Vol 126, No 5 (2019)

The reasons for the discrepancies between the experimental data on the shape of long-wavelength continua near the 6³P₁ → 6¹S₀ resonance line of a mercury atom in a xenon atmosphere obtained by different authors are analyzed. Calculations of spectral profiles for limiting cases of high and low densities of buffer gas atoms lead to the conclusion that the discrepancies are mainly due to the dependence of the rates of three-particle recombination and relaxation processes on the xenon density. Different types of a non-equilibrium or almost equilibrium (at high xenon concentrations) population of rovibrational states of a HgXe(A³0⁺) excimer molecule are formed at different rates of these processes, which affects emission spectral profiles.

GaS thin films have been grown by the SILAR method, their structures have been analyzed, and their optical and photoelectric properties have been investigated. The internal structure of the samples obtained have been studied using X-ray diffraction (XRD) analysis, atomic force microscopy (AFM), energy-dispersive X-ray (EDX) spectroscopy, and scanning electron microscopy (SEM). The GaS band gap has been determined from the absorption spectrum. p-GaS/n-InSe heterojunctions have been formed on the basis of GaS crystals and InSe thin films. Current–voltage, optical, photoelectric, and luminescence characteristics of p-GaS/n-InSe heterojunctions have been experimentally investigated.

Based on the method of continued boundary conditions, a technique is proposed that allows one to model the scattering characteristics, including averaged over orientation angles, for bodies of practically any geometry. A number of examples of solving problems of diffraction on fractal-like bodies of revolution are given. The correctness of the method is confirmed by verifying the implementation of the optical theorem for various bodies and by comparing with the results of calculations obtained by the method of discrete sources.

Processes of localization of field excitations in the form of nonlinear surface waves that propagate along the interface between linear and nonlinear media are considered. The existence conditions of nonlinear surface waves, which are determined by the ratio between the linear characteristics of the media, the Kerr nonlinearity coefficient, and the intensity of interaction of the wave with the interface, are analyzed. The dependences of the energy flux carried by nonlinear surface waves are calculated and analyzed. Two approaches are proposed to selection of the control parameters for determining the energy flux, which are the position of the maximum field perturbation in the nonlinear halfspace and the field amplitude at the interface. Estimates of the fluxes are found in different limiting cases, which correspond to small-amplitude field perturbations and to weak or strong interaction of a nonlinear surface wave with the interface between the media. It is shown that, in the case of small-amplitude field perturbations and the weak interaction of the wave with the interface, the flux in the linear halfspace is directly proportional to the interaction intensity of the wave with the interface, while that in the nonlinear halfspace is inversely proportional to the interaction intensity.

The problem of the propagation dynamics of three-dimensional optical Airy pulses (light bullets) in a photonic crystal with carbon nanotubes is considered. It is numerically shown that the expected type of pulse exhibits a steady and stable propagation.

The structure and optical characteristics of thin films of relaxor ferroelectric Ba_0.5Sr_0.5Nb₂O₆ grown by RF sputtering in an oxygen atmosphere on an Al₂O₃ substrate (c cut) have been studied. X-ray diffraction analysis shows that Ba_0.5Sr_0.5Nb₂O₆ films are c-oriented and unit-cell parameter c is 3.948(1) Å. Ellipsometric measurements confirm that SBN-50 films are characterized by a natural growth direction, which coincides with the direction of the optical crystal axis. An analysis of ellipsometric results shows that there is no transition layer at the film/substrate interface; the damaged layer on the free film surface is 7.5 nm thick, and the volume filling factor is estimated to be 0.625.

The surface waves that propagate along the interface of a dielectric with nonlinear susceptibility of the third order and topological insulator have been considered. The optical nonlinearity of the dielectric ensures the existence of a surface wave. The density of the spin angular momentum of a surface wave has been determined for dielectrics with positive or negative linear permittivity. It has been shown that the spin angular momentum vector has a projection on the normal to the interface, which is different from the usual surface polaritons or plasmon polaritons. The discrete nature of the topological number manifests itself in the discreteness of the values of the normal and tangential components of the spin angular momentum density. The increase in the intensity of the electric field of the wave at the interface of the media changes the value of the spin angular momentum and can lead to its disappearance.

A method for differentiation of pigmented skin lesions is proposed that is based on digital processing of optical images. The optical images are detected using a digital camera and a standard for its color and spatial calibration. The method involves segmentation and automatic recognition of neoplasms on the basis of five parameters: diameter, area, color, shape, and border sharpness. The analysis reveals clinical features of the lesion, required for diagnosis, and allows for calculating the probability of its malignant transformation. The method proposed was tested on 360 digital images of in vivo skin pigment lesions, including ordinary dysplastic nevi and melanoma. The sensitivity and specificity of the method proved to be 97 and 95%, respectively.

A laboratory model of lymphedema development induced by lymphatic vessel resection in rat extremities is presented. In vivo analysis of lymphedema development (monitoring for 4 weeks) employed reflective terahertz spectroscopy with a Dove prism. The incidence angle for an s-polarized electromagnetic wave directed to the boundary of the prism and the biological tissue was close to the Brewster’s angle. Significant changes in the spectral characteristics of the tissue in the animals’ extremities were detected on days 21–28 of lymphedema development. A predictive model for disease diagnostics based on monitoring the changes of the tissue absorbance curve in the 0.4–1.1 THz range was constructed. Principal component analysis and support vector machines were used in the model.

