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We have developed MULTICHARME, a modified Chernin-type multi-pass cell especially designed for IR and THz long-path absorption measurements in the CHamber for Atmospheric Reactivity and Metrology of the Environment (CHARME). By measuring the output power using a near-IR diode-laser and a THz amplified multiplication chain, we have established that the effective reflectivity of MULTICHARME is better than 94 % over approximately three decades of frequency. Absorption measurements of N2O have been performed by probing highly excited rovibrational transitions in the near-IR and ground state rotational transitions at submillimeter wavelengths. In each case the linearity of the absorbance with the path lengths was verified. Finally, we demonstrate that THz spectroscopy is able to study the isotopic composition of greenhouse polar gases such as N2O and to absolutely quantify stable (N2O) and reactive (O3) species at trace levels. At low pressure the ozone concentration was continuously monitored and its decay characterized. The deduced ozone lifetime of 3.4 ± 0.1 h is shorter compared with previous measurements performed in CHARME at atmospheric pressure. For the first time, the ability of THz rotational spectroscopy to monitor, with a very high degree of selectivity, stable and reactive polar compounds at trace level in an atmospheric simulation chamber is demonstrated. However, the sensitivity of the THz monitoring needs to be improved to reach atmospheric trace levels. For this purpose, it is necessary to fully understand the origin of the observed baseline variations caused by the complex multiple standing waves present in MULTICHARME.

A high finesse cavity operating around 620 GHz has been constructed to improve the sensitivity limit for the measurement of weakly absorbing gases. An effective path length of 1 km is achieved significantly improving the available sensitivity. The measurement of the cavity decay time allows the quantification of the absorption coefficient.

oral; International audience; We present an experimental investigation into new continuous-wave laser lines from a terahertz ammonia laser pumped by a quantum cascade laser. These lines were investigated with the help of a molecular figure of merit calculated from spectroscopic parameters available in molecular databases. We also demonstrate the use of this laser as a local oscillator in a heterodyne high-resolution spectrometer between 1 and 3.4 THz.

oral; International audience; We improved the spectral resolution of THz time domain spectroscopy beyond the limit given by the delay line length. The constraint reconstruction algorithm used successfully depicted the 1.21 THz NH 3 doublet separated by ~400MHz in experimental data despite the 1.2 GHz delay line limitation.

Terahertz (THz) Time domain spectroscopy (THz-TDS) is a broadband spectroscopic technique spreading its uses in multiple fields: in science from material science to biology, in industry where it measures the thickness of a paint layer during the painting operation. Using such practical commercial apparatus with broad spectrum for gas spectroscopy could be a major asset for air quality monitoring and tracking of atmospheric composition. However, gas spectroscopy needs high resolution and the usual approach in THz-TDS, where the recorded time trace is Fourier transform, suffers from resolution limitation due to the size of the delay line in the system. In this letter, we introduce the concept of constraint reconstruction for super-resolution spectroscopy based on the modeling of the spectroscopic lines in a sparse spectrum. Light molecule gas typically shows sparse and narrow lines on a broad spectrum and we propose an algorithm reconstructing these lines with a resolution improvement of 10 the ultimate resolution reachable by the apparatus. We envision the proposed technique to lead to broadband, selective, rapid and cheap gas monitoring applications.

International audience; Time Domain Spectroscopy resolution is usually limited by the Fourier Heisenberg criteria (FHC) coming from the length of the delay line. We will present a method to exploit THz-TDS spectra providing a 10-fold resolution improvement reaching 120 MHz for width measurements. These results were obtained by modeling and the implementation of a constraint reconstruction algorithm. We tested the methods on temporal traces from gaseous NH3 and were in very good agreement with the values from literature [1] down to a pressure of 5 mbar.

International audience; We have recently demonstrated a continuous wave molecular laser emission based on ammonia optically pumped by a quantum cascade laser. We report here new results obtained with an optimized laser prototype pumped by a tunable external cavity quantum cascade laser. New laser lines with high output power are demonstrated at frequencies between 1 and 5.5 THz.

