Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide $\left(\mathrm{HCN}\right)$, carbon monoxide $\left(\mathrm{CO}\right)$, formaldehyde $\left({\mathrm{H}}_2\mathrm{CO}\right)$, and water $\left({\mathrm{H}}_2\mathrm{O}\right)$ have been observed from 500 to $2400\mathrm{GHz}$. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of $\mathrm{HCN}$ and $\mathrm{CO}$. The quantification of the measurement was validated by the means of a calibration gas containing $\mathrm{CO}$. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of $\mathrm{HCN}$ equal to 9 parts in ${10}^6$.

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[emsp14]MHz and enhanced spectral accuracy of 8.39 [times] 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.

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.

Coherent spectroscopic techniques in the terahertz (THz) or far-infrared region (frequencies of 0.1 to 10 THz and wavelengths of $30\mu \mathrm{m}$ to 3 mm) are the enabling technology for a wide variety of important applications.^{1 – 3} Among these techniques, photonic-based THz time-domain spectroscopy (THz-TDS)^4,5 has been applied to the study of a diverse range of samples, such as the rotational transitions of polar gas molecules,⁶ the hydrogen bonding signature in aqueous systems,⁷ and self-assembly of proteins,⁸ because it takes advantage of the broad spectral bandwidth of THz emitters/receivers pumped by ultrafast lasers. This well-established THz-TDS scheme uses pump pulses and delayed probe pulses from a femtosecond mode-locked laser for THz radiation and THz detection, respectively. However, it has limited spectral resolution and accuracy due to constraints on the travel range, repeatability, and speed of the mechanical delay lines. Thus, the use of the photonic THz techniques for Doppler-limited spectral measurements and quantitative analysis of molecular rotational transitions, which are attractive for many applications including gas sensing, has not yet been well explored.

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. This work has been supported by the CNRS, the Lille1 University and the “Région Nord Pas de Calais."

Design requirements for an 8000 frame/s dual-wavelength ratiometric chemical species tomography system, intended for hydrocarbon vapor imaging in one cylinder of a standard automobile engine, are examined. The design process is guided by spectroscopic measurements on iso-octane and by comprehensive results from laboratory phantoms and research engines, including results on temporal resolution performance. Novel image reconstruction techniques, necessary for this application, are presented. Recent progress toward implementation, including details of the optical access arrangement employed and signal-to-noise issues, is described. We present first cross-cylinder IR absorption measurements from a reduced channel-count (nontomographic) system and discuss the prospects for imaging.