Despite terahertz (THz) technologies offer a number of applications in medical diagnosis and therapy, their translation into clinics is hampered by the lack of THz endoscopes capable of sensing THz response of hard-to-access tissues. In this paper, we focus on recent attempts to mitigate this difficulty. We consider the two existing principles of THz endoscopy. The first uses the fiber-coupled THz photoconductive antennas (PCAs) for the THz generation and detection in close proximity to an object. The second relies on the THz optical fibers to deliver THz waves to an analyte and then to detect the reflected THz signal. Most recent developments in the area of THz fiber optics pave the way to solve the challenging problem of THz endoscopy. Among them, we emphasize the THz fibers, fiber bundles, waveguides, and endoscopes developed by our research group based on the sapphire shaped crystals obtained by the edge-defined film-fed growth (EFG) technique.

Abstract While the existing terahertz (THz) fiber optics components suffer from high loss and dispersion, low technological reliability, poor environmental resistance and radiation strength, as well as large cross-section, THz applications in different fields still require hardware for the sensing and exposure of hard-to-access objects. To mitigate this difficulty, we develop the two variants of hollow-core THz waveguides, those exploit the antiresonant reflecting optical waveguiding (ARROW) mechanism and use (as a key element) a few-millimeter-diameter sapphire tube produced by the edge-defined film-fed growth (EFG) technique. In the all-dielectric arrangement, the outer surface of this tube is coated by a sub-millimeter-thick polytetrafluoroethylene (PTFE) film, while in the metal-coated one—by a sub-micrometer-thick reflecting copper layer. These coatings increase the guiding efficiency and underlie different performance of the two geometries. Both waveguides are studied numerically and experimentally in the 0.56–0.7 THz frequency range. The observed discrepancies between the theoretical and measured propagation loss are attributed to fluctuation of the cross-section geometry over the waveguide length. In narrow frequency bands, the metal-coated waveguide offers the propagation loss as small as 5.0 dB/m, which is significantly lower than that of the all-dielectric one. Furthermore, the outer metal coating completely prevents mode leakage, whereas in an all-dielectric waveguide, some of the evanescent field extends into the surrounding space and still can be de-coupled. Our findings highlight that the ARROW sapphire THz waveguides provide a reasonable compromise between the guiding efficiency and the cross-section dimensions, thus, forming a favorable platform for the THz sensing and exposure.

Broadband experimental data in the THz-IR region are used to analyze the temperature evolution of optical parameters of sapphire in the temperature range of 77–300 K. Temperature dependences of the refractive index of ordinary and extraordinary rays were obtained and, based on these data, temperature dependence of birefringence in the THz range was calculated. The parameters of optical phonon modes and many-particle processes in the THz-IR region were obtained by using classical oscillator and four-parameter generalized models. Estimates of the contributions of optical phonon modes and many-particle processes to absorption in the THz range in the temperature range of 77–300 K are presented. The obtained data made it possible to establish the operating frequency region and dynamic range of sapphire optical elements for application in THz devices at low and cryogenic temperatures.

In this work, we discuss the advantages of sapphire shaped crystals for manufacturing of medical instruments, in particular, capillary needles for interstitial laser therapy. The application of them for tissue ablation and coagulation is discussed.

The growing demand for composite materials capable of enduring prolonged loads in high-temperature and aggressive environments presents pressing challenges for materials scientists. Ceramic materials composed of silicon carbide largely possess high mechanical strength at a relatively low density, even at elevated temperatures. However, they are inherently brittle in nature, leading to concerns about their ability to fracture. The primary objective of this study was to develop a novel technique for fabricating layered composite materials by incorporating SiC-based ceramics, refractory metals, and their silicides as integral constituents. These layered composites were produced through the liquid-phase siliconization method applied to metal–carbon blanks. Analysis of the microstructure of the resultant materials revealed that when a metal element interacts with molten silicon, it leads to the formation of a layer of metal silicide on the metal’s surface. Furthermore, three-point bending tests exhibited an enhancement in the bending strength of the layered composite in comparison to the base silicon carbide ceramics. Additionally, the samples demonstrated a quasi-plastic nature during the process of destruction.

THz technologies developments into practice are limited by the absence of commercially available THz endoscopic systems. Previously, the transmission properties of waveguides, fibers and even fiber bundles based on shaped sapphire were studied. Sapphire hollow-core waveguides are suitable for efficient radiation transmission with minimal losses and for applications in endoscopy of hard-to-access objects and highresolution imaging.