International audience; High-resolution lines of H2S self-perturbed and perturbed by N2 have been recorded at room temperature using a frequency modulated THz spectrometer at pressures of H2S ranging from 0.1 to 1.0 mbar. A non-linear least squares fitting of these spectra has been performed to retrieve self- and N2-broadening coefficients of pure rotational transitions (mainly in the RQ sub-branch) of H2S in the submillimeter region. We show that the lineshapes are well reproduced by the Speed-Dependent Voigt profile accounting for the speed dependence of relaxation rates which display linear pressure dependencies. Using the Robert and Bonamy formalism, these coefficients were calculated, showing the need to account for all contributions of the intermolecular interaction for N2-broadening. The calculated coefficients are consistent with the measured data and the average accuracy of the line parameters has been estimated to be about 5%. The observed J″ and Ka″ rotational dependences have been analyzed using an empirical model and compared to previous rovibrational studies in the ν2 fundamental band. In our study the self- and the N2-broadening coefficients decreases monotonically with Ka″ and J″, respectively.

Several previously unmeasured transitions of 12 CH 3 D have been recorded by a terahertz photomixing continuous-wave spectrometer up to Q R (10) branch at 2.5 THz. An improved set of rotational constants has been obtained utilizing a THz frequency metrology based on a frequency comb that achieved an averaged frequency position better than 150 kHz on more than fifty ground-state transitions. A detailed analysis of the measured line intensities was undertaken using the multispectrum fitting program and has resulted in a determination of new dipole moment parameters. Measurements at different pressures of the Q R (7) transitions provide the first determination of self-broadening coefficients from pure rotational CH 3 D lines. The THz rotational measurements are consistent with IR rovibrational data but no significant vibrational dependence of self-broadening coefficient may be observed by comparison.

International audience; A new spectrometer on the AILES beamline of the SOLEIL synchrotron facility to achieve sub-MHz resolution in the THz and far-IR regions is currently being developed. Thanks to a dedicated heterodyne detection by use of a new kind of molecular laser as local oscillator along with a Fast Fourier Transform Spectrometer, several pure rotation absorption lines of H 2 S and CH 3 OH around 1.073 THz have been recorded.

International audience; Abstract:Optical probes are among the most important techniques for characterizing new physical states induced by changes of temperature and/or pressure. In link with its high brightness and wide spectral coverage, infrared synchrotron radiation is ideally suited for such studies when the quantity of material to investigate is limited or when the system needs to be probed down to the sub mm domain.

International audience; A new spectrometer allowing both high resolution and broadband coverage in the terahertz (THz) domain is proposed. This instrument exploits the heterodyne technique between broadband synchrotron radiation and a quantum cascade laser (QCL) based molecular THz laser that acts as the local oscillator (LO). Proof of principle for exploitation for spectroscopy is provided by the recording of molecular absorptions of hydrogen sulfide (H 2 S) and methanol (CH 3 OH) around 1.073 THz. Ultimately, the spectrometer will enable to cover the 1-4 THz region in 5 GHz windows at Doppler resolution.

A terahertz (THz) frequency synthesizer based on photomixing of two near-infrared lasers with a sub-THz to THz frequency offset is a powerful tool for spectroscopy of polar gas molecules due to its broad spectral coverage; however, its frequency accuracy and resolution are relatively low. To tune the output frequency continuously and widely while maintaining its traceability to a frequency standard, we developed a photomixing THz synthesizer phase-locked to dual optical frequency combs (OFCs). While the phase-locking to dual OFCs ensured continuous tuning within a spectral range of 120 GHz, in addition to the traceability to the frequency standard, use of a broadband uni-traveling carrier photodiode for photomixing enabled the generation of CW-THz radiation within a frequency range from 0.2 to 1.5 THz. We demonstrated THz frequency-domain spectroscopy of gas-phase acetonitrile CH3CN and its isotope CH3 13CN in the frequency range of 0.600–0.720 THz using this THz synthesizer. Their rotational transitions were assigned with a frequency accuracy of 8.42 × 10−8 and a frequency resolution of 520 kHz. Furthermore, the concentration of the CH3CN gas at 20 Pa was determined to be (5.41 ± 0.05) × 1014 molecules/cm3 by curve fitting analysis of the measured absorbance spectrum, and the mixture ratio of the mixed CH3CN/CH3 13CN gas was determined to be 1:2.26 with a gas concentration of 1014–1015 molecules/cm3. The developed THz synthesizer is highly promising for high-precision THz-FDS of low-pressure molecular gases and will enable the qualitative and quantitative analyses of multiple gases.

