A novel method of investigating the temperature dependent variation of aspects of the complex refractive index n* in samples in the THz range using continuous, non-polarised, synchrotron radiation is presented. The method relies on the use of ATR apparatus, and retains the advantage of minimal sample preparation, which is a feature of ATR techniques. The method demonstrates the viability of rapidly monitoring temperature reflectance whilst continuously heating or cooling samples by using a temperature variable Thermal Sample Stage. The method remains useful when the refractive index of the sample precludes attenuated total reflection study. This is demonstrated with the water reflectance experiments. The temperature dependent ATR reflectance of tissue-representative fats (lard and Lurpak® butter) was investigated with the novel approach. Both are within the ATR range of the diamond crystal in a “true” ATR mode. Lard showed no clear temperature variation between -15 0C and 24 0C at 0.7 to 1.15 THz or 1.70 to 2.25 THz. Lard can be regarded as having invariable, constant, dielectric properties within mixtures when biological substances are being assessed for temperature dependent dielectric variation within the stated THz ranges. Lurpak® butter (water content 14.7%) displayed temperature dependent reflectance features with a steady decline in reflectivity with increasing temperature. This is in line with the temperature-dependent behaviour of liquid water. There is no rapid change in reflectance, even at -20 0C, suggesting that emulsified water retains liquid-water-like THz properties at freezing temperatures.

Capabilities of the Attenuated Total Reflection (ATR) at THz wavelengths for increased sub-surface depth characterisation of (bio-)materials is presented. The penetration depth of a THz evanescent wave in biological samples is dependent on the wavelength and temperature and can reach 0.1-0.5 mm depth due to strong refractive index change ∼0.4 of the ice-water transition; this is quite significant and important when studying biological samples. Technical challenges are discussed when using ATR for uneven, heterogeneous, high refractive index samples with possibility of frustrated total internal reflection (a breakdown of the ATR reflection-mode into transmission-mode). Local field enhancements at the interface are discussed with numerical/analytical examples. Maxwell’s scaling was used to model behaviour of absorber-scatterer inside materials at the interface with ATR prism for realistic complex refractive indices of bio-materials. Modality of ATR with polarisation analysis is proposed and its principle illustrated, opening an invitation for its experimental validation. The sensitivity of the polarised ATR mode to the refractive index between the sample and ATR prism is revealed. Design principles of polarisation active optical elements and spectral filters are outlined. The results and concepts are based on experiments carried out at the THz beamline of the Australian Synchrotron.

A focal plane array (FPA) detector was used for hyperspectral imaging in the infrared (IR) spectral region using thermal and synchrotron light sources. FPA Fourier-transform IR (FTIR) imaging microspectroscopy will be able to monitor real time changes at specific absorption bands when combined with high brightness synchrotron source. In this study, several types of samples with unique structural motifs were selected and used for assessing the capability of the FPA-FTIR imaging technique. It was shown that the time required for polariscopy at IR wavelengths can be substantially reduced by the FPA-FTIR imaging approach. By using natural and laser fabricated polymers with sub-wavelength features, alignment of absorbing molecular dipoles was revealed as well as higher order patterns (laser fabricated structures). Micro-spectroscopy of absorber orientation reveals alignment patterns even when they are not spatially resolved.

THz band-pass filters were fabricated by laser ablation of micro-foils of stainless steel and Kapton. Their spectral performance was tested in transmission and re- flection at the THz beamline at the Australian Synchrotron (AuSy). A 25 μm Kapton film performed as a Fabry-Pérot etalon with a free spectral range of FSR = 119 cm−1, high finesse Fc ≈ 17, and was tuneable over ~10 μm (at ~5 THz band) with β = 30° tilt. The structure of the THz beam focal region as extracted by the first mirror (slit) shows a complex polarisation-wavelength- position dependence across the beam. This is important for polarisation sensitive measurements (in transmission and reflection) and requires normalisation at each orientation of linear polarisation.