Paper published as part of the special topic on Advances in Terahertz Solid-State Physics and Devices ARTICLES YOU MAY BE INTERESTED IN On-chip mid-infrared and THz frequency combs for spectroscopy

We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.

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We report on a terahertz absorption spectrometer, which combines a grating monochromator, a quantum-cascade laser (QCL), and a microbolometer camera. The emission modes of the laser are spectrally resolved by the monochromator and imaged onto the camera. An absorption cell is placed between the QCL and the monochromator, and the absorption spectrum of methanol around 3.4 THz is measured by integrating simultaneously the signal of each of its Fabry-Pérot modes as a function of the laser driving current. The frequency coverage of the spectrometer is about 20 GHz.

Stimulated terahertz emission in the range from 4.5 to 6.4 THz has been realized from a single silicon crystal doped by two hydrogen-like donor centers, phosphorus and antimony, when pumped by midinfrared radiation from a free electron laser. Intracenter as well as Raman lasing has been observed. Simultaneous laser emission from both donors occurs when the pump photon energy is sufficient for photoionization of the antimony donors. The laser processes of both donors are not influenced by each other. Therefore the codoping approach can be extended to other group-V donors including more than two dopants in a single crystal.