Terahercinės difrakcinės optikos ir plazmon-fonon-poliaritonikos komponentų kūrimas

In this dissertation two main topics are discussed, relevant to the development of modern, compact and widespread application spectroscopic terahertz imaging systems. Namely development of terahertz diffractive optics and variable frequency polaritonic infrared optical components is described. Development and performance of direct laser ablation fabricated multilevel phase Fresnel lenses and metal Soret zone plates for focusing of 0.6 and 4.7 THz frequency radiation are discussed in the first part of this dissertation. Characterization of fabricated components was carried out, demonstrating the applicability of the fabrication method in component development. Optimal lens design was also found and application of a fabricated component in an experimental 0.6 THz frequency imaging setup was demonstrated. Second part of this dissertation deals with the development of polaritonic components with specifically designed infrared spectral features in the infrared spectral range described by the Reststrahlen band of the semiconductor. Optical spectra modelling of heavily doped n-GaN surface relief gratings and metal grating coupled heterostructures was carried out using custom modified rigorous coupled wave analysis method. Optimal n-GaN grating height and periodicity were chosen and fabrication was carried out according to the optimal modelled parameters. Close match was demonstrated between the modelled and experimental results therefore validating the modeling and fabrication methods in developing optical components with custom frequency spectral features. Finally, it was shown that thermally excited n-GaN surface relief gratings are applicable in developing coherent, variable frequency infrared thermal sources

Laser-processed diffractive lenses for the frequency range of 4.7 THz

The development of diffractive lenses for the upper terahertz (THz) frequency range above 1 THz was successfully demonstrated by employing a direct laser ablation (DLA) technology. Two types of samples such as the Soret Zone plate lens and the multi-level phase-correcting Fresnel lens were fabricated of a metal foil and crystalline silicon, respectively. The focusing performance along the optical axis of a 4.745 THz quantum cascade laser beam with respect to the positioning angle of the sample was studied by using a realtime microbolometric camera. A binary-phase profile sample demonstrated the values of the focusing gain and focused beam size up to 25 dB and 0.15 mm (2.4λ), respectively. The increase of the phase quantization level to eight led to higher (up to 29 dB) focusing gain values without a measurable increase of optical losses. All the samples were tolerant to misalignment as large as 10 deg of oblique incidence with a focusing power drop no larger than 10%. The results pave the way for new applications of industry-ready DLA technology in the entire THz range

Optical Performance of Two Dimensional Electron Gas and GaN:C Buffer Layers in AlGaN/AlN/GaN Heterostructures on SiC Substrate

Terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy were developed as the method for the investigation of high-frequency characteristics of two-dimensional electron gas and GaN:C buffer layers in AlGaN/AlN/GaN heterostructures grown on a semi-insulating SiC substrate. The reflectance and transmittance spectra of the selected heterostructure layers were studied after the top layers were removed by a reactive ion etching. Results were numerically analyzed using the transfer matrix method taking into account the high-frequency electron conductivity via a Drude model and complex dielectric permittivity of each epitaxial layer via a one-phonon-resonance approximation. Good agreement between the experiment and theory was achieved revealing the temperature dependent electron effective mass in AlGaN/AlN/GaN high electron mobility transistor structures and the small damping factors of optical phonons due to high crystal quality of the epitaxial layers fabricated on the SiC substrate

Long-term Relaxation of Photoconductivity in Cu1-xZnxInS2 solid solutions

На основі уявлень про рівні прилипання проаналізовано результати досліджень релаксації фотопро- відності твердих розчинів n-Cu1-xZnxInS2 в інтервалі температур 30–100 К. Показано, що в Cu1-xZnxInS2 має місце довготривала релаксація фотопровідності (101–103 с), яка контролюється s та r-центрами рекомбінації. Визначена глибина залягання рівнів прилипання становила ~0,1–0,2 еВ.; Photoconductivity kinetics in CuInS2–ZnIn2S4 solid solutions has been studied. An effect of long-term relaxation processes (~101–103 s) at T≈27–100 К was revealed. Data on photoconductivity relaxation are analyzed in terms of attachment levels. Here the nonequilibrium conductivity relaxation kinetic of CuInS2–ZnIn2S4 solid solution corresponds to the case of the exponential recombination. The main parameters characterizing the photoconductivity kinetic were determined. It is shown that as the temperature increases, the relaxation time of the slow component and fast component decreases. The contribution of the slow component also decreases whereas fast component increases. It is shown that relaxation of photoconductivity is controlled by multicenter recombination in which both «fast» and «slow» recombination centers participate