Terahertz gas phase spectroscopy using a high-finesse Fabry–Pérot cavity

Abstract

THz and submillimeter gas phase spectroscopy appears as the perfect tool for analytical chemical analysis due an exceptional degree of resolution. Despite many attractive applications such as breath analysis, environmental surveillance, food spoilage monitoring, or detection of explosive taggants, there is no commercially available versatile THz or submillimeter chemical analyzer. An important factor hindering the development of a THz chemical analyzer are the difficulties encountered to adapt ultrasensitive techniques, such as Cavity-Enhanced Techniques and Cavity Ring Down Spectroscopy to the THz and submillimeter domain. Here, we describe a low-cost high finesse THz resonator based on a low loss oversized corrugated waveguide along with high reflective photonic mirrors, and how those critical components enable a Fabry-Perot THz Absorption Spectrometer obtaining a kilometer equivalent interaction length. In addition, the intracavity optical power have allowed nonlinear interactions such as Lamb-Dip effect to be studied with a low-power emitter.

Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths

“…The second harmonic of the frequency modulation is simultaneously exploited to detect the presence of molecular absorption present in the cavity. The AMC frequency and cavity are thus swept together allowing sensitive absorption measurements to be made [17].…”

Instrumentation for THz Spectroscopy in the Laboratory and in Space

“…With THz time-domain spectrometers (TDS), chirpedpulse methods, and the ability to pulse modulate base-band [90], [91] and CMOS [91] sources, more and more THz spectroscopic techniques are operating with fast, coherent, detections in the time domain, even including dual frequency comb THz generation [92]. With this trend, applications for detections of transient species [93]- [95] are expected to proliferate.…”

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations

“…Figure 8b displays the least energy path for a 120°rotation of À CH . In this case, the situation is more simple since, starting from equilibrium (45,35) towards the equivalent position (165°, 35°), the path of least energy does not pass through a mirror image. The À CH 3 rotation over 120°is accompanied by a smaller back-and-forth movement of the nitro group, with an amplitude of 15°, inducing a rotation barrier of 553 cm À 1 , slightly lower compared to the adiabatic path: 645 cm À 1 .…”

“…However, taking into account the experimental difficulties to measure at room temperature the rotational signatures of these weakly volatile taggants, the ability to perform field measurements of NT vapor traces requires important improvements of the mm-wave spectrometers in terms of sensitivity. Nevertheless, recent technological breakdowns based on intra-cavity techniques [35,36] push the detection limits while preserving the incomparable selectivity of the high-resolution mm-wave rotational spectroscopy.…”