Abstract

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 a “proof of concept” of 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 °C and 24 °C 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 reflected signal intensity 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 reflected signal intensity even at −20 °C, suggesting that emulsified water retains liquid-water-like THz properties at freezing temperatures.

Highlights

  • Terahertz radiation (THz) is highly absorbed by liquid water and this presents the possibility to image biological materials using the difference in water content between normal and pathological lesions [1]

  • This paper examines the viability of investigating biological tissues by employing the temperature dependent variation of THz dielectric parameters using an attenuated total reflection (ATR) apparatus, in both the “true” attenuated total reflection mode and in a novel “reflection/transmission” mode

  • The Beamline was equipped with an attenuated total reflection (ATR) apparatus, a Bruker IFS 125/HR Fourier Transform spectrometer (Bremen, Germany), an Si Bolometer and a diamond prism stage (n = 2.40), and a 45◦ incoming beam angle

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Summary

Introduction

Terahertz radiation (THz) is highly absorbed by liquid water and this presents the possibility to image biological materials using the difference in water content between normal and pathological lesions [1]. Efforts have been made in the past to evaluate THz properties of biological tissues with a view to devising diagnostic imaging techniques [2,3,4]. THz examination can be time consuming and involves highly specialised expertise. These problems have resulted in difficulties in devising a diagnostic test which is both clinically relevant and practical. This paper examines the viability of investigating biological tissues by employing the temperature dependent variation of THz dielectric parameters using an attenuated total reflection (ATR) apparatus, in both the “true” attenuated total reflection mode and in a novel “reflection/transmission” mode. The Australian Synchrotron’s THz-far infrared beamline provides a high brightness source, which results in rapid evaluation, enhanced spectral quality and superior spatial resolution when compared to a conventional source

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