Experimental observation of the enhanced terahertz (THz) emission in a large-area photoconductive antenna-emitter (LAE), boosted by an array of cylindrical sapphire-fiber-based microlenses, is reported. The observed enhancement is achieved, thanks to the sharp focusing of a pump laser beam near the semiconductor surface, for which the high-refractive-index sapphire lenses are used. We predict numerically and confirm experimentally a considerable enhancement in the emitted THz spectral power for such a sapphire-fiber-coupled LAE, as compared to an ordinary one with an equal electrode topology. In fact, a ≃8.5-fold THz power boost is achieved, resulting in a +9.3 dB increase in the dynamic range. The results of our findings can be used to improve the performance of large-area THz devices, aimed at meeting the demands of rapidly developed THz spectroscopy, imaging, sensing, and exposure technologies.

In sapphire needle capillaries, we analyze the form of the internal channel and the needle tip and their influence on the shape of the outcoming beam. We propose some methods of alteration of the capillary shape via growth conditions that contribute to obtaining the required geometry.

Consideration of sapphire shaped crystals as the material for manufacturing of medical instruments expands the opportunities of various approaches for diagnostics, exposure and treatment. Due to physical, mechanical and chemical properties of sapphire, as well as to its complex shape, such instruments are capable to demonstrate better performance for medical applications comparing to common tools. However, the manufacturing of high quality sapphire crystal with such geometry is still a complex issue, that usually requires application of various crystal growth techniques assisted with the automated weight control system. In this work, we consider one of such cases, that is the growth of a sapphire crystal, which can be applied for cryosurgery as an applicator due to a hollow-monolithic shape transition. Its hollow part can be filled with coolant in order to enable fast freezing of biological tissue during application. For this aim, it is of high importance to exclude the appearance of inclusions during the shape transition. To overcome this problem, we suggest using of noncapillary shaping (NCS) technique of crystal growth and study the weight signal measured during the manufacturing. We obtain the analytical description of the weight signal alteration that can be used as the program equation to control the crystal shape. We experimentally demonstrate the advantage of using such crystal for cryosurgery and obtaining faster ice-ball formation inside the model gelatin-based medium in comparison with the usage of the monolithic sapphire applicator of the same diameter. The demonstrated ability can be applied for future development of cryosurgical tools, while the analytical description of the weight signal could find its application for NCS manufacturing of sapphire crystals for other purposes.

Compact sapphire capillary needles for laser-assisted therapy connected to optical fibers by the means of internal channel closed from one side transmit laser radiation to the tissues with minimal loss of energy and simultaneously protect the fibers. The geometry of the internal capillary channel with the curved bottom is a key factor in the formation of the output beam’s shape. Various curvatures of the bottom in combination with the different angle of conical needle’s tip are studied in detail. When the tissue is covered in a liquid, we have observed forming the ring radiation pattern with a sharp axial peak without additional diaphragms or spatial light modulators. Coagulation of ex vivo liver tissue samples using continuous laser source with 1.06 μm wavelength, offered opportunity to obtain small coagulated spots without carbonization with the diameter of at least 0.8 mm and to enlarge them evenly by tuning the source output power.

Solid immersion microscopy is a near‐field imaging modality that overcomes the Abbe diffraction limit by focusing the light beam behind a high refractive index lens. It offers high energy efficiency, thanks to the absence of any sub‐wavelength probes or apertures in the optical path. A favorable combination of superresolution and high optical throughput opens up a variety of imaging applications in different branches of science and technology. The spatial resolution of solid immersion microscopy is mostly limited by the refractive index value of the lens, with optically denser lenses offering higher resolutions. In this paper, bulk rutile (TiO 2 ) crystal is used as a material for the solid immersion lens, which offers an impressive refractive index of ≈10 in the terahertz range. This is the highest value of refractive index ever used in solid immersion microscopy. A continuous wave impact ionization avalanche transit‐time diode‐emitter at the 0.2 THz frequency (the λ = 1.5 mm wavelength) and a Golay detector are used for building a solid immersion microscope. Numerical and experimental studies reveal 0.06–0.11λ resolution of the developed microscope. This is the highest normalized resolution ever reported for any solid immersion imaging systems.