Mode-resolved dual-comb spectroscopy (DCS) is an emerging spectroscopic tool with the potential to simultaneously achieve a broad spectral coverage and ultrahigh spectral resolution in terahertz (THz) spectroscopy. However, the need for two independently stabilized ultrafast lasers significantly hampers the potential application of DCS techniques. In this article, we demonstrate mode-resolved DCS in the THz region based on a free-running single-cavity dual-comb fiber laser with adaptive sampling. Low-pressure spectroscopy of acetonitrile gas with absorption features approaching the Doppler limit is demonstrated by comb-mode-resolved measurements with a spectral sampling spacing of 48.8 MHz, a spectral resolution of less than 5 MHz and a signal-to-noise ratio of ~50 dB. The successful demonstration of the proposed method clearly indicates the great potential for the realization of low-complexity, MHz-resolution THz spectroscopy instrumentation.

A terahertz sensor system based on a commercially available multiplier chain operating in the range 600–900 GHz is used to monitor the photolysis of formaldehyde H2CO. The exposure of formaldehyde to solar radiation provokes dissociation via two pathways: 1) the molecular channel producing molecular carbon monoxide CO and 2) the radicalar channel giving formyl HCO. Monitoring the concentrations of CO and H2CO via pure rotational transitions, allowed us to obtain the kinetic rate constants for these two channels. In order to establish the detection limit of HCO, a modified spectrometer configuration employing the Zeeman modulation was used with acetaldehyde CH3CHO and photosensitization by mercury.

The fine-structured spectrum of a terahertz (THz) frequency comb has been observed using asynchronous-optical-sampling THz time-domain spectroscopy with an extended time window covering multiple THz pulses. Fourier transformation of 10 consecutive THz pulses enables us to obtain the spectrum of THz comb mode having a linewidth of 25 MHz at intervals of 250 MHz. Further expansion of time window up to 100 consecutive THz pulses reduces the linewidth of THz comb mode down to 2.5 MHz while conserving interval of 250 MHz. The observed THz comb mode can be used as a precise and accurate frequency marker for broadband THz spectroscopy because it is phase-locked to a microwave frequency standard by laser control. The spectroscopy of pharmaceutical tablets and low-pressure molecular gas is used to demonstrate the utility of this approach, indicating a spectral resolution of 250 MHz. The proposed method enables frequency calibration of the THz spectrometer based on a microwave frequency standard.

A THz sensor system based on a commercially available multiplier chain is applied to the analysis of trace gases. Frequency modulation of the source is used to achieve the high sensitivity required. Particular attention is paid to the line broadening effect caused by the modulation as this can hamper the quantification of the various chemical species being monitored. A numerical model using a Voigt absorption profile along with a single calibration step is used to provide quantitative information. The sensitivity obtained by this prototype instrument permits the analysis of various samples extracted during the industrial sintering of iron ore as part of the manufacturing of steel.

