A new instrumentation for simultaneous terahertz and mid-infrared spectroscopy in corrosive gaseous mixtures

The correct interpretation of infrared (IR) observations of planetary atmospheres requires an accurate knowledge of temperature and partial and global pressures. Precise laboratory measurements of absorption intensities and line profiles, in the 200-350 K temperature range, are, therefore, critical. However, for gases only existing in complex chemical equilibria, such as nitrous or hypobromous acids, it is not possible to rely on absolute pressure measurements to measure absolute integrated optical absorption cross sections or IR line intensities. To overcome this difficulty, a novel dual-beam terahertz (THz)/mid-IR experimental setup has been developed, relying on the simultaneous use of two instruments. The setup involves a newly constructed temperature-controlled (200-350 K) cross-shaped absorption cell made of inert materials. The cell is traversed by the mid-IR beam from a high-resolution Fourier transform spectrometer using along a White-cell optical configuration providing absorption path lengths from 2.8 to 42 m and by a THz radiation beam (82.5 GHz to 1.1 THz), probing simultaneously the same gaseous sample. The THz channel records pure rotational lines of molecules for which the dipole moment was previously measured with high precision using Stark spectroscopy. This allows for a determination of the partial pressure in the gaseous mixture and enables absolute line intensities to be retrieved for the mid-IR range. This new instrument opens a new possibility for the retrieval of spectroscopic parameters for unstable molecules of atmospheric interest. The design and performance of the equipment are presented and illustrated by an example of simultaneous THz and mid-IR measurement on nitrous acid (HONO) equilibrium.

Unlocking synchrotron sources for THz spectroscopy at sub-MHz resolution

Synchrotron radiation (SR) has proven to be an invaluable contributor to the field of molecular spectroscopy, particularly in the terahertz region (1-10 THz) where its bright and broadband properties are currently unmatched by laboratory sources. However, measurements using SR are currently limited to a resolution of around 30 MHz, due to the limits of Fourier-transform infrared spectroscopy. To push the resolution limit further, we have developed a spectrometer based on heterodyne mixing of SR with a newly available THz molecular laser, which can operate at frequencies ranging from 1 to 5.5 THz. This spectrometer can record at a resolution of 80 kHz, with 5 GHz of bandwidth around each molecular laser frequency, making it the first SR-based instrument capable of sub-MHz, Doppler-limited spectroscopy across this wide range. This allows closely spaced spectral features, such as the effects of internal dynamics and fine angular momentum couplings, to be observed. Furthermore, mixing of the molecular laser with a THz comb is demonstrated, which will enable extremely precise determinations of molecular transition frequencies.

Characterization of the Observed Electric Field and Molecular Relaxation Times for Millimeter-Wave Chirped Pulse Instrumentation

In a chirped pulse experiment, the strength of the signal level is proportional to the amplitude of the electric field, which is weaker in the millimeter-wave or submillimeter-wave region than in the microwave region. Experiments in the millimeter region thus require an optimization of the coupling between the source and the molecular system and a method to estimate the amplitude of the electric field as seen by the molecular system. We have developed an analytical model capable of reproducing the coherent transient signals obtained with a millimeter-wave chirped pulse setup operated in a monochromatic pulse mode. The fit of the model against the experimental data allowed access to the amplitude of the electric field and, as a byproduct, to the molecular relaxation times T ₁ and T ₂.

Broadband terahertz heterodyne spectrometer exploiting synchrotron radiation at megahertz resolution

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-based molecular THz laser that acts as the local oscillator. Proof of principle for exploitation for spectroscopy is provided by the recording of molecular absorptions of hydrogen sulfide (H₂S) and methanol (CH₃OH) around 1.073 THz. Ultimately, the spectrometer will enable to cover the 1-4 THz region in 5 GHz windows at Doppler resolution.

Conformational landscape and inertial defect of methoxyphenol isomers studied by mm-wave spectroscopy and quantum chemistry calculations

Because methoxyphenols (MP) are emitted in significant quantities during biomass fires and contribute to the secondary organic aerosols formation which impacts the climate, their gas phase monitoring in the atmosphere is crucial and requires accurate rovibrational cross sections determined with a good knowledge of their ground state (GS) and vibrationally excited state (ES) molecular parameters. Therefore, the rotational spectra of the two isomers, 2-MP (guaïacol) and 4-MP (mequinol), have been measured in absorption and in emission at room temperature using a frequency multiplication chain and a mm-wave Fourier transform chirped-pulse spectrometer, respectively. Guided by quantum chemistry calculations, the conformational landscape has been characterised and the observation of only one rotamer in the spectra of 2-MP and 4-MP has been explained. For 2-MP, the most stable conformation is justified by an intramolecular O-H⋯OCH₃ hydrogen-bond which has been characterised by a topology analysis of the electron density. In a global fit including more than 30 000 line assignments, rotational and quartic centrifugal constants of the GS and the three lowest energy ES have been determined allowing to reproduce the millimeter-wave spectra at the experimental accuracy. The same work has been performed on the cis-rotamer of 4-MP highlighting some perturbations marring the fit quality for two vibrationally ES. Finally, the isomeric dependence of the negative inertial defect ΔI agrees with that of the lowest energy out of plane mode ν₄₅, and the variation of ΔI with the degree of vibrational excitation allows a fine estimation of v₄₅ = 1 vibrational wavenumber.

