

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Outline

Коррелляция и автокорреляция

Нелинейность и интерференция в

автокорреляционных измерениях

электромагнитных полей- электрическое поле- интенсивность- Crosstalk

Получение коротких терагерцовых

импульсов

Результаты измерений

Fluorescent Correlation Spectroscopy

Magde, D., Elson, E., and Webb, W.W. (1972) Phys. Rev. Lett. 29, 705

Flu

ore

scen

t C

orr

elat

ion

Sp

ectr

osc

op

y

Autocorrelation function

22,

2

4181

111

)(

)()()(

zyx w

D

w

DNtI

tTtIG

N – average number of the molecules in the focal volumeD – diffusion coefficient

Wx,y

Wz

Flu

ore

scen

t C

orr

elat

ion

Sp

ectr

osc

op

y

Diffusion coefficient

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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

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Correlation function with a single photon detector

Tim

e co

rrel

atio

n o

f p

ho

ton

s

Tkoh is the measure for thedegree of coherence inthermal light sources

C. Zinoni et al., APL 2007

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Hanbury-Brown/Twiss-Experiment

Tim

e co

rrel

atio

n o

f p

ho

ton

s

Finite response time and/or dead time of a single photon detector brought upthe HBT method

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Outline






импульсов


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Femtosecond pulse lasers

Autocorrelator

How to measure the pulse duration?

SHD – second harmonic generator(non-linear optical crystal)

D – any slow detector

Interferometric autocorrelation

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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.In

terf

ero

met

ric

auto

corr

elat

ion

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Fast optical detectors

Interferometer

How to measure the response time of the detector?

L – femtosecond pulse laser

P – polarizer

V – slow voltmeter

D –detector under studyA. Semenov et al., JLTP 1996

Use the nonlinearity V(P) of the detectorresponse and do not forget to eliminateinterference

P V

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Intensity autocorrelation

Interferometer

P. Probst et al., PRB <2012>

P V

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

-30 -20 -10 0 10 20 30

Delay (ps)

Co

rre

lati

on

sig

na

l (r.

u.)

R1fl

R2fl

GAUSS 7 ps

YBCO superconducting detector and Ti-Sapphire laser

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Intensity autocorrelation

Inte

nsi

ty a

uto

corr

elat

ion

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

22

2

2

1

2

)(t)E(t)E(aV(t)

IaV(I)

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Fast optical detectors

L. Shi et al., APL 1992

Use the mutual current drain of two identicaldetectors

How to measure the linear response time?

Crosstalk correlation

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Crosstalk correlation

Cro

ssta

lk c

orre

lati

on

I = const0

I1

R1

I2

R2

(t) (t+ )

021

2211

02,1

)(

),()(

))(1()())(()(

iII

tVRIRI

ItRRtR

tPItIItItR nC

nC

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Outline






импульсов


THz Synchrotron Radiation

Synchrotronradiation

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Signal appearance

J. Feikes et al., PR ST AB 2011

Bending magnet

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Synchrotron radiation

Typical valuesfst

ps

e

e

035.0

25

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Coherent synchrotron radiation

re(x)

orbit acceptance portion

c

v

25 ps

bunch length ( x) 10 ps

Coherence condition x < l

Coherent THz Radiation from a Synchrotron

momentum compaction factor: p/p = L/L fs

2

reference orbit: L = 240 m

L

bunch, p

intensity vs. number of electronslongitudinal bunch length

h

h

z > l

z l

intensity vs. number of electronslongitudinal bunch length

low alpha optics

z 1 mm

t < 7 ps

10-4

normal user optics

z > 5 mm

t > 35 ps

= 7·10-3

h

h

z > l

z l

v c

Single electron1 ps window

THz -pulse

10 ps

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MLS data sheet

Syn

chro

tro

n

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Outline






импульсов


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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

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 inAntenne eingekoppeltund nur wenig reflektiert

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Martin-Puplett Interferometer

Output

Input 1

Input 2

Typical autocorrelation signal

Detector signals seem to overlap over the whole scan lengthNegative autocorrelation signalNeither the peak at 0 nor the whole response corresponds with the

streak camera measurementsPeriod of about 20 psPeak at zero shorter than the other peaks

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

0.0

0.5

1.0

Normalize to [0, 1] of B GaussAmpFit von C

Nor

mal

ized

Aut

ocor

rela

tion

sign

al

Time (ps)

Beam parameter: 629 MeV, 480 kV, 7.05 kHz, 100mA beam current

Streak camera: FWHM) = 26ps

Combination from crosstalk correlationand field correlation

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Field detector

0.0 0.2 0.4 0.6 0.8 1.0-5

0

5

10

TH

z re

spo

nse

(m

V)

Time (ns)

Dielectric mirror Metallic mirror

I = const0

I1

R1

I2

R2

(t) (t+ )

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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).

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Autocorrelation with superlattice detector

S. Winnerl et al., APL 1998

Combination from field and intensity correlation

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Thank you

Documents

N. Novgorod, IFM, 20.09.2011 Корреляционные методики измерения коротких импульсов терагерцового излучения Alexej