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Coherent transients from carbonyl sulfide excited by terahertz radiation

Apologises for my absence but the writing up of my phD manuscript is

taking all my time!Nevertheless, I’m looking for a

postdoctoral position for the next year. If you’re interest, contact me

by e-mail : damien.bigourd@univ-littoral.fr

D.Bigourd, A.Cuisset, G. Mouret, S. Matton, F. Hindle, E. Fertein , R. Bocquet

Laboratory of Physics and Chemistry of the Atmosphere, UMR CNRS 8101,

Université du Littoral Côte d’Opale, Dunkirk

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The terahertz waves:1 THz 33 cm-1 300 µm 4.1 meV 48 K

1012108 109 1010 1011 1013 1014 1015 1016 1017Frequencies

(Hz)

T-rayRadio-TVwaves

Micro - Waves

Infra-Red VisibleUltra-Violets X-Ray

(electronic technologies ) (optic technologies)« spectral gap »

Submillimeter-waves rotational spectroscopy

Atmospheric spectroscopy Astronomic spectroscopy

Far-infrared vibrational spectroscopy

Low-frequency modes (Large amplitude motions, torsion, bending…)

Spectroscopic applications :

BiomoleculesBiomolecules Toxically agentsToxically agentsWeakly bounded Weakly bounded complexescomplexes

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THz spectroscopy in the LPCA• Located at Dunkirk, in the north of France

• Dunkirk, very big industrial harbour with significant air pollution

• Development of two complementary THz spectrometer for molecular compounds monitoring in gas phase

• Generation of a THz continuum

for Terahertz-Time Domain Spectroscopy

(THz-TDS) applications

• Continuous Wave Terahertz

(CW-THz)

radiation produced by photomixing( “Rendez-Vous” this afternoon 1015 McPherson lab 4. pm)

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Generation of a broadband THz radiation

FocalizatioFocalization of a fs n of a fs

laser pulse laser pulse on a on a

dipolar dipolar antenna antenna

V

THz pulse

t

tjtTHzE

)()(

10V

37.5µm

5µm

Institut of Electronic, Microelectronic and Nanotechnology Université des Sciences et Technologies, Lille

Collaboration withCollaboration with

Optical pulse« pump » THz pulse time structure THz pulse time structure dependsdepends on : on :

Optical pulse (Sa:Ti MIRA 900) Active layer material : LT Ga-As(subpicosecond life-time of charge carriers)Ultrafast dipolar antenna

(Different geometries tested

)

Emitted power : Emitted power :

A few tens of nW on the completeA few tens of nW on the completebroadband spectrumbroadband spectrum

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Coherent detection by THz photoconductive sampling

Optical pulse « probe »

THz pulse from the emitter

antennaA

)()( tEtj zTH

THzE

Alternative: detection with an electro-optical Zn : Te crystal

Measurement of a nA currentdirectly proportional to the

THz field

Coherent detection

Information on the amplitudeabsorption studies

Information on the phasedispersion studies

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Lock-in Amplifier

Labview Acquisition

EmissionDetection

Beam splitter 50/50

« Pump »beam

« Probe »Beam

Delay-Line

MIRA Oscillator( 100fs)

Sa:Ti laser 800 nm

(nA)

t(ps)

PhotoconductivesamplingOptical Pulse

THz pulse

THz-TDS set-up

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Time-shape of the THz field / Broadband spectrum

Time separation max min ~1psSNR>1000

0 25 50 75 100-6

-4

-2

0

2

4

6

8

Ave

rage

Cur

rent

(nA

)

Time delay (ps)

0 20 40 60 80 100 120

-4

-2

0

2

4

6 Sample Reference

Ave

rage

cu

rren

t (n

A)

Time delay (ps)

Absorption

optical thickness

Coherent measurement principle

Fourier transform

Spectral Range: 100 GHz – 1300 GHz(limited by the antenna cut-off frequency) Resolution ~ 2.2GHz(limited by the measurement duration)

0 250 500 750 1000 1250

1E-4

1E-3

0,01

Am

plit

ude

spec

trum

(u.

a)

Frequency (GHz)

H2O transitions

110- 101

211- 202

202- 111

312- 303

312- 303

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THz-TDS of a linear molecule OCSCarbon sulfide : a molecule very well-known in spectroscopy

Atmospheric interest (Significant VOC in the troposphere)

Astrophysic interest (presence in the interstellar medium)Theorical interest

(Linear molecule with few atoms)

