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) June 2010

2. 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) June 2010

3. 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) June 2010

4. 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) June 2010

5. 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
11th International HITRAN Conference (united with the 10th ASA conference) June 2010

6. 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 (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) June 2010

7. 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) June 2010

8. 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) June 2010

9. 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) June 2010

10. Development of a THz synthesizer based on a frequency comb
(Mouret et al., Opt. Express, vol.17, pp , (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) June 2010

11. Development of a THz synthesizer based on a frequency comb
(Mouret et al., Opt. Express, vol.17, pp , (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 : Hz
Standard déviation: 13.5 Hz
Frequency of the locked repetitin rate:
Mean frequency: Hz
Standard deviation: 0.01 Hz
fTHz=n.frr+fa+fb
11th International HITRAN Conference (united with the 10th ASA conference) June 2010

12. First spectroscopic measurements of theTHz synthesizer
(Mouret et al., Opt. Express, vol.17, pp , (2009))
fOCS = 8133 * MHz
= / MHz
fJPL = / 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) 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) June 2010

14. Thanks for your attention!