Terahertz (THz) spectroscopy is a promising method for analysing polar gas molecules mixed with unwanted aerosols due to its ability to obtain spectral fingerprints of rotational transition and immunity to aerosol scattering. In this article, dynamic THz spectroscopy of acetonitrile (CH3CN) gas was performed in the presence of smoke under the atmospheric pressure using a fibre-based, asynchronous-optical-sampling THz time-domain spectrometer. To match THz spectral signatures of gas molecules at atmospheric pressure, the spectral resolution was optimized to 1 GHz with a measurement rate of 1 Hz. The spectral overlapping of closely packed absorption lines significantly boosted the detection limit to 200 ppm when considering all the spectral contributions of the numerous absorption lines from 0.2 THz to 1 THz. Temporal changes of the CH3CN gas concentration were monitored under the smoky condition at the atmospheric pressure during volatilization of CH3CN droplets and the following diffusion of the volatilized CH3CN gas without the influence of scattering or absorption by the smoke. This system will be a powerful tool for real-time monitoring of target gases in practical applications of gas analysis in the atmospheric pressure, such as combustion processes or fire accident.

Pure rotational transitions of OH and SH radicals have been recorded in the THz spectral range using cw-THz and synchrotron-based FT-FIR techniques. Line lists on these radicals have been completed in the three and two lowest vibrational states for OH and SH, respectively. Furthermore, the hyperfine structure of OH and SH has been observed for the first time using infrared IR FT-spectroscopy, and at frequencies higher than 1 THz, respectively. A combined fit has been made for each of these radicals including v = 0, 1 and 2 for OH and v = 0 and 1 for SH.

Approximately 60 pure rotational frequency transitions of formaldehyde in its ground state have been measured with sub-MHz uncertainty in the 0.7–1.8 THz frequency range using a photomixing THz synthesizer locked onto a frequency comb. The frequencies associated with previous submillimeter and infrared data have been included in a fit providing a new set of improved molecular parameters. The assignment of each line was checked using the usual statistical diagnostics. Finally, the ability of the continuous-wave spectrometer coupled to a multipass-cell to measure THz rotational transitions of H 2 CO in the 3 1 , 4 1 and 6 1 vibrational states was demonstrated.

A continuous-wave terahertz (CW-THz) source generated by photomixing has been employed to detect and quantify radicals produced in a cold plasma probing their spin-rotation transitions. Due to their dual interest for both atmospherists and astrophysicists, the hydroxyl OH and the mercapto SH radicals have been chosen. The photomixing technique which can access the largest range of THz frequencies of any known coherent source, allowed to resolve the Doppler-limited hyperfine transitions of OH in the 2.5 THz frequency region. Line profile analysis of the hyperfine components demonstrated that OH radicals have been detected in this region at a ppm level at a temperature close to 490 K. The hyperfine structure of SH has been resolved for the first time above 1 THz. Ten new frequency transitions have been measured in the 1.3–2.6 THz frequency range using the CW-THz synthesizer based on a frequency comb. With relative uncertainties better than 10 −7 , the CW-THz frequencies measured in this study are now competitive with those measured by other instruments such as frequency multiplication chains or FT-FIR spectrometers and are now capable to improve the predictions of the complete high-resolution spectra of these radicals collected in the atmospheric and astrophysical spectroscopic databases. versioncorrigeeAC 2011-07-18 17:32 © 2011 Arnaud Cuisset.

International audience; The generation of cw-THz radiation by photomixing is particularly suited to the high resolution spectroscopy of gases; nevertheless, until recently, it has suffered from a lack of frequency metrology. Frequency combs are a powerful tool that can transfer microwave frequency standards to optical frequencies and a single comb has permitted accurate (10−8) THz frequency synthesis with a limited tuning range. A THz synthesizer composed of three extended cavity laser diodes phase locked to a frequency comb has been constructed and its utility for high resolution gas phase spectroscopy demonstrated. The third laser diode allows a larger tuning range of up to 300 MHz to be achieved without the need for large frequency excursions, while the frequency comb provides a versatile link to be established from any traceable microwave frequency standard. The use of a single frequency comb as a reference for all of the cw-lasers eliminates the dependency of synthesized frequency on the carrier envelope offset frequency. This greatly simplifies the frequency comb stabilization requirements and leads to a reduced instrument complexity.

