Chemical Sensing using Terahertz Time Domain Spectroscopy

Summary form only given. Terahertz (THz) radiation or T-rays corresponds to the frequency band ranging from 0.3 THz to 10 THz on the electromagnetic (EM) wavespectrum. It bridges the gap between the electronic microwaves and photonic infrared radiations. The photon energy of T-rays (4.14 meV at 1 THz), leads to rotational and vibrational transitions in molecules [1]. These transitions are molecule specific and hence the molecules exhibit spectral signatures in THz region. Some of the other important properties of the THz waves are - it is transparent to non - polar and non - metallic materials like cloth, paper, wood and plastic. This range of EM waves also undergoes extreme water absorption. These properties make THz time domain spectroscopy (THz-TDS) suitable for many applications like - chemical identification, security screening, medical imaging and remote sensing [2]. Unlike other spectroscopic techniques, the THz TDS measures THz electric field; therefore, it is relatively straightforward to extract the amplitude as well as phase information. This information can be further utilized to extract the material parameters like refractive index and extinction coefficient [3]. In the present work, liquid chemicals including acetone, ethanol, glycerin and nitro -glycerin, in the form of a pharmaceutical drug (each tablet contained 2.6 mg of nitro -glycerin) have been investigated using THz TDS. Nitro -glycerin was not used in its pure form, primarily owing to safety considerations, as it is a high energetic material that undergoes phase transformations even for very small changes in temperature and pressure. The experiments have been performed in a controlled environment having 21%RH and temperature of -21°C. The experimental time domain spectrum (having a bandwidth of over 5.5 THz), was converted to frequency domain and the spectral signature of the chemicals were obtained using MATLAB. Representative spectra of glycerin and nitro glycerin, thus obtained