In our work we demonstrate a reflection-mode polarization-sensitive terahertz (THz) microscope based on solid immersion effect. We apply it to study THz anisotropy of test media with $0.15 \lambda$ spatial resolution, including freshly excised rat brain, where the most pronounced birefringence is observed in the Corpus callosum. The obtained results show the prospects of applying THz polarization-sensitive microscopy in medical imaging.

In this research, we aim to quantitatively study the changes in optoelectronic properties of single-walled carbon nanotubes (SWCNTs) induced by defects. We employ various spectroscopic techniques, including Raman spectroscopy, UV-vis-NIR absorbance spectroscopy, Fourier-transform infrared spectroscopy (FTIR), THz time domain spectroscopy, and nonlinear terahertz spectroscopy. By analyzing the spectroscopic data, we strive to establish empirical relationships between the observed parameters and the concentration of oxygen defects. Our findings can be applied to predict and control the optoelectronic behavior of SWCNTs in various applications

The development of terahertz (THz) imaging methods is hampered by the low spatial resolution of traditional diffraction-limited imaging systems, mainly due to the large wavelength of used radiation (from a few of mm to tens of µm). To solve this problem, we have proposed a new method of THz endoscopy with subwavelength spatial resolution, which is designed to study hard-to-reach areas of living organisms in vivo. A hollow-core sapphire tube with polytetrafluoroethylene outer coating is used as a waveguide, in which the antiresonant principle of radiation transmission is implemented. The waveguide and the immersion lens are optimized to provide high optical characteristics in a given wavelength range to ensure the best focusing. Two immersion lenses made of sapphire and silicon were developed and fabricated, which were then mounted on plane-parallel windows fixed on the rear end of the waveguide. The study of the field intensity distribution on the shadow side of the “waveguide–lens” system revealed a focal spot diameter of ≃0.2λ in the case of a lens made of sapphire and ≃0.3λ in the case of a lens made of crystal silicon at a wavelength λ = 500 µm, which significantly exceeds the Abbe diffraction limit. This agrees with our numerical predictions and demonstrates the promise of using the proposed endoscope for measurements with subwavelength resolution.

Bundles or bundles of optical fibers are finding increasing applications in various areas of fiber optics, despite the relatively low resolution of such devices, which does not exceed the wavelength λ. One way to solve this problem is to use materials with a high refractive index, which will allow for strong localization of radiation modes in the fiber. This review discusses the use of sapphire fibers with a high refractive index of n>3 for these purposes. They are used as the basis for fiber bundles operating in the terahertz (THz) range and provide visualization with spatial resolution exceeding the Abbe diffraction limit for free space. Bundles of sapphire fibers of various configurations have been manufactured, consisting of arrays of parallel and non-parallel fibers, and theoretical and experimental evaluations of their spatial resolution have been obtained using both analysis of the paired correlation function of disordered fiber packing and THz imaging. In particular, for a bundle consisting of parallel fibers with metallic coating, the resolution varies with aperture with a mean value of 0.53.λ and can reach 0.3 λ in individual areas. For a bundle with diverging fibers with dielectric coating, the resolution is 0.35 λ, which significantly exceeds the Abbe limit. The developed principles can be transferred to any spectral range where materials for fiber optics with a high refractive index are available. Finally, methods for reconstructing THz images of test binary objects obtained using the proposed bundles are described.

The task of monitoring the condition of the tissue during its cryodestruction is extremely relevant for cryosurgery. Previously, the concept of a sapphire cryoprobe was proposed, which makes it possible to detect diffusely scattered light from a tissue during an ice ball formation. This probe combines the advantages of sapphire as a promising material for cryosurgery, as well as the possibility of assessing the depth of tissue freezing in the contact area. The use of several light source channels inside the applicator, spaced at different distances from the detector channel, makes it possible to analyze the scattering properties of the medium using the methods of diffusion theory. In this paper, we consider the influence of the position and number of analyzed source channels on the signals recorded by the detector channel and the determined effective scattering coefficient of a two-component medium consisting of an iceball and unfrozen tissue. Differences in the scattering coefficient obtained for various channel configurations are shown, as well as the advantages of analyzing a large number of channels to describe the effective properties of the medium with a complex iceball boundary.