A standard DFB diode laser operating at 1.55 μm has been used with an EDFA to construct a hybrid cw-THz spectrometer. Frequency steps of as small as 4 MHz are demonstrated by current control whereas larger spectra in excess of 400 GHz have been realized using temperature control. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 991–994, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24207

International audience; The room-temperature nitrogen- and oxygen-broadening coefficients of hydrogen cyanide spectral lines have been measured in the 0.5–3 THz (17–100 cm1) frequency range (purely rotational transitions with 5pJp36) by a continuous-wave terahertz spectrometer based on a photomixing source. An improved version of the Robert and Bonamy semiclassical formalism has been used to calculate the oxygen-broadening coefficients and resulted in a good agreement with these measurements. The nitrogen and oxygen data are combined to provide the air-broadening coefficients as used by theHITRAN database. A significant difference is observed between the measured and tabulated values for transitions with high values of the rotational quantum number. A new polynomial representation is suggested for inclusion in HITRAN. A similar polynomial expression has been derived for the nitrogen broadening to aid the studies of Titan’s atmosphere.

A terahertz time domain spectroscopy experiment is used to study the coherent re-emission after exciting more than 60 energy rotational states of OCS molecules in gas phase. Due to the regular structure of the absorption spectrum of such linear molecules, a set of subsequent pulses separated by 82.6 ps is re-radiated from the vapour and recorded up to 450 ps. A model based on a linear response of the gas and by use of “Maxwell–Bloch” equations has permitted the re-emitted free induced decay to be investigated. Spectroscopic parameters, such as rotational constant, centrifugal distortion coefficient and relaxation times are responsible for the temporal shape and so can be evaluated. The influence of the optical thickness to access the relaxation times is discussed.

Recent advances in the development of monochromatic continuous-wave terahertz sources suitable for high resolution gas phase spectroscopy and pollution monitoring are reviewed. Details of a source using an ultra fast opto-electronic photomixing element are presented. The construction of a terahertz spectrometer using this source has allowed spectroscopic characterisation and application studies to be completed. Analysis of H2S and OCS under laboratory conditions are used to demonstrate the spectrometer performance, and the determination of the transition line strengths and pressure self broadening coefficients for pure rotational transitions of OCS. The spectral purity 5 MHz, tunability 0.3 to 3 THz, and long wavelength ≈ 200 μm of this source have been exploited to identify and quantify numerous chemical species in cigarette smoke. The key advantages of this frequency domain are its high species selectivity and the possibility to make reliable measurements of gas phase samples heavily contaminated by aerosols and particles. To cite this article: F. Hindle et al., C. R. Physique 9 (2008).

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. In addition, all frequency modes of a THz comb can be phase-locked to a microwave frequency standard, providing excellent traceability. However, the need for stabilization of dual femtosecond lasers has often hindered its wide use. To overcome this limitation, here we have demonstrated adaptive-sampling THz-DCS, allowing the use of free-running femtosecond lasers. To correct the fluctuation of the time and frequency scales caused by the laser timing jitter, an adaptive sampling clock is generated by dual THz-comb-referenced spectrum analysers and is used for a timing clock signal in a data acquisition board. The results not only indicated the successful implementation of THz-DCS with free-running lasers but also showed that this configuration outperforms standard THz-DCS with stabilized lasers due to the slight jitter remained in the stabilized lasers.