Towards the Detection of Explosive Taggants: Microwave and Millimetre-Wave Gas-Phase Spectroscopies of 3-Nitrotoluene

The monitoring of gas-phase mononitrotoluenes is crucial for defence, civil security and environmental interests because they are used as taggant for TNT detection and in the manufacturing of industrial compounds such as dyestuffs. In this study, we have succeeded to measure and analyse at high-resolution a room temperature rotationally resolved millimetre-wave spectrum of meta-nitrotoluene (3-NT). Experimental and theoretical difficulties have been overcome, in particular, those related to the low vapour pressure of 3-NT and to the presence of a CH₃ internal rotation in an almost free rotation regime (V₃ =6.7659(24) cm^-1 ). Rotational spectra have been recorded in the microwave and millimetre-wave ranges using a supersonic jet Fourier Transform microwave spectrometer (T_rot <10 K) and a millimetre-wave frequency multiplication chain (T=293 K), respectively. Spectral analysis of pure rotation lines in the vibrational ground state and in the first torsional excited state supported by quantum chemistry calculations permits the rotational energy of the molecule, the hyperfine structure due to the ¹⁴ N nucleus, and the internal rotation of the methyl group to be characterised. A line list is provided for future in situ detection.

High density terahertz frequency comb produced by coherent synchrotron radiation

Frequency combs have enabled significant progress in frequency metrology and high-resolution spectroscopy extending the achievable resolution while increasing the signal-to-noise ratio. In its coherent mode, synchrotron radiation is accepted to provide an intense terahertz continuum covering a wide spectral range from about 0.1 to 1 THz. Using a dedicated heterodyne receiver, we reveal the purely discrete nature of this emission. A phase relationship between the light pulses leads to a powerful frequency comb spanning over one decade in frequency. The comb has a mode spacing of 846 kHz, a linewidth of about 200 Hz, a fractional precision of about 2 × 10(-10) and no frequency offset. The unprecedented potential of the comb for high-resolution spectroscopy is demonstrated by the accurate determination of pure rotation transitions of acetonitrile.

Photomixing at 1.55 microm in ion-irradiated In(0.53)Ga(0.47)As on InP

We report the first demonstration of a terahertz photomixer made of ion-irradiated In(0.53)Ga(0.47)As lattice-matched to InP and fiber-optic coupled with the drive lasers. A continuous-wave radiation is generated at frequencies up to 0.8 THz by photomixing two continuous-wave laser diodes around 1.55 microm. The measured 3dB-down bandwidth of 300 GHz yields a carrier lifetime of 0.53 ps, in agreement with the value of 0.41 ps measured in pump probe experiments. The detected signal is at the most 15 dB lower than the one obtained from similar photomixers fabricated from low-temperature-grown GaAs.

Mid-infrared trace gas detection using continuous-wave difference frequency generation in periodically poled RbTiOAsO4

A tunable mid-infrared continuous-wave (cw) spectroscopic source in the 3.4-4.5 micrometers region is reported, based on difference frequency generation (DFG) in a quasi-phase-matched periodically poled RbTiOAsO4 (PPRTA) crystal. DFG power levels of 10 microW were generated at approximately 4 micrometers in a 20-mm long PPRTA crystal by mixing two cw single-frequency Ti:Al2O3 lasers operating near 713 nm and 871 nm, respectively, using a laser pump power of 300 mW. A quasi-phase-matched infrared wavelength-tuning bandwidth (FWHM) of ~ 12 cm-1 and a temperature tuning rate of 1.02 cm-1/degree C were achieved. Experimental details regarding the feasibility of trace gas detection based on absorption spectroscopy of CO2 in ambient air using this DFG radiation source are also described.

Enhancement of sum frequency generation near the photonic band gap edge under the quasiphase matching conditions

We analyze theoretically and study experimentally the mechanisms of enhancement of the sum frequency and second harmonic generation in a finite one-dimensional photonic band gap structure with second order nonlinearity under Bragg diffraction conditions. It is shown that, near the photonic band gap edge, the efficiency of conversion in sum frequency and second harmonic generation processes can be significantly enhanced if two conditions are fulfilled simultaneously: grating assisted phase matching, or quasiphase matching, and an increase of the electromagnetic field density at the fundamental frequencies near the photonic band edges. The role of each mechanism is discussed.

Compression of femtosecond laser pulses in thin one-dimensional photonic crystals

We demonstrate experimentally the effect of compression of femtosecond laser pulses in thin (a few micrometers) one-dimensional photonic crystal. We show that the compression effect is reasonably described by the linear dispersion properties of the photonic crystal itself and the quadratic dispersion approximation cannot be efficiently used for the description of interaction of the femtosecond laser pulses with the thin photonic crystal. For given parameters of the femtosecond pulse it leads to the existence of the optimal dimension of the photonic crystal from the point of view of the compression efficiency. Due to the wide spectral width of the femtosecond laser pulses the high-order dispersion effects play an important role in pulse propagation in photonic crystals and as a result the pulse compression occurs for both positive and negative signs of chirp of the incoming femtosecond pulses.

Gas filter correlation instrument for air monitoring at submillimeter wavelengths

A gas filter correlation (GFC) instrument for air monitoring at submillimeter wavelengths has been developed. We used a high-power terahertz radiation source in combination with a specific gas filter cell to obtain a highly selective instrument to differentiate species present in an unknown mixture. This approach provides a new method for survey measurements in the spectrum from 100 to 1000 GHz, in which many molecular rotational lines appear. Basic operational considerations and preliminary experiments with hydrogen sulfide are described.