While terahertz (THz) technology offers a variety of applications in medical diagnosis, nondestructive testing, and quality control, its acceptance in these practical fields is hampered by the absence of endoscopic systems, capable of sensing the complex refractive index of the hard-to-access objects. In this paper, we develop the THz endoscope based on the hollow-core antiresonant waveguide, formed by a polytetrafluoroethylene (PTFE)-coated sapphire tube with the outer end closed by a monolithic sapphire window. The endoscope is attached to the backward wave oscillator spectrometer to measure the sample reflectivity. By studying the well-known liquid and solid samples, we demonstrate that analysis of the Fabry–Pérot resonance in the measured reflection spectra makes it possible to quantify the complex refractive index of an analyte. Thanks to the advanced chemical inertness and thermal strength of sapphire and PTFE, the developed endoscope is capable of operation in harsh environments, which broadens the range of its applications. Our findings pave the way for the THz technology use in a number of demanding practical fields.

— The tribological properties of carbon–carbon antifriction composite materials reinforced with carbon fabric based on polyacrylonitrile and viscose raw materials have been studied. Tribological tests were carried out according to the ring–disc scheme paired with silicon carbide ceramics under dry friction conditions with different orientations of the composite fabric layers relative to the friction surface in the temperature range of 80–100°C. Dependences of the friction coefficient and wear rate on the fabric orientation relative to friction surface, structure of the composite, and properties of its structural components were obtained at a fixed load and sliding speed. The surface of composites was analyzed after tribological tests using scanning electron microscopy and optical profilometry. The composites friction and wear mechanisms for different contact configurations and different material properties have been revealed. The characteristic features of composite individual structural (fibers, fiber bundles, layers of reinforcing fabric) frictional destruction have been determined. It has been established that the film of wear products formed on the friction surface has a decisive influence on the tribological characteristics of the studied materials. Combinations of the fabric base of the composite, its orientation relative to the friction surface, and the heat treatment mode of the material were determined, which simultaneously provide increased wear resistance and reduced friction in tandem with a ceramic counterbody.

In this work, sapphire fibers formed using the modified Stepanov/EFG method were tested for strength. A study of the surface of sapphire fibers obtained from the melt shows that the roughness of the fibers arises mainly due to its fluctuation in ascending gas flows during the growth process. The paper investigates the effect of fiber surface roughness on its strength. To reduce the roughness, the fiber diameter stabilization system was used, which enable to reduce the roughness parameters to tens of nanometers. When testing according to the original method, it was found that a decrease in the surface roughness of the fiber leads to an increase in its strength. And the strength of the fibers decreases with length in a power law.

The processes occurring on the contact surface during friction of experimental carbon–carbon composite materials reinforced with a carbon fabric based on polyacrylonitrile (PAN) and a viscose precursor were studied. Tribological tests of the composites were carried out on a tribometer using a ring–disk contact scheme. Two materials were used as counterbodies: hardened steel and silicon carbide ceramics. It has been established that the wear rate and friction coefficient under the load-speed modes used in operation mainly depend on the choice of the counterbody material. Heat treatment (carbonization or graphitization) and precursor material (PAN or viscose) also have an effect, but depending on the selected counterbody. It is shown that when tested with a ceramic counterbody, the tribological characteristics (friction coefficient and wear resistance) are better compared to friction paired with a steel counterbody. The surfaces of counterbodies and composites were studied before and after tribological tests by scanning electron microscopy, X-ray spectral analysis, and optical profilometry. It is shown that during friction a film of secondary structures is formed on the surface of carbon composites from wear products. This is the determining factor that affects tribological characteristics. After testing with a steel counterbody, a significant amount of iron was found on the surface of the composites in the film, which indicated wear of the counterbody. This process negatively affects the tribological properties of the composites. At the same time, the ceramic counterbody practically does not wear out, which makes it a more preferable material for working in friction units paired with a carbon composite.

One of the urgent tasks of modern medicine is to detect microcirculation disorder during surgery to avoid possible consequences like tissue hypoxia, ischemia, and necrosis. To address this issue, in this article, we propose a compact probe with sapphire tip and optical sensing based on the principle of spatially resolved diffuse reflectance analysis. It allows for intraoperative measurement of tissue effective attenuation coefficient and its alteration during the changes of tissue condition, caused by microcirculation disorder. The results of experimental studies using (1) a tissue-mimicking phantom based on lipid emulsion and hemoglobin and (2) a model of hindlimb ischemia performed in a rat demonstrated the ability to detect rapid changes of tissue attenuation confirming the feasibility of the probe to sense the stressful exposure. Due to a compact design of the probe, it could be useful for rather wide surgical operations and diagnostic purposes as an auxiliary instrument.