In order to obtain accurate terahertz center frequencies for SO and its isotopologues, we have studied the absorption spectrum of SO, 34SO, and 33SO up to 2.5 THz using continuous-wave terahertz photomixing based on a frequency comb providing an accuracy down to 10 kHz. Sulfur monoxide was produced in a radio frequency discharge of air in a cell containing pure sulfur. Together with the strong absorption signal of the main isotopologue, transitions of 34SO (34S: 4.21%) and 33SO (33S: 0.75%) were observed in natural abundance. The newly observed transitions constitute an extension of the observed rotational quantum numbers of the molecule toward higher N values, allowing an improvement of the molecular parameters for the three species. An isotopically invariant fit has been performed based on pure rotational and ro-vibrational transitions of all SO isotopologues, enabling their accurate line position prediction at higher frequencies. Thanks to this new set of parameters, it is now possible to predict with very high accuracy the frequencies of the ro-vibrational lines. This should enable the research of SO in the mid-IR using ground-based IR telescopes, space-based telescope archives (Infrared Space Observatory, Spitzer), and future space missions such as the James Webb Space Telescope. This set of parameters is particularly well adapted for the detection of SO lines in O-rich evolved stars or in molecular clouds in absorption against bright IR sources., The THz spectrometer was funded by the Communauté urbaine de Dunkerque, the Région Nord-Pas de Calais, the Ministére de l'éducation nationale, de l'enseignement supérieur et de la recherche, and European funds in the context of the IRENI (Institut de Recherche en ENvironnement Industriel) and InterregIVA >Cleantech> programs. J. Cernicharo thanks the Spanish MINECO program for funding support through grants CSD2009-00038 (CONSOLIDER program >ASTROMOL>), AYA2009-07304, and AYA2012-32032, and the European Research Council for funding under the European Union's Seventh Framework Program (FP/2007-2013) / ERC-2013-SyG, Grant Agreement No. 610256 NANOCOSMOS.

Terahertz time domain spectroscopy and photomixing have been used alongside one another for the detection and the quantification of small polar species in mainstream cigarette smoke. The broadband submillimeter source used in time domain spectroscopy allowed a rapid and simultaneous detection of several pure rotational transitions of hydrogen cyanide (HCN) and carbon monoxide (CO) in realistic conditions of pressure and temperature. The spectral purity of the continuous wave terahertz source produced by photomixing, permitted the concentrations of these molecules to be measured at pressures of tens of hPa. Moreover, at lower pressure, traces of formaldehyde (H2CO) have been unambiguously identified at frequencies above 1 THz. A comparison with chemical analytical methods has been completed for each molecule highlighting the advantages of the direct measurement by THz spectroscopy.

A spectrometer operating in the 100–2000 GHz range and allowing for absolute line strength measurements has been developed. The continuous wave terahertz radiation is generated by mixing two Ti:Sapphire laser beams in a vertically integrated low temperature grown GaAs (LTG-GaAs) photomixer. Pure rotational lines of 16 O 12 C 32 S in the ground vibrational state have been considered for J values up to 90. Observed self-broadening parameters are in agreement with those deduced from infrared experiments. For the first time in the submillimeter range, absolute line strengths have been determined, allowing for a determination of the electric dipole moment in good agreement with the value previously obtained from Stark effect measurements.

The generation of continuous coherent THz radiation by mixing two cw Ti:Sa laser beams with a well-controlled frequency separation for a new scheme of vertically integrated low temperature grown GaAs (LTG-GaAs) spiral photomixer is reported. For this new photomixer device used in THz emission, the LTG-GaAs active layer is sandwiched between the two parallel metal plates of a high-speed photodetector loaded by a broadband spiral antenna. We have exploited the advantage of a higher delivered power in the low part of the spectrum (

Optical frequency combs are innovative tools for broadband spectroscopy because a series of comb modes can serve as frequency markers that are traceable to a microwave frequency standard. However, a mode distribution that is too discrete limits the spectral sampling interval to the mode frequency spacing even though individual mode linewidth is sufficiently narrow. Here, using a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region, we achieved a spectral sampling interval equal to the mode linewidth rather than the mode spacing. The spectrally interleaved THz comb was realized by sweeping the laser repetition frequency and interleaving additional frequency marks. In low-pressure gas spectroscopy, we achieved an improved spectral sampling density of 2.5 MHz and enhanced spectral accuracy of 8.39 × 10(-7) in the THz region. The proposed method is a powerful tool for simultaneously achieving high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.

