Recent progress of THz generation and detection in ambient air or gases

THz time domain spectroscopy has been largely applied on the measurement of semiconductor, electro-optic crystals, and selected chemical, biological and explosive materials. The objective of this paper is to report THz gas photonics and its applications, with an emphasis on remote sensing capabilities. The most recent results of using air (and selected gases) as the emitter and sensor material for both generation and detection of broadband THz waves will be reported. Air, especially ionized air (plasma), has been used to generate intense peak THz waves (THz field > 1.5 MV/cm) with a broadband spectrum (10% bandwidth from 0.1 THz to 46 THz). THz-enhanced-fluorescence (TEF) and THz-enhanced acoustic (TEA) techniques have been developed for remote sensing purpose. By "seeing" the fluorescence, or "hearing" the sound, coherent detection of THz waves at standoff distance is feasible.

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