Quantitative analysis of organics based on terahertz frequency-domain spectroscopy

Traditional terahertz time-domain spectroscopy (THZ-TDS) can provide a broadband spectral response of the measured object and has been widely used. Notwithstanding, for applications that require real-time quantitative detection such as security inspection, more attention should be paid to the terahertz response around the absorption characteristics of the sample. Terahertz frequency domain spectroscopy (THz-FDS) using frequency modulation continuous wave can better meet the real-time requirements of security applications. In this paper, an analytical method is established to achieve accurate prediction of molar concentration for organics. The absorption coefficient spectra of samples with different molar concentrations are obtained by using traditional THz-TDS in a wide frequency range, and then the THz-FDS based on photomixing technology is applied to locking a narrow band range near the absorption peak for rapid quantitative analysis. The scheme was verified by taking α-lactose monohydrate as an example, and the results showed that the mean square error of concentration prediction was only 0.025 under the interference of water vapor environment. It may shed light on terahertz rapid quantitative detection of organic compounds in realistic security scene.