A THz spectrometer based onto the photomixing technique associated with a femtosecond frequency comb is a perfect tool for the study of radicals, which usually required a very large tunability. The spectra of OH, SH and SO have been revisited and updated data have been obtained.

International audience; It is shown that a continuous wave output power of 0.35 mW at 305 GHz can be generated by photomixing in a low temperature grown GaAs photoconductor using a metallic mirror Fabry-Pérot cavity. The output power is improved by a factor of about 100 as compared to the previous works on GaAs photomixers.

A THz spectrometer based onto commercially available blocks has been implemented to identify and quantify gas mixtures at trace levels. Specifications of the prototype in a context of gas trace analysis have been addressed by spectroscopic investigations onto SO2.

Rovibrational absorption spectra of methyl chloride in the spectral region between 2800 and 3200 cm −1 were recorded with a high-resolution Fourier transform spectrometer. A multispectrum fitting procedure was used to analyze 527 transitions of the ν 1 band and to retrieve the self-broadening coefficients for various J - and K- values with an estimated accuracy around 8%. Pure rotational transitions of CH 3 Cl in the submillimeter/terahertz region (0.2–1.4 THz) were also investigated using two complementary techniques of frequency-multiplication and continuous-wave photomixing. Forty-three pure rotational self-broadening coefficients were extracted with the accuracy between 3 and 5%. The whole set of measured values was used to model the J - and K -rotational dependences of the self-broadening coefficients by second-order polynomials. In addition, semi-classical calculations were performed, based on the real symmetric-top geometry of the active molecule, an intermolecular potential model including not only the dominant electrostatic but also the short-range forces, as well as on an exact classical treatment of the relative translational motion of the colliding partners. Comparison of all experimental and theoretical results shows similar rotational dependences and no significant vibrational dependence, so that extrapolations to other spectral regions should be straightforward.

Room-temperature N 2 -broadening coefficients of methyl chloride rotational lines are measured over a large interval of quantum numbers (6≤ J ≤50, 0≤ K ≤18) by a submillimeter frequency-multiplication chain ( J ≤31) and a terahertz photomixing continuous-wave spectrometer ( J ≥31). In order to check the accuracy of both techniques, the measurements of identical lines are compared for J =31. The pressure broadening coefficients are deduced from line fits using mainly a Voigt profile model. The excellent signal-to-noise ratio of the frequency-multiplication scheme highlights some speed dependence effect on the line shape. Theoretical values of these coefficients are calculated by a semi-classical approach with exact trajectories. An intermolecular potential including atom–atom interactions is used for the first time. It is shown that, contrary to the previous theoretical predictions, the contributions of short-range forces are important for all values of the rotational quantum numbers. Additional testing of modifications required in the semi-classical formalism for a correct application of the cumulant expansion is also performed. It is stated that the use of the cumulant average on the rotational states of the perturbing molecule leads, for high J and small K values, to slightly higher line-broadening coefficients, as expected for the relatively strong interacting CH 3 Cl–N 2 system. The excellent agreement between the theoretical and the experimental results ensures the reliability of these data.

International audience; It is shown from on-wafer measurement that a continuous wave output power of 0.35 mW at 305 GHz can be generated by photomixing in a metal-metal Fabry-Pérot GaAs photoconductor.

Generation of cw-THz by photomixing is an attractive solution for high resolution spectroscopy, nevertheless it has suffered from a lack of frequency metrology to determine the line centre frequencies. The phase locking of two extended cavity laser diodes (ECLD) to a frequency comb (FC) has allowed THz frequencies to be synthesized and an accuracy of Δν THz /ν THz = 10−8 obtained, however, the continuous tuning range is limited to 10 MHz. A new configuration based on a single FC and three cw-lasers has been implemented and provides a tuning range in excess of 300 MHz. The use of a single FC eliminates the need to measure and stabilize the carrier envelope offset frequency of the FC. The system also has the advantage of establishing a direct link between the THz frequency and any microwave frequency standard without the need of any calibration steps.

