For biomedical applications in the terahertz band, composites such as macromolecule compounds, biotissues and phantoms are studied. A description of dielectric properties of composite materials using mathematical models has its own fundamental and technological importance. In this work, we present an iterative effective medium theory for multi-component materials. The model has good performance in describing composite materials with more than two components. The theory is evaluated by comparing with the complex permittivity of three different composite materials. A comparison with other commonly used models is given in the form of relative errors.

The optical properties of the polymer composites consisting of polyvinyl chloride nanofibers and polypropylene films in the frequency range of 0.2–1.0 THz were studied, and the mechanical properties of polyvinyl chloride nanofibers and the structure porosity were investigated. An iterative mathematical model based on effective medium theory was used to describe the effective refractive index and absorption coefficient of the polymer composites. The permittivity tensors of the composites were calculated using the Rytov method. We found that the refractive indices of the composites increased with the increase of polypropylene contents, while absorption coefficients remained the same. The polarization-dependencies of THz optical properties of the composites were relatively low. The proposed composites have the potential to be used as materials for terahertz optical components.

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With the fast development of terahertz technology in medical diagnosis and monitoring, it has become important to investigate the application of THz radiation in the cancer treatment assessment during the therapy. In this paper, a buccal drug delivery system is studied as the first step towards this application. The drug delivery system is based on a gelatin-starch biopolymer matrix filled with plasticizing glycerol and various contents of reinforcing particles of bentonite clay. The biopolymers were subjected to morphology analysis using optical microscopy, analysis of mechanical tensile properties, and analysis of terahertz optical properties, followed by a theoretical approach of the experiment. The results show a visible effect of the bentonite content on both of the mechanical and terahertz optical properties of the biopolymer. These findings allow us to confirm the feasibility of using THz radiation for cancer assessment during therapies. The proposed biopolymer also has the potential to be applied as a substrate when carrying out in-vivo optical property measurement of biotissue in terahertz frequency range.

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The hyperbolic materials are strongly anisotropic media with a permittivity/permeability tensor having diagonal components of different sign. They combine the properties of dielectric and metal-like media and are described with hyperbolic isofrequency surfaces in wave-vector space. Such media may support unusual effects like negative refraction, near-field radiation enhancement and nanoscale light confinement. They were demonstrated mainly for microwave and infrared frequency ranges on the basis of metamaterials and natural anisotropic materials correspondingly. For the terahertz region, the tunable hyperbolic media were demonstrated only theoretically. This paper is dedicated to the first experimental demonstration of an optically tunable terahertz hyperbolic medium in 0.2–1.0 THz frequency range. The negative phase shift of a THz wave transmitted through the structure consisting of 40 nm (in relation to THz wave transmitted through substrate) to 120 nm bismuth film (in relation to both THz waves transmitted through substrate and air) on 21 µm mica substrate is shown. The optical switching of topological transition between elliptic and hyperbolic isofrequency contours is demonstrated for the effective structure consisting of 40 nm Bi on mica. For the case of 120 nm Bi on mica, the effective permittivity is only hyperbolic in the studied range. It is shown that the in-plane component of the effective permittivity tensor may be positive or negative depending on the frequency of THz radiation and continuous-wave optical pumping power (with a wavelength of 980 nm), while the orthogonal one is always positive. The proposed optically tunable structure may be useful for application in various fields of the modern terahertz photonics.