Study of broadband THz time-domain spectroscopy at different relative humidity levels

Abstract

Two detection techniques of broadband terahertz (THz) time-domain spectroscopy-THz air-biased coherent detection (THz-ABCD; from 0.3 to 14 THz) and electro-optical (EO) detection (from 0.3 to 7 THz) -are both performed at several different relative humidity levels. The THz power exponentially decays with the increase in relative humidity. The dynamic range of the main pulse in the time domain linearly decreases as the relative humidity increases from 0% to 40%, and linear fittings show that the slopes are -0.017 and -0.019 for THz-ABCD and EO detection, respectively. Because of the multiple reflections caused by the crystal in the common EO detection, THz-ABCD has better spectral resolution (17 GHz) than that of EO detection (170 GHz). The spectrum of water vapor absorption measured by THz-ABCD is also compared with that measured by the Fourier transform infrared spectroscopy (FTIR).OCIS codes: 300.6495, 320.7150, 300.6500. doi: 10.3788/COL201210.043001.Terahertz (THz) waves have brought benefits to a variety of applications, including biomedical investigation, non-destructive inspection, and material characterization [1−3] . This is because most rotational and vibrational resonances of molecules and chemical compounds are located in such frequency range. However, water vapor attenuates THz waves when it propagates in the atmosphere. Grischkowsky et al. have reported a number of research about water vapor absorption in the THz range, especially from 0.2 to 2 THz [4,5] . On the other hand, the astronomical observations at THz frequencies are significantly impacted by daily weather, varying seasons, as well as different altitudes and geographical locations. To reduce the attenuation by water vapor over an absorbing propagation path, many groups have driven submillimeter-wave telescopes to the highest, driest, and coldest sites for ground-based astronomical telescopes operating at THz frequencies [6,7] . THz time-domain spectroscopy (THz-TDS) using selected gases as an emitter and sensor now provides an intense THz field, a broadband spectrum, and sensitive detection capabilities for studying material properties [8,9] . In terms of remote sensing, a promising technique is the use of gases as the emitter and the sensor [10,11] . To investigate the performance of broadband THz-TDS in various environments, this study uses nitrogen gas as THz emitter along with two broadband detection techniques -THz air-biased coherent detection (THz-ABCD) and electro-optical (EO) sampling with a GaP crystal [12,13] -to measure the absorption spectra of water vapor at different relative humidity (RH) levels.The broadband THz spectrometer with THz-ABCD and EO sampling with a GaP crystal is schematically shown in Fig. 1. A Ti-sapphire amplified laser (Coherent Libra) with a central wavelength of 800 nm, 50-fs pulse duration, 1.3-mJ pulse energy, and 3-kHz repetition rate is used as the optical source. The laser beam is split into two by an optical beam splitter. One beam has 85% of the energy passed through a 100-µm-thick type I beta b...

High-resolution frequency-domain spectroscopy for water vapor with coherent and continuous terahertz wave

“…Water vapor is the largest contributor to atmospheric attenuation for THz waves due to lots of vibrational and rotational states. It is important to investigate the THz properties of water vapor, especially for THz telecommunications and environment monitoring [22,23] . In comparison with THz-TDS, we verify that THz-FDS is even more powerful for gas-phase molecule spectroscopy applications.…”

Terahertz radiation from pentacene organic diode at room temperature

“…Due to the T-rays spectrum's special sensitivity to molecular structures, it is especially useful for dynamic analysis and nondestructive detection in the safety check [1,2] . Currently, T-rays have attracted great deal of attention in both fundamental research and industrial applications [3][4][5][6][7] . T-rays can be generated by the interactions of ultrashort pulse laser with nonlinear optics crystal [8,9] , or interactions of high-power laser with gas-like organic molecules such as methanol (CH 3 OH) and ammonia (NH 3 ) [10,11] .…”