High-resolution terahertz spectra of trace gases and aerosolized biological agents*

High-resolution terahertz spectrometry can identify the gases or biological agents in real-time if the spectroscopy can be done over a short time with high signal-to-ratio (SNR) and superior resolution. We performed time-domain terahertz (THz) spectroscopy on gas (or vapor) phase samples of carbon monoxide, methanol, water, and acetonitrile at concentrations less than 10ppm, and demonstrated 50-60dB SNR on a single measurement. Also, we measured unambiguous terahertz spectra of various types of biological agents which were aerosolized. Using data measured at different sample concentration and terahertz probe beam power we investigated the interplay between SNR, probe beam power, sample concentration, and the electric dipole moment of molecules, for which we extended an absorbance theory to THz frequencies. Comparison of our data with references and a theoretical model revealed 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. These results were possible largely due to our high power THz source capable of generating up to 3 mW, an order of magnitude greater than those available commercially.