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Arnaud Cuisset ,

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.

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Arnaud Cuisset ,

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  1. 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

  2. THz spectroscopy using optoelectronic methods 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

  3. THzgeneration 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

  4. The cw-THzspectrometer 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

  5. High resolutioncw-THzspectroscopy 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) H2S P=0.5mbar 616 - 505 Low pressure (< 10 mbar) line by line measurement 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

  6. Determination of spectroscopicparameters athigh 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-broadeningparameters of HCN (Yang et al, JQSRT vol.109, pp 2857-2868 (2008)) Rotational transitions measured at different pressures up to 3 THz Systematic studies at high J values: Contribution to spectroscopic databases (HITRAN 2008) Determination of broadening coefficient and comparison to theoretical values Δν=γΡ New measurements for J>28 J=31 ■: cw-THz * : Ref Hitran 2004. 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

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

  8. THzfrequencystudies 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 DiMethylSulfOxyde Vinyl chloride Synchrotron FTIR spectrum L=150m Cuisset et al. Chem. Phys. Lett., Vol.492 pp 30, (2010) Hindle et al. Sensors, Vol.9 pp 9039, (2009) 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

  9. Photomixing : How does one obtain the THzfrequency? 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 !!!) Frequencymetrologybased on femtosecond lasers (Th. Hansch) Periodicity in time = periodicity in frequency f0: Carrier-envelope offset frequency 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

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

  11. Development of a THz synthesizer based on a frequency comb (Mouret et al., Opt. Express, vol.17, pp. 22031, (2009)) Bolometer fs fiber laser Photomixer DL 1 DL 2 PLL BS BS G x2 Off axis parabolic mirror Synthesizer PLL PD1 PD2 Frequency of the locked repetitin rate: Mean frequency: 100 000 000.992 Hz Standard deviation: 0.01 Hz fTHz=n.frr+fa+fb Frequency of the phase locked beat note Mean frequency : 20000000. 2 Hz Standard déviation: 13.5Hz 11th International HITRAN Conference (united with the 10th ASA conference) 16 - 18 June 2010

  12. First spectroscopic measurements of theTHz synthesizer (Mouret et al., Opt. Express, vol.17, pp. 22031, (2009)) All the J → J + 1 transitions with 65 < J < 106 of OCS have been measured in the 0.8 – 1.3 THz frequency range fTHz=n.frr+fa+fb fOCS = 8133 * 100.001 097 8+ 44.821 MHz = 813 353.748 +/- 0.012 MHz fJPL = 813 353.706 +/- 0.06 MHz 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

  13. 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

  14. Thanks for your attention!

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