Quantum Electronics 46 (6) 488 – 495 (2016) © 2016 Kvantovaya Elektronika and Turpion Ltd Abstract. We report a study of aqueous solutions of glucose and bovine serum albumin using THz time-domain spectroscopy. To describe the permittivity of the solutions of these substances, we use a simplified model being applicable in the frequency range of 0.05 – 2.7 THz. On the assumption that most of the water molecules become bound at high concentrations of glucose and protein in the solution, the changes in water characteristics are investigated. To improve the reliability of the results, the measurements are per- formed by two independent methods: the method of attenuated total internal reflection and the transmission method. Combination of the results obtained by these two methods allows expanding the spectral range towards lower frequencies. Keywords: THz radiation, THz time-domain spectroscopy, attenu- ated total internal reflection, water, glucose, bovine serum albumin, solutions, dielectric function, Debye model. 1. Introduction Determination of concentrations of sugars and proteins in blood and tissues is important for medical and biological appli- cations. It is known that the terahertz (THz) signal is changed in in vivo studies of THz radiation reflection from human or animal skin having a high glucose concentration in blood [1]. The main component of tissues and fluids in a body is water. The THz and sub-THz frequency bands can be used for the diagnosis of relative changes in the concentration and proper- ties of water, since these frequencies correspond to the relax- ation times of water molecules [2]. The state of the water itself, which is prevailing in these samples [3], represents a character- istic that is measured using THz spectroscopy. Changes in pro- portions of free and bound water, changes in relaxation times for each of these states of water – they are all manifested in the THz frequency range. Unfortunately, THz time-domain spec- troscopy usually captures not the centre of a low-frequency ‘peak’ of water absorption (in the region near 0.02 THz), but its high-frequency ‘tail’, starting from the frequency of 0.1 THz (Fig. 1). Nevertheless, THz time-domain spectroscopy has become an established method which complements the existing methods of diagnostics of biological tissues and solutions. However, there is no consensus as to how to interpret the THz signal contrast in scanning the skin in vivo and how to predict the changes in the THz response of solutions. Originally, the spectral response of a substance is given by its complex permittivity e(w). In the case of a homogeneous smooth object, absorption, refraction and reflection are uniquely determined by e(w). Historically, theory of the per- mittivity of water and water solutions has gained a greatest momentum in its development in the frame of dielectric spec- troscopy when describing the changes observed in the experi- mental spectra of dielectrics [2, 4]. The relaxation processes in dielectrics, in the frequency range from kilohertz to gigahertz, can be divided into four types: a-, b-, g- and d-relaxations [2, 4, 5]. Each of these pro- cesses corresponds to its own wide frequency maximum which overlaps with neighbouring peaks and describes a specific relaxation process. For example, the a-relaxation describes the dielectric losses in glass-like dielectrics, and its relaxation times are equal to a few nanoseconds [2]. The g-relaxation dominates for water, and its characteristic time amounts to tens of picoseconds (Fig. 1) [5, 6]. The external electromag- netic field sets the orientation of dipoles of water molecules. Under the impact of heat and entropy, molecules undergo a Study of the dielectric function of aqueous solutions of glucose and albumin by THz time-domain spectroscopy M.M. Nazarov, O.P. Cherkasova, A.P. Shkurinov DOI: 10.1070/QEL16107 M.M. Nazarov Institute of Laser and Information Technologies, Russian Academy of Sciences, Svyatoozerskaya ul. 1, 140700 Shatura, Moscow region, Russia; e-mail: nazarovmax@mail.ru; O.P. Cherkasova Institute of Laser Physics, Siberian Branch, Russian Academy of Sciences, prosp. Akad. Lavrent’eva, 13/3, 630090 Novosibirsk, Russia; e-mail: o.p.cherkasova@gmail.com; A.P. Shkurinov Faculty of Physics and the International Laser Centre, M.V. Lomonosov Moscow State University, Vorob’evy Gory 1, 119991 Moscow, Russia; Institute on Laser and Information Technologies, Russian Academy of Sciences, Svyatoozerskaya ul. 1, 140700 Shatura, Moscow region, Russia; e-mail: ashkurinov@gmail.com Received 21 April 2016 Kvantovaya Elektronika 46 (6) 488 – 495 (2016) Translated by M.A. Monastyrskiy sum of contributions ‘slow’ relaxation ‘fast’ relaxation Lorentz term –Ime 10 1 0.1 0.01 0.1 1 f/ THz Figure 1. Spectra of the imaginary part of the permittivity Im e( f ) of water in the THz frequency range for a ‘two-component’ relaxation model.