3  
[PERSON]  
3 Optical Sciences Centre and ARC Training Centre in Surface Engineering for Advanced Materials (SEAM), School of Science, Swinburne University of Technology, Hawthorn, VIC 3122, Australia; [EMAIL_ADDRESS] (S.  
Harnessing the THz portion of synchrotron radiation, high brightness THz beams are extended to \(<\)300 cm\({}^{-1}\) wavenumbers (single digit THz frequencies), where conventional tabletop Fourier transform IR (FTIR) spectrometers are rendered less useful, due to the low signal-to-noise ratio (SNR). The high brilliance of THz synchrotron radiation makes it especially promising for real-time monitoring of phase transitions. Due to the high transparency of diamond at THz frequencies, the characterisation of water at high pressure was demonstrated in [3]. An anvil and pressure cell (using diamond windows) experiment with materials at high pressure and temperature was recently demonstrated with a 35 MPa cell at sub-THz spectral window [4]. Charge separation and its control over the photosynthetic reaction centres upon illumination and is another active area of research where THz spectroscopy of bio-molecules and proteins can provide new insights [5]. The THz properties of protein structures and their hydration shells were studied extensively [6; 7; 8]. The complexity increases when coupled with the temperature-dependent properties of water [9]. When the ATR technique is used with a high brilliance THz-IR beam (Australian Synchrotron), it becomes possible to measure the THz response of both simple and complex composition samples and to reveal temporal changes. High quality scans can be produced every 0.3-0.4 s, allowing for the monitoring of effects caused by rapid temperature changes. The typical signal-to-noise ratio (S/N) ratio is \(\sim\)20 times higher as compared with Global sources widely used in table-top FTIR spectrometers. The water-ice transition is of particular interest for cryogenic applications in biological materials. It can also be studied and exploited using the ATR technique for deeper characterisation of bio-samples. Due to both a large refractive index change from the water-ice transition (\(\sim\)0.4) and change in absorption coefficient through the THz range, the ATR condition can be not fulfilled, i.e., instead of reflection from the diamond-sample interface, the THz beam propagates in transmission. The total internal reflection occurs for the light propagating from the medium with a higher refractive index into the lower index material (ATR diamond to sample). A higher refractive index of the sample (with ice) invalidates the ATR performance. The refractive index of ice is \(\sim\)1.7 at 1-to-3 THz, increases to \(\sim\)1.9 at 4.2 THz, reduces slowly to 1.75 at 6.5 THz, then quickly drops to 1.1 by 6.9 THz [10]. This means that between 1 and 3 THz, there is a good refractive index contrast between liquid water and ice. Due to the air gaps between the sample and the ATR diamond window, light tunnelling can take place, also contributing to the transmission mode. Addressing these issues and taking advantage of (some of) them is discussed in this prospective/concept paper with numerical examples and required analytical expressions. In this perspective/concept paper, based on recent experiments at the THz beamline at the Australian Synchrotron, technical peculiarities of the ATR operation mode are detailed. We introduce the concept of polarisation analysis for the ATR mode and validate it numerically and experimentally. Challenges of multi-reflection beam delivery to (and from) the sample in ATR measurements, as well as ATR condition changes with the liquid-solid water transition, are scrutinised in order to extract the useful orientation characterisation of samples in ATR experiments. Polarisation, intensity and the spectral filtering of complex synchrotron THz radiation can provide more flexibility in material characterisation.