PHOTONICS-BASED TERAHERTZ SOURCES FOR MOLECULAR SPECTROSCOPY

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.3]{Spectrum.eps} \end{wrapfigure} Since twenty years photonics-based terahertz (THz) sources have made great progress. They are now usable for low- and high-resolution molecular spectroscopy in the 0.1-4 THz range (3-130 \wn). We will present the principles of near-infrared laser-based THz set-ups: time-domain spectroscopy (see figure showing ambient pressure \chem{H_2O} lines) and frequency-domain photomixing. Then we will review the different types of high bandwidth semiconductors photodetectors used to convert laser beams into THz beams. Two families are mainly used: photoconductors and photodiodes. Advantages and drawbacks of each will be presented. The subject of THz antennas will be also discussed and a state of the art will be given including the devices developed at IEMN. Some example of molecular spectroscopy measurements using these devices at various frequencies and resolutions will be given. Finally new ways for photonics-based THz sources will be exposed

HIGH-RESOLUTION TERAHERTZ GAIN SPECTRA OF MID-INFRARED PUMPED NH3

Inversion of population in the terahertz (THz) range can be obtained thanks to the optical pumping of polar molecules in the mid-infrared range. Generally it is done with \chem{CO_2} lasers but recently we have demonstrated the first molecular laser pumped by a quantum cascade laser (QCL). It is based on the optical pumping of the \chem{NH_3} molecule in the \nub{2}=1 state. The gain is obtained by the stimulated emission on pure inversion transitions of \chem{NH_3} (large amplitude motions) around 1 THz that are not accessible to continuous-wave (CW) \chem{CO_2} lasers. We present here CW high-resolution gain measurements of two strong lines: the (3,3) around 1.073 THz and the (4,4) around 1.083 THz. The measurements are done with a THz multiplication chain and an InSb bolometer. The gain profiles are recorded at different pressure and different QCL frequencies as for an IR/THz double resonance experiment. The highest gain at the best conditions are obtained with the (3,3) line: 10 dB/m for a pump power of about 40 mW. To our knowledge this gain is highest measured in the THz range for a CW-pumped molecule. These measurements will help the understanding and the design of our \chem{NH_3} lasers. This kind of laser will find applications in THz molecular spectroscopy/astronomy as a source or as a local oscillator for heterodyne detection

Plasmonic behavior of III-V semiconductors in far-infrared and terahertz range

Background: In this article, III-V semiconductors are proposed as materials for far-infrared and terahertz plasmonic applications. We suggest criteria to estimate appropriate spectral range for each material including tuning by fine doping and magnetic field. Methods: Several single-crystal wafer samples (n, p-doped GaAs, n-doped InP, and n, p-doped and undoped InSb) are characterized using reflectivity measurement and their optical properties are described using the Drude-Lorentz model, including magneto-optical anisotropy. Results: The optical parameters of III-V semiconductors are presented. Moreover, strong magnetic modulation of permittivity was demonstrated on the undoped InSb crystal wafer in the terahertz spectral range. Description of this effect is presented and the obtained parameters are compared with a Hall effect measurement. Conclusion: Analyzing the phonon/free carrier contribution to the permittivity of the samples shows their possible use as plasmonic materials; the surface plasmon properties of semiconductors in the THz range resemble those of noble metals in the visible and near infrared range and their properties are tunable by either doping or magnetic field.Web of Science13art. no. 1

High-resolution THz gain measurements in optically pumped ammonia

This study is aimed at the evaluation of THz gain properties in an optically pumped NH3 gas. NH3 molecules undergo rotational-vibrational excitation by mid-infrared (MIR) optical pumping provided by a MIR quantum cascade laser (QCL) which enables precise tuning to the NH3 infrared transition around 10.3 mu m. Pure inversion transitions, (J = 3, K = 3) at 1.073 THz and (J = 4, K = 4) at 1.083 THz were selected. The THz measurements were performed using a THz frequency multiplier chain. The results show line profiles with and without optical pumping at different NH3 pressures, and with different MIR tuning. The highest gain at room temperature under the best conditions obtained during single pass on the (3,3) line was 10.1 dBxm(-1) at 26 mu bar with a pumping power of 40 mW. The (4,4) line showed lower gain of 6.4 dBxm(-1) at 34 mu bar with a pumping power of 62 mW. To our knowledge these THz gains are the highest measured in a continuous-wave MIR pumped gas.Web of Science2616212482124

Analysis of a narrowband terahertz signal generated by a unitravelling carrier photodiode coupled with a dual-mode semiconductor Fabry–Pérot laser

A narrowband terahertz signal generated by a unitravelling carrier photodiode (UTC-PD) interfaced with a dual-mode Fabry–Pérot laser diode is demonstrated. A beat tone corresponding to the free spectral range is generated on the UTC-PD, and radiated by a transverse-electromagnetic-horn antenna. A terahertz signal at a frequency of 372 GHz, featuring a linewidth of 17 MHz is recorded by a subharmonic mixer coupled to an electrical spectrum analyzer. All components involved in this experiment operate at room temperature. The linewidth and the frequency of the emitted terahertz wave are analyzed, along with their dependency on dc-bias conditions applied to laser diode

Magneto-optical properties of InSb for terahertz applications

Magneto-optical permittivity tensor spectra of undoped InSb, n-doped and p-doped InSb crystals were determined using the terahertz time-domain spectroscopy (THz-TDS) and the Fourier transform far-infrared spectroscopy (far-FTIR). A Huge polar magneto-optical (MO) Kerr-effect (up to 20 degrees in rotation) and a simultaneous plasmonic behavior observed at low magnetic field (0.4 T) and room temperature are promising for terahertz nonreciprocal applications. We demonstrate the possibility of adjusting the the spectral rage with huge MO by increase in n-doping of InSb. Spectral response is modeled using generalized magneto-optical Drude-Lorentz theory, giving us precise values of free carrier mobility, density and effective mass consistent with electric Hall effect measurement.Web of Science611art. no. 11502

Recent Developments of an Opto-Electronic THz Spectrometer for High-Resolution Spectroscopy

A review is provided of sources and detectors that can be employed in the THz range before the description of an opto-electronic source of monochromatic THz radiation. The realized spectrometer has been applied to gas phase spectroscopy. Air-broadening coefficients of HCN are determined and the insensitivity of this technique to aerosols is demonstrated by the analysis of cigarette smoke. A multiple pass sample cell has been used to obtain a sensitivity improvement allowing transitions of the volatile organic compounds to be observed. A solution to the frequency metrology is presented and promises to yield accurate molecular line center measurements