## 2 Experimental: Setups and Samples  
### Synchrotron Radiaton and ATR  
THz/Far-IR beamline at the Australian Synchrotron was used in this study. Polarisation of synchroton radiation has a combination of a linear (along the extraction mirror slit; y-orientation in Figure 1) and circular polarisations [27]. The origin of those two components is due to the bending magnet caused emission (inside the magnet) and the edge-emission (at the entrance/exit magnet edge). The THz and IR microscopy beamlines share the synchrotron radiation extracted from the magnet. Polarisation is defined in the plane of incidence as \(E_{s}\) and \(E_{p}\), which are \(\perp\) and \(\parallel\) to the plane (or TE and TM modes), respectively. For anisotropic samples, those two components will be absorbed differently, i.e., alignment of the absorbing dipoles. This opens up the possibility for determining anisotropy of absorbance in the sample. The s-pol. (TE or \(E_{y}\) in Figure 1b) is probing orientation of dipoles in the plane of ATR crystal. The reflected light at typical 45\({}^{\circ}\) incidence usually does not experience phase change upon reflection at the sample-prism interface (see Appendix A for phase change upon reflection) for low refractive index \(n_{1}<1.6\) samples, which is typical for bio-materials. However, the p-pol. (TM; \(E_{z}\) in Figure 1b), which probes the absorbance in the direction perpendicular to the prism-sample interface has a strong dependence of the reflected phase on \(n_{1}\). At \(\theta_{i}=45^{\circ}\) incidence, a phase change of \(\pi\) occurs when \(\theta_{B}<\theta_{i}<\theta_{c}\) (\(\theta_{B}\) and \(\theta_{c}\) are the Brewster and critical angle, respectively. See Appendix A  
Figure 1: **(a)** Photo of the THz/Far-IR radiation injection port and distribution chamber between two beamlines at the Australian synchrotron; the chamber is in vacuum. Trajectory of electrons (and slit of bending magnet) are in xy-plane (see the inset on top where dipole and edge radiation extraction from synchrotron is shown schematically). The beam extraction mirror had an intermediate focus before entering beam-splitting chamber and is shown schematically. This caused the left-right and top-bottom flip of the impinging radiation (dipole on the right hand side and edge on the left hand side). **(b)** The attenuated total reflection (ATR) geometry based on a prism (diamond) with a typical 3 mm cross section of the area of measurements (diameter of the THz beam). Sample is pressed against the window during measurements. This setup allows THz experiments in out-of-vacuum conditions normally required to cancel strong water absorbance in air. for definitions) and the reflected \(E_{z}\) component becomes \(E_{y}\). The consequence of the phase dependence of the TM mode makes polarisation analysis of ATR signals complicated, and most of the published data does not discriminate polarisation. We followed this approach in the current study and will carry out future experiments with a defined incident polarisation combined with polarisation analysis at the exit. A Bruker IFS 125/HR Fourier Transform spectrometer (Bremen, Germany), with a Si bolometer, 75 \(\mu\)m Mylar beamsplitter and a diamond prism stage (refractive index of \(n_{1}=2.42\)) was used. Data analysis was carried out with OPUS 8.0 software (Bruker Optik GmbH, Ettlingen, Germany). We used modified frustrated internal reflection (FTIR) technique. The evanescent wave reflection by a gold plated mirror with a varied separation from the ATR diamond crystal surface was installed. In this way, the wave is redirected back to the detector. The experiments were conducted over the frequency range of 0.91 to 1.01 THz due to high brightness of synchrotron radiation at this frequency window. It is noteworthy, that Australian Synchrotron has the capability to generate coherent synchrotron radiation (CSR) at the spectral window of 15-25 cm\({}^{-1}\) (0.45-0.75 THz in frequency or 0.667-0.4 mm wavelength) which is available at the THz beamline.