Electromagnetic Analysis of Attenuated Total Reflection Spectra of Water in Bulk and Films Forms at Terahertz Frequencies

Terahertz attenuated total reflectance (THz-ATR) is being extensively used for spectroscopy and imaging applications in materials science, chemistry, and biomedical research. One of the driving motivations is its potential for studying protein dynamics, identifying living tissues, or detecting herbicide traces, as a non-destructive and non-ionizing technique. It is well known that studying biological and chemical samples in the terahertz range represents a real challenge because of the presence of water, which is highly absorbing. In this work, we present experimental data from THz-ATR spectroscopy of water in the 1-15 THz frequency range and propose an insightful interpretation of the optical phenomenon that is taking place in this experiment, supported by finite-element-method simulations and an analytical model. Interestingly, this study reveals that there is no evanescent field at the interface between the crystal and water, but that a refracted beam is propagating through the water, independently from the value of the angle of incidence. These findings improved the signal intensity after a simple modification of the accessory, by creating a thin film of micrometer thickness with a top mirror. We believe that this technique could lead to highly sensitive terahertz microfluidic systems for chemical and biomolecular detection.