This work describes a sapphire cryo-applicator with the ability to sense tissue freezing depth during cryosurgery by illumination of tissue and analyzing diffuse optical signals in a steady-state regime. The applicator was manufactured by the crystal growth technique and has several spatially resolved internal channels for accommodating optical fibers. The method of reconstructing freezing depth proposed in this work requires one illumination and two detection channels. The analysis of the detected intensities yields the estimation of the time evolution of the effective attenuation coefficient, which is compared with the theoretically calculated values obtained for a number of combinations of tissue parameters. The experimental test of the proposed applicator and approach for freezing depth reconstruction was performed using gelatin-based tissue phantom and rat liver tissue in vivo. It revealed the ability to estimate depth up to 8 mm. The in vivo study confirmed the feasibility of the applicator to sense the freezing depth of living tissues despite the possible diversity of their optical parameters. The results justify the potential of the described design of a sapphire instrument for cryosurgery.

Application of optical coherence tomography (OCT) in neurosurgery mostly includes the discrimination between intact and malignant tissues aimed at the detection of brain tumor margins. For particular tissue types, the existing approaches demonstrate low performance, which stimulates the further research for their improvement. The analysis of speckle patterns of brain OCT images is proposed to be taken into account for the discrimination between human brain glioma tissue and intact cortex and white matter. The speckle properties provide additional information of tissue structure, which could help to increase the efficiency of tissue differentiation. The wavelet analysis of OCT speckle patterns was applied to extract the power of local brightness fluctuations in speckle and its standard deviation. The speckle properties are analysed together with attenuation ones using a set of ex vivo brain tissue samples, including glioma of different grades. Various combinations of these features are considered to perform linear discriminant analysis for tissue differentiation. The results reveal that it is reasonable to include the local brightness fluctuations at first two wavelet decomposition levels in the analysis of OCT brain images aimed at neurosurgical diagnosis.

Compact and cost-effective spectrometers and imaging systems in the terahertz (THz) frequency range based on optical-THz photoconductive converters of ultrashort laser pulses (photoconductive antennas PCAs) are actively being developed and widely used to solve fundamental and applied problems in a variety of fields of science and technology. This high activity of research and development is associated with the PCAs' reliability and compact size, the easy scalability of a single element to 1D and 2D arrays, and PCAs' ability to provide a wide spectral range and high dynamic range of recorded THz signals without cooling. Recently, systems for multi-pixel detection of THz radiation based on matrix PCA detectors, designed to greatly increase the speed of THz imaging, have been of particular interest. This review presents the latest trends in the development of PCA-based THz devices, PCA-based methods of THz pulsed spectroscopy and imaging, as well as alternative approaches to THz pulse recording and THz imaging.

Single-crystal sapphire fibers are obtained by the Stepanov/edge-defined film-fed growth (EFG) method. The procedure for obtaining them is described. Mechanical testing of the fibers is carried out according to the presented scheme, and the dependences of the limiting deformation and strength of the fibers on the length are determined. The dependences are of the power-law type and decrease with the length of the fibers. The strength of the obtained fibers corresponds to the international standard and meets the conditions for their use as reinforcing fibers for high-temperature composite materials. From blanks containing layer-by-layer unidirectionally arranged sapphire fibers, niobium powder, and metal foils of molybdenum and aluminum, layered-fibrous composites are obtained by solid-phase diffusion welding under load. Using scanning electron microscopy with X-ray analysis, the structure of the composites is studied. It is found that, in addition to the initial components, it includes intermetallic compounds of niobium, molybdenum, and aluminum, as well as solid solutions of these metals formed during the technological process. As a result of mechanical testing of the composite samples, deformation curves of load–deformation dependences are obtained, which, together with the developed fracture surfaces, indicate the nonbrittle nature of the fracture of composites containing brittle components. The dependences of the strength of the composites on temperature in the range of 20–1400°C are obtained, which meet the requirements for high-temperature structural materials of this kind.