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

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

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)

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

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

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)

Time-shape of the THz field / Broadband spectrum Fourier transform Spectral Range: 100 GHz – 1300 GHz (limited by the antenna cut-off frequency) Resolution ~ 2.2GHz (limited by the measurement duration) H2O transitions 110- 101 211- 202 202- 111 312- 303 Time separation max min ~1ps SNR>1000 Coherent measurement principle

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

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 = 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 linear and symmetric molecules

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

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

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, 101108, (2005))

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)