A THz PHOTOMIXING SYNTHESIZER BASED ON A FIBER FREQUENCY COMB DEDICATED TO HIGH RESOLUTION SPECTROSCOPY OF ATMOSPHERIC COMPOUNDS Arnaud Cuisset, Laboratoire de Physico-Chimie de l’Atmosphère, Maison de la Recherche en Environnement Industriel, Dunkerque, France. 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

THz spectroscopy using optoelectronic methods Optoelectronic THz generation using two methods High spectral purity THz radiation produced by photomixing Continuous-Wave THz spectroscopy: Broadband technique for low resolution gas phase spectroscopy THz - Time Domain Spectroscopy: 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

THz generation by photomixing: global overview Main idea:  To convert an optical frequency beatnote towards the THz spectral range by focusing two continuous lasers on a photomixer. V Spatial overlapping of two continuous lasers with THz frequency separation THz wave emission via a specially designed antenna Optoelectronic conversion of the THz beatnote into photocurrent P1+P2: incident laser power VDC: Bias voltage (RaC)-1 :cut-off frequency antenna t : charge carrier lifetime 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

The cw-THz spectrometer in the LPCA The photomixer: A specific antenna connected to an interdigitated electrode array on a LTG-GaA substract 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

High resolution cw-THz spectroscopy by photomixing A large tunability: 0.3 THz – 3.3 THz ( 10 cm-1 – 110 cm-1) A good spectral purity: < 3 MHz ( 10-4 cm-1) Low pressure (< 10 mbar) line by line measurement H2S P=0.5mbar 616 - 505 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

Determination of spectroscopic parameters at high J values for linear polar molecules New line profil analysis (broadening and intensities) from collisional to Doppler limit Have been performed by means of the cw-THz spectrometer Air-broadening parameters of HCN (Yang et al, JQSRT vol.109, pp 2857-2868 (2008)) Rotational transitions measured at different pressures up to 3 THz J=31 Determination of broadening coefficient and comparison to theoretical values Δν=γΡ Systematic studies at high J values: Contribution to spectroscopic databases (HITRAN 2008) New measurements for J>28 ■: cw-THz * : Ref Hitran 2004. 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

Determination of spectroscopic parameters at high J values for linear polar molecules Absolute line intensities measurements of OCS (Matton et al, JMS vol. 239, pp.182 (2006))) 848 850 GHz 852 1000 2000 3000 4000 16O12C34S: J=71->72 1 Amplitude, u.a 16O12C32S: J=69 70 16O12C32S: J=6970, 2 band POCS = 10 mbar (cell length = 125 cm) Possible studies of the different isotopic species and of the rotational lines in the vibrationnally excited states. Intensity versus J =0,710  0,005 D Fine determination of the permanent dipole moment 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

THz frequency studies of VOC Long optical path are required for these spectroscopic studies. A good frequency metrology is also necessary in order to be competitive with common submillimeter measurements Vinyl chloride Hindle et al. Sensors, Vol.9 pp 9039, (2009) DiMethylSulfOxyde Synchrotron FTIR spectrum L=150m Cuisset et al. Chem. Phys. Lett., Vol.492 pp 30, (2010) 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

Frequency metrology based on femtosecond lasers (Th. Hansch) Photomixing : How does one obtain the THz frequency? fTHz= f1-f2 mais f1(775 nm) ≈ f2(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 f0: Carrier-envelope offset frequency Frequency metrology based on femtosecond lasers (Th. Hansch) 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

Development of a THz synthesizer based on a frequency comb (Mouret et al., Opt. Express, vol.17, pp. 22031, (2009)) Principle of the frequency measurement in a photomixing experiment DL1 DL2 f1 f2 fTHz=n.frr+f1+f2 frr Frequency I(f) offset 5 nm ↔ 2.5 THz Turn-key commercial femtosecond laser source. 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

Development of a THz synthesizer based on a frequency comb (Mouret et al., Opt. Express, vol.17, pp. 22031, (2009)) Bolometer Off axis parabolic mirror Photomixer DL 1 DL 2 fs fiber laser G PD1 PD2 BS PLL Synthesizer x2 Frequency of the phase locked beat note Mean frequency : 20 000 000. 2 Hz Standard déviation: 13.5 Hz Frequency of the locked repetitin rate: Mean frequency: 100 000 000.992 Hz Standard deviation: 0.01 Hz fTHz=n.frr+fa+fb 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

First spectroscopic measurements of theTHz synthesizer (Mouret et al., Opt. Express, vol.17, pp. 22031, (2009)) fOCS = 8133 * 100.001 097 8+ 44.821 MHz = 813 353.748 +/- 0.012 MHz fJPL = 813 353.706 +/- 0.06 MHz fTHz=n.frr+fa+fb All the J → J + 1 transitions with 65 < J < 106 of OCS have been measured in the 0.8 – 1.3 THz frequency range The fit of OCS parameters demonstrated that the properties of the THz spectrometer may be compared with competing techniques. 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

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 ground state molecular parameters from THz frequency measurements. Gaël Mouret Francis Hindle Sophie Eliet Robin Bocquet 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

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