3)1(4)1(22/ JDJBJJ kHzDGHzB 3.10815.6

100 200 300 400 500 600 700 800 900 1000-2

-1

0

1

Frequency(GHz)

k L

(rad

ian

)

100 200 300 400 500 600 700 800 900 10000

1

2

3

4

Frequency(GHz)

Ab

sorp

tion

(L

/2)

Experimental absorption and dispersion profile at a pressure of 200 mbar

More than 60 pure rotational transitions are excited simultaneously by the THz field

The weak resolution of the THz-TDS (~2.2GHz) do not allowed a correct measurement of the molecular parameters Analysis of the THz field in the temporal domain

10 < J < 80

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Coherent transient behavior of the OCS molecules after a broadband THz excitation

0 100 200 300 400

-2

0

2

4

6

82.6ps 82.6ps 82.6ps 82.6ps

5432

1

Av

era

ge c

urren

t (n

A)

Time (ps)

082.6ps

P=100 mbar

Observation of a series of THz pulses until 450 ps at a rate equal to frequency separation of the absorption lines (2B = 12.107 GHz ↔ 82.6 ps)

Free induction decay (F.I.D.) signal with a characteristic time inversely proportional to the collisional broadening

Coherent transient phenomena observable in the case of linearand symmetric molecules

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Theoretical model for the F.I.D. signalReference to the previous studies on N2O by Harde & Grischkowsky, (J.O.S.A.B, 8, 1642-1651 (1991))

²),(²

²),(²

²²

²),(²

0

ttzP

ttzE

cn

ztzE

Model based on the Maxwell-Bloch equation in a case of a linear polarization (weak intensities)

with dttitEE )exp(),0(21)(

is given by the summation of the absorption coefficients including all the rotational

transitions excited by the THz field

A lorentzian profile has been chosen for the function

),())1(

exp()1()(6

²²)( 3

0

0JAJ G

kT

JhBJJ

kTnc

hBpf

)()( J

),( JAG

An equivalent expression may be establishedfor the dispersion term , the change of the

vector wave is summed over all the transitions too.

),())1(

exp()1(²)²()(6

²²)( 3

0

0JN

J

JJ G

kT

JhBJJ

kTnc

hBpfk

)(k)(JkE(0,t) corresponds to the experimental

reference pulse including THz pulse profile, absorptions on the beam propagation, noise…

dzzkizktiEtzE ))(2

1exp())(exp())((exp)(),( 0

One

solution

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Experimental F.I.D. signal

Theoretical F.I.D. signal

Comparison theory - experience

Good agreement between theory and experience: the pulse shape changes, due to the gradual dephasing induced by anharmonicity, are well reproduced

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Determination of molecular parameters in the time domain

82 84 86 88 90 92 94 96 98 100 102 104-2

-1

0

1

2 Experimental pulse Calculated pulse in the

rotator rigid approximation

Time(ps)

Ave

rage

cu

rren

t (n

A)

Measure of the rotational constant B with the rate of the radiated coherent pulses with an absolute uncertainty inferior to 30 MHz ( relative uncertainty < 0.5%) Clear observation of the influence on the pulses shape of the centrifugal distortion (D=1.3 kHz)

82 84 86 88 90 92 94 96 98 100 102 104-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

(Isi

gnal-I

mod

el)

(nA

)

Time (ps)

with centrifugal distorsion without centrifugal distorsion

In the case of this low frequency resolution technique, the analysis of THz F.I.D.,may be a good way for the determination of molecular parameters.

Dynamical information may be obtained by the relaxation time measurements (T2 and T1) from the F.I.D. signal (B. L. Yu & al. App. Phys. Lett., 86, 101108, (2005))

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Summary : • The low frequency resolution of the THz Time Domain Spectroscopy is well

matched to the small quality factor of resonances with the molecules ( studies in condensed phase, probe of intermolecular low-frequency vibrational modes…)

• Nevertheless the experimental and theoretical analysis of the coherent transients produced by the excitation of a linear or symmetric molecule by a THz field allow the determination of molecular parameters in the gas phase.

Current study in the continuity of this work: Collaboration with Alexander Skhurinov from the department of Physics and International Collaboration with Alexander Skhurinov from the department of Physics and International

Laser Center of the Moscow universityLaser Center of the Moscow university

•Application of the spectrochronography technique* based on the windowedFourier transform procedure for studies of temporal dynamics of THz pulse in interaction with the carbonyl sulfide molecules

*Nazarov & al., Laser Phys. Lett. 2, No. 10, 471–475 (2005)