HIGH RESOLUTION SPECTROSCOPY USING A TUNABLE THz SYNTHESIZER BASED ON PHOTOMIXING Arnaud Cuisset, Laboratoire de Physico-Chimie de l’Atmosphère, Maison de la Recherche en Environnement Industriel, Dunkerque France 66 th International Symposium on Molecular SpectroscopyJune 23, 2011

Motivation Development of spectrometers using optoelectronic sources dedicated to gas phase THz analysis of molecular systems with environmental interest Line position and Line profile analysis of atmospheric compounds from their rotational signatures in the THz range Detection, quantification and kinetic studies of pollutants in realistic media 66 th International Symposium on Molecular SpectroscopyJune 23, 2011

CW-THz Photomixing (Hindle et al Comptes Rendus Phys. 9, pp , (2008)) 66 th International Symposium on Molecular SpectroscopyJune 23, 2011 Spatial overlapping of two continuous lasers with THz frequency separation V Optoelectronic conversion of the THz beatnote into photocurrent THz wave emission via a specially designed antenna

The photomixer: A specific antenna connected to an interdigitated electrode array on a LTG-GaAs substract CW-THz Photomixing 66 th International Symposium on Molecular SpectroscopyJune 23, 2011 A large tunability: 0.3 THz – 3.3 THz ( 10 cm -1 – 110 cm -1 ) A good spectral purity: < 3 MHz ( cm -1 ) A THz output power limited from 1THz to 3 THz An absolute THz frequency accuracy limited to 100 MHz using a commercial wavelengthmeter.

66 th International Symposium on Molecular SpectroscopyJune 23, 2011 f THz = f 1 -f 2 mais f 1 (775 nm) ≈ f 2 (775 nm) ≈ 387 THz An accuracy of 1 MHz for the frequency of the THz radiation require the same order of accuracy on the frequency of Ti:Sa laser (10 -9 !!!) Periodicity in time = periodicity in frequency f 0 : Carrier-envelope offset frequency

f syn DL 2 66 th International Symposium on Molecular SpectroscopyJune 23, 2011 CW-THz synthesizer 1 st generation: : 2 ECDL are locked in phase on 2 lines of the frequency comb 2 ECDL are locked in phase on 2 lines of the frequency comb ☺ THz frequency accuracy and spectral purity are better than 100kHz ☹ The continuous tunability is reduced to the tens of MHz CW-THz synthesizer 2 nd generation: : The fixed base frequency is generated by locking DL0 and DL1 to a FC. The phase locked tuning of the synthesized frequency is performed by PLL3 locking DL2 to DL1. The optical beatnote between DL0 and DL2 is converted to THz radiation ☞ 300 MHz of continuous tunability CW-THz Synthesizer based on a frequency comb (Mouret et al., Opt. Express, vol.17, pp , (2009)) f THz = f synt + n.f rr (with f 0 =-f 1 )

The CW-THz spectrometer 66 th International Symposium on Molecular SpectroscopyJune 23, 2011

THz line position measurements on H 2 CO 66 th International Symposium on Molecular SpectroscopyJune 23, 2011 In order to test the frequency metrology of the new version of the THz synthesizer, 61 new ground state rotational transitions of H 2 CO have been measured in the 1.1 – 1.7 THz region. Parametersglobal fitfit CDMS (only)fit LPCA(only)* A(MHz) (128) (129) (139) B (MHz) (32) (32) (105) C(MHz) (31) (32) ( 92) DJ(MHz) ( 67) ( 71) (133) DJK (MHz) (163) (176) ( 37) DK(MHz) (282) (287)17.64( 78) dl(MHz) (213) (218) (138) d2(KHz) ( 63) ( 64) (113) HJ (mHz)3.95( 48) 4.38( 52) -5.3(134) HJK (Hz) 7.446( 37) 7.458( 42) (110) HKJ(KHz) (201) (206) ( 33) HK (KHz) 4.019(301) 3.944(311) (101) h1 (Hz) (155) (159)0.0410(180) h2 (Hz) (133) (135) 0.053( 37) h3 (Hz) ( 46) ( 46) -5.3(140) LJJK (mHz)0.0926(183) (224) -0.12( 87) LJK (mHz)-0.67(286) -1.65(296) (115) LKKJ (mHz)-1.9( 70) 0.3( 72)0.386(266) LK (Hz)3.3(112) 7.1(116) 0.76( 43) l2 (µHz) (108) (112) -9( 40) l3 (µHz) ( 38) ( 38) -6.8(140) l4 (µHz) (136) (143) -8.0( 52) RMS (MHz) MHz Number of lines A new list of frequencies is available for the spectroscopic databases. The molecular parameters in the ground state are fitted at the experimental accuracy, only the highest order terms are sensitive to the new measurements 9 4,6 -8 4,5 9 4,5 -8 4,4 24 4, ,20

66 th International Symposium on Molecular SpectroscopyJune 23, 2011 THz line position measurements on H 2 CO Recent measurements of rotational transitions in v4 and v6 excited states have been performed in the 0.7 – 1.5 THz frequency region by means of a THz propagation in a mulipass cell (L=8.8m) v4: Out of plane bending cm -1 v6: CH2 rocking cm -1 Goal: To characterize the vibrational dependence of the rotational constants providing in particularly the vibration-rotation constants. Still in progress: 25 new THz lines have been measured with an average precision of 40 kHz An average uncertainty of 240 kHz is obtained relatively to the predictions of Agnès Perrin

66 th International Symposium on Molecular SpectroscopyJune 23, 2011 THz line profile analysis on CH 3 Cl Large scans allowed a multifit procedure applied on K series of J → J + 1 transitions of CH 3 Cl An accuracy lower than 3% is obtained on the broadening coefficients J=31

66 th International Symposium on Molecular SpectroscopyJune 23, 2011 THz line profile analysis on CH 3 Cl Collisional analysis : Self, O 2, N 2  Collisional analysis : Self, O 2, N 2  Studies from 0.84 THz up to 1.4 THz  Studies on the J and K dependency of the broadening coefficients  Work from J down = 31 up to J down = 50 and for K=0;1;2;3;4;5;6;9; and 12 K dependency of the self-broadening Self pressure shift Excellent agreement with theory (collaboration with Besançon)

66 th International Symposium on Molecular SpectroscopyJune 23, 2011 THz line recognition in methanol/ethanol gas phase mixture The synthesizer is able to discriminate ethanol and methanol spectra from a mixture at THz frequency The sensitivity of the measurement can be increased by implementing a frequency modulation scheme.

THz detection and quantification of OH and SH radicals produced in a cold plasma 66 th International Symposium on Molecular SpectroscopyJune 23, 2011 OH is detected in a cold plasma produced by means of a continuous flow of precursor excited in a RF discharge. OH Hyperfine components resolved in the 2,5 THz region FM has been used for the recording of new SH transitions in the 1,3 – 1,6 THz region 2 Π 3/2 : 5/ /2 - RF off RF on

Conclusion Photomixing technique is now an alternative solution to implement THz tunable solid source The photomixing technique has the greatest tuning range of any known coherent source in the THz region, and has a spectral purity inferior to the Doppler broadening limit. Therefore, the photomixing technique allow to generate a THz radiation very promising for gas phase spectroscopic applications: Line profil analysis up to high excited rotational levels Accurate determination of molecular parameters from THz frequency measurements. Gas phase monitoring of stable and unstable atmospheric species 66 th International Symposium on Molecular SpectroscopyJune 23, 2011

Gaël MouretFrancis HindleSophie ElietRobin Bocquet Mickael Guinet