## 3 Conclusions and Outlook  
In this prospective/concept paper analysis of ATR measurements, we present most of the examples based on experiments at the THz beamline at the Australian Synchrotron. Explicit formulae pertinent to the ATR data analysis are presented for the discussion and illustration of practical examples as well as their tutorial value. Challenges in using THz  
Figure 9: (**a**) Image of super-pixel mosaic design with \(5\times 5\) elements. (**b**) A block scheme of the Gerchberg–Saxton algorithm with scattering degree \(\sigma=d/D\). Images of spatially randomly multiplexed cross-filters with for (**c**) \(\sigma=0.1\), (**d**) \(\sigma=0.15\) and (**e**) \(\sigma=0.2\). SEM images of the fabricated (**f**) mosaic type and (**g**) randomly-multiplexed cross filters on gold-coated SU8 films on silicon substrate. ATR units for the polarisation analysis are due to the large number of internal reflections: five to the ATR prism and an extra five to the output port. Moreover, focusing is carried out on the beam propagation to the sample, and those reflections have complex angular dependence and distribution of angles of incidence. This is additionally complicated by the complex polarisation composition of the synchrotron beam. However, for the unique spectral window at <3 THz, where broadband table top THz sources are not available for high brilliance/intensity, synchrotron polarisation is mostly linear (\(\sim\)90%; along the slit of the first mirror used to extract THz radiation from the synchrotron) and can be utilised for THz polariscopy, which is still at the very early stage of development. Spectral filters for specific spectral bands can be made using photolithography on free-standing SU8 laminates as shown above. A metal coating of the developed SU8 micro-sheets can produce single frequency transmission filters as well as intensity and bandwidth comb filters. The potential of polarisation analysis in the ATR mode and development of spectral/polarisation filters is presented by concept examples. In addition, polarisation optics for SAM and OAM manipulation and the analysis of THz radiation have to be developed for the interrogation of complex orientational structures, especially in biological/medical samples. Conceptualization, M.R., S.J. and J.M. ; methodology, M.R., D.A., Z.V., A.W.W., S.J., J.M. ; validation, M.R., S.H.N., V.A., Z.V. and J.M. ; formal analysis, M.R., Z.V. and J.M. ; investigation, S.L., J.H., T.K., D.A. and Z.V. ; data curation, M.R., Z.V., S.J. and J.M. ; writing--original draft preparation, S.J. and M.R. ; writing--review and editing, all the authors; visualization, [PERSON] All authors have read and agreed to the published version of the manuscript. This work was supported by JST CREST Grant Number JPMJCR19I3, Japan, and the ARC Discovery DP190103284 grants. The project was carried during EU16010, M15121 beamtimes at the Australian Synchrotron. [PERSON] and [PERSON] are grateful for the support via the NHMRC grant 1042464. [PERSON] and [PERSON] are grateful for the support via the ARC Linkage LP190100505 project. Not applicable. The data presented in this study are available on request from the corresponding author. The authors declare no conflict of interest. The amplitude and phase of reflected light from the diamond prism interface with the sample changes (Equation (4)) and has to be accounted for in the analysis of light using the analyser. First, the changes without the sample (from ATR prism-air interface) are considered at different angle of incidence \(\theta_{i}\).