1. Coherent transients from carbonyl sulfide excited by terahertz radiation
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
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

2. The terahertz waves: Spectroscopic applications : 1012 108 109 1010
1 THz  33 cm-1  300 µm  4.1 meV  48 K
1012
108
109
1010
1011
1013
1014
1015
1016
1017
Frequencies
(Hz)
T-ray
Radio-TV
waves
Micro - Waves
Infra-Red
Visible
Ultra-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 :
Biomolecules
Toxically agents
Weakly bounded
complexes

3. 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)

4. Generation of a broadband THz radiation
37.5µm
5µm
Institut of Electronic, Microelectronic and Nanotechnology Université des Sciences et Technologies, Lille
Collaboration with V
10V
Optical pulse
« pump »
THz pulse time structure depends 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 :
A few tens of nW on the complete
broadband spectrum
THz pulse
Focalization of a fs laser pulse on a dipolar antenna

5. Coherent detection by THz photoconductive sampling
Optical pulse
« probe »
THz pulse from the emitter antenna A
Alternative: detection with an electro-optical Zn : Te crystal
Measurement of a nA current
directly proportional to the
THz field
Coherent detection
Information on the amplitude
absorption studies
Information on the phase
dispersion studies

6. THz-TDS set-up (nA) Photoconductive sampling t(ps) THz pulse
Lock-in Amplifier
Labview
Acquisition
Emission
Detection
Beam splitter 50/50
« Pump »
beam
« Probe »
Beam
Delay-Line
MIRA Oscillator
( 100fs)
Sa:Ti laser   800 nm
(nA)
THz pulse
Photoconductive
sampling
Optical Pulse
t(ps)

7. Time-shape of the THz field / Broadband spectrum
Fourier
transform
Spectral Range: GHz – 1300 GHz
(limited by the antenna cut-off frequency)
Resolution ~ 2.2GHz
(limited by the measurement duration)
H2O transitions
Time separation max min ~1ps
SNR>1000
Coherent measurement
principle

8. THz-TDS of a linear molecule OCS
Carbon sulfide : a molecule very well-known in spectroscopy
Astrophysic interest
(presence in the interstellar medium)
Atmospheric interest
(Significant VOC in the troposphere)
Theorical interest
(Linear molecule with few atoms)
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

9. Coherent transient behavior of the OCS molecules after a broadband THz excitation
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 = 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 linear
and symmetric molecules

10. Theoretical model for the F.I.D. signal
Reference to the previous studies on N2O by Harde & Grischkowsky, (J.O.S.A.B, 8, (1991))
Model based on the Maxwell-Bloch equation
in a case of a linear polarization (weak intensities)
One solution
with
An equivalent expression may be established
for the dispersion term , the change of the
vector wave is summed
over all the transitions too.
 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 ) , ( J A G w
E(0,t) corresponds to the experimental
reference pulse including THz pulse profile,
absorptions on the beam propagation, noise…

11. Comparison theory - experience
Experimental F.I.D. signal
Theoretical F.I.D. signal
Good agreement between theory and experience:
the pulse shape changes, due to the gradual dephasing induced by anharmonicity, are well reproduced

12. Determination of molecular parameters in the time domain
 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.
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)
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, , (2005))

13. Current study in the continuity of this work:
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 Laser Center of the Moscow university
Application of the spectrochronography technique* based on the windowed
Fourier 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)