Abstract
Spectroscopic measurements in the millimeter, submillimeter, or THz range, with resolutions exceeding a MHz, provide for highly specific detections of gas-phase absorption and emission by atoms and molecules. Due to relatively low excitation energies involved in the transitions, multiple features are observable in most physical systems, and thus such observations dominate the scientific discovery of molecules in space and contribute significantly to remote sensing of the Earth and planetary bodies. The methods and techniques of THz spectroscopy continue to evolve as capabilities and technologies expand. In this article, we review the genesis of THz spectroscopy in both the laboratory and in space, and follow its development to date, providing background on the challenges, and context for the current developments that promise to extend both remote and in-situ gas composition sensing.
Highlights
The THz region, between 0.5 and 10 THz (600-30 μm or 16.7-333.5 cm−1), is one of the few remaining regions of the electromagnetic spectrum where the generation, control, and to a lesser extent, detection, of coherent radiation remains far from routine
The development of Doppler limited or velocity resolved THz spectroscopy has been completely tied to the available sources of radiation and how well these sources are controlled
Almost all the techniques have found their way into some sort of laboratory system, but no technique has enabled the entire region to be completely exploited in either the laboratory or in the space environment
Summary
The THz region, between 0.5 and 10 THz (600-30 μm or 16.7-333.5 cm−1), is one of the few remaining regions of the electromagnetic spectrum where the generation, control, and to a lesser extent, detection, of coherent radiation remains far from routine. A number of methods to generate tunable radiation were developed including laser sideband [23], tunable klystrons [24], tunable backward-wave oscillators (BWOs) [25], tunable far-infrared (TuFIR)[26], and optical difference frequency generation (photomixing)[27] These techniques are discussed in more detail in the section on THz laboratory spectroscopy. The first demonstration of laser sideband spectroscopy [23] proved that Schottky barrier diodes could work as mixers in the THz. In addition to the drift of a free running molecular laser, laser sideband suffers from the challenges of relatively low power in the sideband relative to the carrier and the presence of a second sideband that moves oppositely in frequency relative to the first. Heterodyne systems exploiting photomixing devices are available commercially as THz Time Domain Spectrometers (THz-TDS, e.g., [68]), these systems routinely provide spectral coverage from 0.1 to 1.0 THz, and when used in dry environments, up to 3 THz, practical limitations for timedelay lines and time resolution have limited high resolution efforts
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