We report the seminal approach for localization of photocarriers in a photoconductive antenna (PCA)-emitter via the focusing element comprising the sapphire fiber. Using numerical simulation, we showed that at the certain ratio between the fiber diameter and the gap size, i.e. d/g = 22.5, one can attain a 35-fold enhancement of the laser irradiation in vicinity of the PCA electrodes. This provides the formation of subwavelength EM wave caustics located at the edges of the PCA electrodes which potentially gives rise to an increase of optical-to-terahertz conversion efficiency.

We report on the experimental study of the photoconductive antennas (PCAs) - detectors based on superlattice heterostructures (SLS) InGaAs/InAs/InAlAs with different types of elastic stresses in their functional layers. By using our laboratory time-domain THz spectrometer we measured and compared the detected THz signals, noise characteristics, and signal-to-noise ratios of the developed bow-tie PCA-detectors at different average probe power. We showed that the SLS-based PCA-detector with elastic stresses of both compression and tension in the layers demonstrates increased THz detection bandwidth compared to that for the SLS-based PCA-detector with only compression stresses in the SLS layers in the whole range of the optical probe power. We thus demonstrate that modification of the SLS via introduction of the elastic strain in the crystalline lattice of its layers could become an efficient approach to enhance the PCA-detectors performance leading to implementation of these PCAs to the spectroscopic THz setups.

We report on experimental study of the photoconductive antennas (PCA)-emitters featuring conventional topology as well as metallic metasurface with plasmonic grating, based on the InGaAs/InAlAs superlattices. We measure and determine the photocurrents and the emission spectra of the PCAs, as well as the energy characteristics of the terahertz (THz) radiation, and the efficiency of the optical-to-THz conversion for different bias voltages and average laser excitation power. The integral THz emission power of 10 μW as well as conversion efficiency of 0.2% are demonstrated for the PCA with the metasurface, which are not reachable for the conventional due to thermal breakdown of the antenna. Therefore, the PCAs with the metasurface can be considered to be the attractive THz emitters that can potentially become among the elemental base for modern THz spectroscopic systems for biomedical applications.

Terahertz (THz) radiation and related technologies are rapidly developed and find their application in different branches of science and technology, including medical diagnostics and therapy. This poses a problem of determining the safety limits of the human body exposure to THz radiation, which is closely related to a problem of biological activity of THz waves. Therefore, in this work, the current state of research in the area of THz radiation – cells interaction is overviewed.

With a change in temperature, a-lactose monohydrate crystals undergo changes in the molecular structure due to dehydration and decay of intramolecular bonds. The transmission spectra of the pressed microcrystalline samples of a-lactose monohydrate were measured using terahertz pulsed spectroscopy in the temperature range of the existence of the solid phase 10 – 475 K. An analysis of the observed absorption lines using the classical oscillator model made it possible to reveal the complex temperature evolution of the eigenfreuencies of the resonances, as well as to determine the region of existence of response phase of a-lactose monohydrate. Data obtained can find practical application in various fields of terahertz optics, including pharmacology, food industry, analytical chemistry and biophotonics.

The transmission spectra of the most common hyperosmotic agents, such as pure glycerol, propylene glycol (PG), dimethyl sulfoxide (DMSO), polyethylene glycol (PEG) with 200, 300, 400 and 600 Da molecular weights, and their aqueous solutions, as well as aqueous solutions of sucrose, glucose, fructose, dextran 40 and 70 were measured. The experiments were carried out using a THz pulsed spectrometer with a vacuum measuring compartment to reduce the effect of water vapor on spectral measurements. The dielectric properties of hyperosmotic agents were restored in the spectral range from 0.1 to 2.5 THz and the dependence of the amplitude absorption coefficient on the concentration of considered agents at 0.5 THz frequency was constructed. The obtained results make it possible to choose the optimal agents for immersion optical clearing in the THz range.

