Stable, compact terahertz ATR time-domain spectroscopy apparatus for quantitative solute measurements and real-time monitoring

We developed a stable, compact, terahertz attenuated total reflected (ATR) spectrometer using an integrated prism, which unifies a terahertz emitter, a terahertz detector, and an ATR prism. Because the prism confines terahertz wave propagation within it and shortens the terahertz path length, our spectrometer has the following two advantages: 1) because water vapor does not disturb the terahertz wave propagation, a high-quality terahertz spectrum is obtained even without a nitrogen purge, and 2) the shorter propagation length provides distinct stability and compact features. To verify our spectrometer's stability, we successively measured water absorption for 9 days and found the relative error to be ±3%. We also provide three distinctive examples by adapting our spectrometer. The first is the quantitative measurement of a solute. We determined the detection limit of H2SO4 to be 0.21% using the calibration curve of the refractive index. The second example is monitoring fermentation. Because terahertz absorption is a useful, new indicator against the potential of hydrogen (pH) as a standard indicator, we confirmed that the absorption reduction agreed with the pH during the fermentation of milk to yogurt by our prism for 1 day. The third is the observation of a crystal transition. The transition process of theophylline anhydrate to hydrate was measured on the prism because of the sensitivity of terahertz to the crystal and our spectrometer's stability. Therefore, our apparatus has potential application to different types of new quantitative measurements and real-time monitoring.