Optical and structural experiments have been carried out on Si/ZnO thin films modified with ultra-thin gold layers of different thicknesses. ZnO was produced via Atomic Layer Deposition (ALD) and Au via Physical Vapor Deposition (sputtering). The structural properties of nanostructures were studied by XRD and AFM. Optical characterization was performed by absorbance, photoluminescence (PL) and spectroscopic ellipsometry (SE). A transition from cluster-to-thin films with the increase of Au thickness has been revealed from an analysis of optical and structural parameters. The analysis of optical features of the system has shown that slight changes of the localized plasmon absorption peaks in spectra make a significant contribution to complex refractive index of gold film and, as a result, leads to a strong enhancement of UV PL peak in the ZnO layer. The mechanism of the tailoring of ZnO optical features changes by varying the Au layer thickness was discussed. Our studies have shown that through the changes of structural properties of thin gold layer between the Si substrate and the ZnO film, we can tune the optical dispersion of each layer and hence the control of ZnO PL spectra enhancement and quenching in UV-Vis wavelengths region is possible. In order to apply the hybrid structure under consideration in various optical applications, such as LED, the dispersion of the complex refractive index of the components should be optimized taking into account a particular target.

The adsorption of Eu(III) on composites synthesised from graphene oxide (GO), maghemite (MGH), and chitosan (CS) has been studied using different approaches. The physicochemical and morphological characteristics of the composites GO-MGH, GO-CS, GO-MGH-CS I, II, and III were determined by XRD, Mössbauer spectroscopy, FTIR, Raman spectroscopy, and TEM. According to the results of batch experiments, the maximum experimental adsorption capacity was 52, 54, 25, 103, and 102 mg/g for GO-MGH, GO-CS, GO-MGH-CS I, II, and III, respectively. The data obtained are in better agreement with the Langmuir, pseudo-second-order, and pseudo-first-order models only for GO-MGH. Thus, the adsorption of Eu(III) on the composites was a favourable, monolayer, and occurred at homogeneous sites. The nature of adsorption is chemical and, in the case of GO-MGH, physical. Tests of the composites in natural waters showed a high removal efficiency for Eu(III), Pu(IV), and Am(III), ranging from 74 to 100%. The ANFIS model has quite good predictive ability, as shown by the values for R², MSE, SSE, and ARE. The GO-MGH-CS composites with the high adsorption capacity could be promising candidates for the removal of Eu(III) and the pre-concentration of Pu(IV) and Am(III) from natural waters.

In this study, polarized reflection and emission spectroscopy was employed in order to describe experimentally and numerically the coherent behavior of surface plasmon phonon polaritons in shallow n-type gallium nitride grating. For the grating with a $1\hspace{0.17em}\mu$ m depth and an $11\hspace{0.17em}\mu$ m period, polariton coherence was observed measuring the reduction of the linewidth and the redshift of the resonance position in both reflection and emission spectra with the increase in the incident angle. Maximum coherence conditions were found at a frequency of 562 cm⁻¹ and an angle of 37°, demonstrating the coherence length value up to 724 μm (41λ).

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.