A novel method of terahertz (THz) microscopy was proposed for imaging of biological tissues with sub-wavelength spatial resolution. It allows for overcoming the Abbe diffraction limit and provides a sub-wavelength resolution thanks to the solid immersion effect – i.e. to the reduction in the dimensions of electromagnetic beam caustic, when the beam is focused in free space, at a small distance (smaller than the wavelength) behind the medium featuring high refractive index. An experimental setup realizing the proposed method was developed. It uses a backward wave oscillator, as a THz-wave emitter, and a Golay cell, as a THz-wave detector. In this setup, the radiation is focused behind the silicon hemisphere in order to realize the solid immersion effect. The spatial resolution of 0.15λ was demonstrated for the developed microscope, while the measurements were carried out at the wavelength of λ=500 μm, with the metal-air interface as a test object. Such a high spatial resolution represents a significant advantage over that of the previously reported arrangements of solid immersion microscopes. The solid immersion microscopy does not imply using any diaphragms or other near-field probes for achieving the sub-wavelength spatial resolution; thus, it eliminates the energy losses associated with such elements. The proposed methods were applied for imaging of biological tissues, and the observed results highlight its potential in biology and medicine.

A method for differentiation of pigmented skin neoplasms based on digital processing of optical images has been proposed. The optical images are detected using a digital camera and an etalon for its color and spatial calibration. The method implies segmentation and automatic differentiation of neoplasms based on 5 parameters –the diameter, area, color, shape and smoothness of margins of the neoplasm. It reveals clinical features of the neoplasm, required for diagnosis, and allows for calculating a probability of the neoplasms malignizaion. The proposed method has been verified using 360 imaging of pigmented neoplasms of the skin in vivo; among them: ordinary, dysplastic nevi of the skin and skin melanoma. The observed sensitivity and specificity of the proposed technique are 97% and 95%, correspondingly.

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.

AbstractWe propose a terahertz (THz) plasmonic photoconductive antenna (PCA) with a record height of its metal electrodes of h = 100 nm and a high aspect ratio of h / p = 0.5 ( p is the period of the plasmonic grating) that can be used as a source is THz pulsed spectroscopic and imaging systems. We experimentally demonstrate that the power of the THz radiation generated by the proposed plasmonic PCA is two orders of magnitude higher than that of an equivalent ordinary PCA without a plasmonic grating. Current–voltage measurements of the thus developed plasmonic PCA under femtosecond laser excitation show that the photocurrent of the PCA increases 15-fold, up to i _p ≈ 1.2 mA. To reduce the leakage currents of the PCA, we propose a fabrication technology that is based on the etching of windows in a thin Si_3N_4 passivation dielectric layer deposited on the photoconductor surface, which makes it possible to reduce the dark current to i _d ≈ 5 μA.

AbstractA 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.

AbstractA 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.

В последние десятилетия растет интерес к применению терагерцовой (ТГц) техники в ранней неинвазивной, малоинвазивной и интраоперационной диагностике злокачественных новообразований различной нозологии и локализации. Методы ТГц диагностики основаны на ТГц спектроскопии и визуализации тканей, при этом используются естественные (эндогенные) маркеры новообразования. Отмеченное делает ТГц диагностику привлекательной в сравнении с другими методами, зачастую включающими введение в организм экзогенных маркеров. Несмотря на значительный прогресс в рассматриваемой области, ТГц инструменты далеки от клинической практики из-за высокой стоимости, громоздкости, низкой эргономичности и отсутствия эффективной ТГц элементной базы. В данной работе, состоящей из двух частей, обсуждается современное состояние исследований в области диагностики злокачественных новообразований с помощью ТГц спектроскопии и визуализации. В первой части рассматриваются свой­ства ТГц излучения, специфика его взаимодействия с биологическими тканями. Отдельное внимание уделяется ТГц импульсной спектроскопии, находящей все большие применения в ТГц биофотонике.

Предложена широкоапертурная асферическая линза для фокусировки пучка терагерцового (ТГц) электромагнитного излучения в кружок субволнового размера.Расчет линзы и оценка размера формируемой ей каустики осуществлялись с помощью вычислительных методов геометрической оптики и электродинамики.Линза изготовлена из полиэтилена высокой плотности с помощью токарного станка.Для экспериментальной оценки пространственного разрешения, обеспечиваемого линзой, создана ТГц изображающая система, основанная на растровом сканировании поверхности объекта.Изображающая система в сочетании с