Photomixing is an attractive solution for high resolution spectroscopy, nevertheless it has suffered from a lack of frequency metrology to determine the line centre frequencies. Frequency combs can provide excellent metrology of lasers, hence we propose a photomixing source that is phase locked to a frequency comb to overcome this problem. The resulting instrument has an accuracy of Δν THz /ν THz = 10−8, a spectral purity in the order of 50 kHz and can be continuously tuned over 200 MHz.

Specific investigations onto the hydroxyl radical (OH) and the mercapto radical (SH) are proposed by using a high resolution photomixing spectrometer. The presented results point out clearly the availability of the proposed technique to investigate radical compounds.

International audience; It is shown from accurate on-wafer measurement that continuous wave output powers of 1.2 mW at 50 GHz and 0.35 mW at 305 GHz can be generated by photomixing in a low temperature grown GaAs photoconductor using a metallic mirror Fabry-Pérot cavity. The output power is improved by a factor of about 100 as compared to the previous works on GaAs photomixers. A satisfactory agreement between the theory and the experiment is obtained in considering both the contribution of the holes and the electrons to the total photocurrent.

A transverse Electromagnetic Horn Antenna is monolithically integrated with a low-temperature-grown GaAs vertical photoconductor on a silicon substrate forming a vertically integrated photomixer. Continuous-wave terahertz radiation is generated at frequencies up to 3.5 THz with a power level reaching 20 nW around 3 THz.

A THz synthesizer has been realized by phase locking the photomixer pump lasers to a frequency comb effectively locking the THz emission frequency of the source. This allowed the centre frequency of a molecular transition at 813 GHz to be measured with an accuracy of 20 kHz, (δυ/υ ≈10−8).

We have developed a new generation of optoelectronic larg e bandwidth terahertz sources based on TEM horn antennas monolithically integrated with several types of photodetectors: low-temperature grown GaAs (LTG-GaAs) planar photoconductors, vertically integrated LTG-GaAs photoconductors on silicon substrate and uni-travelling-carrier photodiodes. Results of pulsed (time-domain) and photomixing (CW, frequency domain) experiments are presented. Keywords: Terahertz source, photomixing, time-domain spectroscopy, low-temperature grown GaAs, uni-travelling-carrier photodiode, TEM horn antenna 1. INTRODUCTION Powerful room-temperature solid-state terahertz sources are key elements for promising applications such as spectroscopy, imagery, and telecommunications. Recently increasing performances have been published concerning Schottky diodes harmonic generation 1 , quantum cascade lasers 2 and also transistor-based circuits 3 . Terahertz optoelectronic generation is an alternative technique with specific advantages. The most obvious is the bandwidth or the tunability available thanks to this approach. The main drawbacks are the limited amount of power generated, the efficiency and also the complexity of the system due to the optical aspect. Since several years we have developed a new generation of large bandwidth terahertz sources based on optoelectronics. The devices were designed to improve the generated power, the efficiency and to decrease the complexity while keeping a large bandwidth. They are based on two kinds of ultrafast photodetectors: short lifetime photoconductors and photodiodes. The preferred shot lifetime semiconductor is LTG-GaAs. Lifetimes in the 0.1-1 ps range are obtained with a good mobility, making this material very efficient as a phot oswitch. In part 2, a new type of THz antenna is exposed and integrated with standard LTG-GaAs photoswitches. In part 3 we present our results concerning the integration of uni-travelling-carrier photodiodes (UTC-PD) with this antenna. The main advantages of this photodiode are that it works at 1.55 µm and it has a high efficiency. In part 4 we introduce the vertically integrated LTG-GaAs photoconductor which is a new approach to realize photoswitches on silicon substrate. *jean-francois.lampin@isen.iemn.un iv-lille1.fr; www.iemn.univ-lille1.fr