Regular trends of Raman scattering in microcrystalline diamond powders as a function of size of the diamond microcavities are investigated in the range of the latter between 1 to 600 μm. The observed effect of anomalously high intensity of spontaneous Raman scattering is attributed to “trapping” of electromagnetic radiation with a wavelength shorter than the size of the diamond microcrystals in diamond microcavities. The electromagnetic energy density for the driving and secondary radiation increases as a result of photon “trapping” in the diamond microcavities. A high Q factor of the fundamental optical mode in the vibrational sp-ectrum of diamond and anomalous increase in the Raman scattering intensity in diamond microcavities pave the way for observation of the low-threshold multifrequency Raman scattering in microcrystalline diamond powders. Using the radiation of a pulsed solid state YAG:Nd³⁺ laser at the fundamental wavelength (λ = 1064 nm) and its optical harmonics (λ = 532, 355, 266 nm) as sources of driving radiation opens up a possibility for creation of an array of equidistant (with respect to frequency shift) laser sources with wavelengths extending from the ultraviolet to the terahertz range promising for investigation of biological and medical objects.

A contact sapphire neurosurgical probe for the removal of brain tumors with the possibility of intraoperative exogenous fluorescence diagnostics and laser coagulation of adjacent blood vessels has been developed. The geometry of the sapphire neuroprobe has been optimized to increase the sensitivity of fluorescence diagnostics. For this purpose, a series of computational experiments have been performed using the Monte Carlo method. The technique for growing a sapphire shaped crystal with a variable cross section has been developed. Using this technique, a pilot prototype of the sapphire neuroprobe has been manufactured. The sample has been approbated experimentally.

We propose a new method of terahertz microscopy for imaging of biological tissues with a subwavelength spatial resolution. It makes it possible to surmount the Abbe diffraction limit and ensures a subwavelength resolution due to the solid immersion effect, i.e., due to decreasing dimensions of the electromagnetic beam caustic as the beam is focused in free space at a small distance (smaller than the wavelength) behind a medium with a high refractive index. An experimental setup that realizes the proposed method is developed. It uses a backward-wave oscillator and a Golay cell as a source and a detector of the terahertz radiation, respectively. In this setup, the radiation is focused behind a silicon hemisphere to realize the solid immersion effect. A record-high spatial resolution of 0.15λ is demonstrated experimentally for optical systems based on the solid immersion effect (the measurements have been performed at a wavelength of λ = 500 μm using a metal–air interface as a test object). Microscopy based on the solid immersion effect does not imply using diaphragms or near-field probes of other types for achieving the subwavelength spatial resolution, and, correspondingly eliminates energy losses associated with these elements. The proposed method has been applied for imaging of soft biological tissues, which has made it possible to demonstrate its potential for the use in biology and medicine.

The spectral dependences of the absorption coefficient, scattering coefficient, scattering anisotropy factor, and transport scattering coefficient for human eye lens in vitro at different stages of cataracts were studied. The spectra of the absorption coefficient and transport scattering coefficient were obtained for the spectral range 400–2300 nm, and the spectra of the scattering coefficient and the scattering anisotropy factor were obtained for the spectral range 400–1800 nm. The wavelength regions in which the spectra of the studied optical characteristics of human eye lens do not differ significantly and, conversely, differ significantly for the samples with different stages of cataract were identified.

A method for wavelet filtration procedure training for optical coherence tomography (OCT) images using the experimental measurements of test objects that were constructed by means of water solutions of monodisperse nanoparticles and several microscopic inclusions has been described in the present paper. The choice of test-object parameters (concentration of water solution, size of nanoparticles, and shape, dimensions, and mutual position of inclusions) has allowed the modeling of various working conditions of OCT and setting different criteria for estimation of filtration efficiency. In the present work, the optimal filter for the considered example of a test object has been selected among the combinations of various basic functions of five wavelet families, soft and hard threshold filtering methods, four decomposition levels, and threshold values in a range of 0.05–3.05. The mutual position of the micro-inclusions has been used as a criterion for evaluating the filtration efficiency. As a result, it has been shown that the determined wavelet filter leads to effective suppression of the scattering noise in OCT images and preserve information about the structure of the object under study.

A new kind of square lattice porous core microstructure fiber is proposed for promising low loss terahertz applications. To analyze the guiding characteristics of proposed fiber Finite Element Method (FEM) based Comsol V4.2 software is used. The proposed fiber structure is very simple and easy to realize. The numerical results ensures that this proposed microstructure fiber exhibits low effective absorption loss of 0.06 cm^–1, low confinement loss of 9.2 × 10^–3 cm^–1, low bending loss of 8.8 × 10^–9 dB/m, and very high power fraction of 47% through the core at 1 THz simultaneously. Moreover, our proposed fiber offers very low dispersion variation of 0.85 ± 0.12 ps/THz/cm over a wide range of frequency from 0.7 to 1.15 THz. The investigated results indicates that this fiber will be a good candidate in terahertz signal transmission as well as different terahertz devices.