THz detection of small molecule vapors in the atmospheric transmission windows

Abstract

Using a low power beam of ultrashort THz pulses that propagate in the ambient laboratory environment we have measured the rotational signatures of small molecule vapors at frequencies within the atmospheric transmission windows. We investigate two types of apparatus. In the first type the THz beam propagates along a 6.7 meter round trip path that is external to the spectrometer, and which contains a long sample tube (5.4 meter round trip path) that holds the analyte vapor. The environment of the tube is controlled to simulate dry or humid conditions. In the second apparatus the THz beam propagates over a much longer 170 meter round trip path with analyte vapor contained in a relatively short 1.2 meter round trip path sample chamber. We describe the rotational signatures for each apparatus in the presence of the strong interference from water vapor absorption. For the shorter path long-tube apparatus we find that the peak detection sensitivity is sufficient to resolve a 1% absorption feature. For the more challenging 170 meter path apparatus we find that the peak detection sensitivity is sufficient to resolve a 3-5% absorption feature. The experiments presented here represent a first step towards using ultrashort THz pulses for coherent broad band detection of small molecule gases and vapors under ambient conditions.

Terahertz volatile gas sensing by using polymer microporous membranes

“…THz gas sensing techniques have been validated by using two fundamental methods. One method is to radiate THz waves directly on the gaseous analytes and acquire their spectral information, such as the fingerprint feature or decay signals of a pulse [9] as response signals for sensing purposes. As reported in literature, the systems include photo-mixing [10], heterodyne detection [1,11], and chirped-pulse THz spectroscopy [12].…”

Analysis and design of efficient passive components for the millimeter-wave and THz bands

“…Regarding to the aforementioned classification of aspirin and methamphetamine, see how the f = 1. 39 THz results unclear for that purpose, whereas the f = 1.69 THz image shows a determinant contrast between both samples.…”

“…By observing the curves corresponding to the different frequencies, certain dispersive features are observed, although quite moderate (the different curves are quite close to each other despite of the relative bandwidth in [29][30][31][32][33][34][35][36][37][38][39][40][41] GHz] is RBW = 34%). Notice that, due to the periodic structure, it is obvious that the impedance dispersion of the structure must be higher than other quasi-TEM transmission lines, but it is low enough for moderate-wide bandwidth applications.…”

“…An intense research is being done these days to proper characterize the THz propagation in our atmosphere [39]- [42]. On the one hand, calculation of atmospheric transmission requires line positions, intensities, pressure broadening coefficients, and pressure shift coefficients.…”

“…Due to the atmospheric absorption, current state of the art points to the use of [37][38][39][40][41][42] inside the Q-band, and [71][72][73][74][75][76] inside the E-band [669], although some previous commercial prospects considered as well [46][47][48][49][50][51][52][53][54][55][56] GHz] in the V-band. Serious commercial proposals indicate as possible geostationary orbits, medium earth orbit, low earth orbit, and Molniya orbit, or in some cases combinations of two of these orbits.…”