Broadband and Intense Terahertz Time-Domain Spectroscopy for Investigating Liquid Solutions

Over the past two decades, terahertz (THz) spectroscopy has demonstrated remarkable potential for the investigation of liquids, including studies of living organisms and biological components in their natural, aqueous environments. The main advantages of THz radiation lie in its ability to interact with collective and low-energy vibrational modes of macromolecules and microorganisms, while being non-harmful due to the low photon energy involved. These characteristics make THz spectroscopy particularly valuable for research in liquids compared to other well-established techniques such as Raman and infrared spectroscopy. In this study, we offer a concise overview and comparison of two case studies from our earlier publications, highlighting how Ultrabroadband THz spectroscopy and Intense THz Spectroscopy serve as complementary methods for advancing research in liquids. Ultrabroadband THz spectroscopy enables simultaneous probing of both intermolecular and intramolecular interactions in a single experiment. On the other hand, intense THz spectroscopy greatly simplifies the determination of the optical constants of liquid solutions, eliminating the need for additional assumptions or prior knowledge. Moreover, it offers high sensitivity, allowing the detection of dilute solutions and subtle spectral variations. Currently, these two techniques typically rely on different THz sources, as achieving both broadband coverage and high intensity in a single setup remains challenging. In fact, the experimental results reviewed here were obtained at two different times and within two distinct scientific collaborations. In particular, the intense source was accessed through a collaboration with Prof. Novelli at Ruhr University in Bochum. Integrating both capabilities into a single apparatus would be highly desirable. Therefore, we also present a theoretical investigation of a novel experimental approach that could enable combined ultrabroadband and intense THz spectroscopy, merging the strengths of both methods.