## 4 Conclusions and Outlook  
The THz-ATR technique with a top-reflecting gold mirror was used to measure absorbance of the bio-material--rendered pork lard sample. At the chosen \(\sim\)1 THz synchrotron radiation band, polarisation is mostly linear and corresponds to the s-pol. at the sample-prism interface. The experimentally determined absorption coefficient of lard is 25 cm\({}^{-1}\) (or absorption depth of 0.4 mm) for the thinnest samples where the evanescent light fields dominates light-matter interaction at the interface between the lard and the ATR diamond prism.  
Future improvement of the THz-ATR technique can benefit from use of band-pass filters [33], which can also be designed to act as polarisers. Geometrical parameters of a cross opening in a metallic mask can control polarisation, spectral bandwidth, central wavelength and transmittance of the filter. We demonstrated bandpass filters based on pinhole-arrays in a gold film layer for  
Figure 5: FDTD simulations and experimental results of the reflected signal (polarisation was not discriminated for incidence nor for the reflected power) from the rendered porcine lard sample. Normalisation was carried out for FDTD data while the experimental data were normalised to the Au mirror reflectivity, which is taken as \(R=100\%\). Lines are drawn as eye-guides.  
the mid-IR spectral range [34]. Due to a mass density alignment (real part of the refractive index), the orientation of absorbers or structural patterns can be recognised even when spatial resolution is not achieved [31]. Polarisation resolved imaging at THz wavelengths is expected to further improve diagnostics of malignant skin formations. Uniquely to the ATR geometry, the E-field component \(E_{z}\) oriented perpendicular to the sample-prism interface can be used to detect orientation of patterns and structures along that field due to a stronger absorption. Sub-wavelength patterns can also contribute to the formation of \(E_{z}\) field, which cannot be present in the paraxial beam propagating in a free space but can have a strong contribution at the near and evanescent modes [35]. Laser-induced breakdown of liquid targets (e.g., water micro-jet) are a promising THz radiation source which can be localised with sub-wavelength (at THz) precision with intensity scalable by using multi-foci [19]. THz field enhancement at sub-wavelength formations cause an increased absorption and is a promising direction to explore for removal of skin lesions. For the visible spectral range, the enhancement was demonstrated by laser ablation of tens-of-nm wide grooves using \(\lambda\approx 800\) nm laser light [36]. The most obvious advantage of ATR THz measurements of biological samples is elimination of a strong absorption by atmospheric water during beam delivery to the sample.  
Conceptualization, Z.V. and A.W.W.; methodology, Z.V., D.A.; software, Z.V. and A.L.; validation, Z.V and A.L.; formal analysis, Z.V., A.L. and S.H.N.; investigation, Z.V., A.L., S.H.N. and D.A.; resources, A.W.W.; data curation, Z.V. and S.J.; writing--original draft preparation, Z.V. and S.J.; writing--review and editing, all the authors; visualization, Z.V. and S.J.; supervision, Z.V., D.A. and A.W.W. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the ARC Linkage LP190100505 grant. The project was carried during EU16010, M15121, M15578 beamtimes at the Australian synchrotron, ANSTO.  
The authors declare no conflict of interest.