High SNR single measurements of trace gas phase spectra at THz frequencies

Abstract

Fast detection and identification of trace gases in ambient conditions demand a high signal-to-ratio (SNR) and superior resolution from a single measurement. We performed time-domain terahertz (THz) spectroscopy on gas (or vapor) phase samples of carbon monoxide, methanol, water, and acetonitrile at concentrations less than 10 ppm and demonstrated 50–60 dB SNR on a single measurement. Using data measured at different sample concentrations and terahertz probe beam powers, we investigated the interplay between the SNR, the probe beam power, the sample concentration, and the electric dipole moment of molecules, for which we extended an absorbance theory to THz frequencies. When comparing our data with references and a theoretical model, we found some discrepancy in certain spectral line intensities, suggesting that certain rotational resonance quantum state may have higher populations or transition probabilities than our model predicts at atmospheric conditions. We could achieve the above results largely due to our high power THz source capable of generating up to 3 mW, an order of magnitude greater than those available commercially.