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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 (TiO2) 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. [ABSTRACT FROM AUTHOR]

This article describes a sapphire cryoprobe as a promising solution to the significant problem of modern cryosurgery that is the monitoring of tissue freezing. This probe consists of a sapphire rod manufactured by the edge‐defined film‐fed growth technique from Al2O3 melt and optical fibers accommodated inside the rod and connected to the source and the detector. The probe's design enables detection of spatially resolved diffuse reflected intensities of tissue optical response, which are used for the estimation of tissue freezing depth. The current type of the 12.5‐mm diameter sapphire probe cooled down by the liquid nitrogen assumes a superficial cryoablation. The experimental test made by using a gelatin‐intralipid tissue phantom shows the feasibility of such concept, revealing the capabilities of monitoring the freezing depth up to 10 mm by the particular instrumentation realization of the probe. This justifies a potential of sapphire‐based instruments aided by optical diagnosis in modern cryosurgery. [ABSTRACT FROM AUTHOR]

In this work, a thorough analysis of hyperosmotic agents for the immersion optical clearing (IOC) in terahertz (THz) range was performed. It was aimed at the selection of agents for the efficient enhancement of penetration depth of THz waves into biological tissues. Pulsed spectroscopy in the frequency range of 0.1 to 2.5 THz was applied for investigation of the optical properties of common IOC agents. Using the collimated transmission spectroscopy in visible range, binary diffusion coefficients of tissue water and agent in ex vivo rat brain tissue were measured. IOC agents were objectively compared using two‐dimensional nomogram, accounting for their THz‐wave absorption coefficients and binary diffusion coefficients. The results of this study demonstrate an interplay between the penetration depth enhancement and the diffusion rate and allow for pointing out glycerol as an optimal agent among the considered ones for particular applications in THz biophotonics. [ABSTRACT FROM AUTHOR]

A sapphire shaped capillary needle designed for collimating and focusing of laser radiation was proposed and fabricated by the edge‐defined film‐fed growth technique. It features an as‐grown surface quality, high transparency for visible and near‐infrared radiation, high thermal and chemical resistance and the complex shape of the tip, which protects silica fibers. The needle's geometrical parameters can be adjusted for use in various situations, such as type of tissue, modality of therapy and treatment protocol. The focusing effect was demonstrated numerically and observed experimentally during coagulation of the ex vivo porcine liver samples. This needle in combination with 0.22NA optical fiber allows intensive and uniform coagulation of 150 mm3 volume interstitially and 30 mm3 superficially by laser exposure with 280 J without tissue carbonization and fiber damaging along with delicate treatment of small areas. The demonstrated results reveal the perspectives of the proposed sapphire microfocusing needle for laser surgery and therapy. [ABSTRACT FROM AUTHOR]

While fiber bundles attract much interest in optics, their resolution is limited by the individual fiber size, which can be as large as a wavelength λ for common fiber optics materials. In order to increase fiber bundle resolution, in this work, high‐refractive‐index sapphire fibers with n > 3 are employed in the terahertz (THz) range, which ensures strong confinement of the guided modes and enables a fiber bundle resolution beyond the 0.62λ Abbe limit of free space optics. A sapphire fiber bundle is fabricated and its advanced resolution is experimentally demonstrated, using both analysis of the pair correlation function of the disordered fiber packing and continuous‐wave THz imaging. The resolution vares across the aperture with the mean value of 0.53λ, while, at certain regions of the bundle, it is as low as 0.3λ. The developed principles can be translated to any spectral range where high‐refractive‐index fiber optics materials are available. [ABSTRACT FROM AUTHOR]

Crystals are the elementary constituents of Laue lenses, an emerging technology which could allow the realization of a space-borne telescope 10-100 times more sensitive than existing ones, in the 100 keV-1.5 MeV energy range. This paper addresses the development of efficient crystals for the realization of a Laue lens. In the theoretical part, 35 candidate crystals, both pure and two-component crystals, are considered. Their peak reflectivity at 100, 500 keV and 1 MeV is calculated assuming they are mosaic crystals. It is found that, by careful selection of crystals, it is possible to achieve a reflectivity above 30% over the whole energy range, and even up to 40% in the lower part of the energy range. In the experimental part, three different materials (Si1-xGex with a gradient of composition, mosaic Cu and Au) have been measured at both ESRF and ILL using highly monochromatic beams ranging from 300 to 811.6 keV. The aim was to check their homogeneity, quality and angular spread (mosaicity). These crystals have shown outstanding performance, such as reflectivity up to 31% at ∼600 keV (Au) or 60% at 300 keV (SiGe) and angular spread as low as 15 arcsec for Cu, fulfilling very well the requirements for a Laue lens application. An unexpected finding is that there are important discrepancies with Darwin's model when a crystal is measured using various energies. [ABSTRACT FROM AUTHOR]

This contribution presents the growth of Si1−xGex crystals with constant gradient of composition for X‐ray or γ‐ray diffraction optics using automated control of the crystal shape during the growth process. The key idea is to achieve a constant concentration gradient by continuously decreasing the crystal diameter during the main crystal growth phase. For this purpose a concept has been developed for a proper planning of the radius trajectory matching all technological requirements. Si1−xGex single crystals with constant gradients between 0.10 and 0.16 at% cm−1 have been grown using an automated control system. [ABSTRACT FROM AUTHOR]

Terahertz (THz) frequency range opens significant opportunities in various fundamental and applied fields including condensed matter physics and chemistry, biology and medicine, public security and nondestructive testing. Despite significant advances in THz instrumentation, the problem of THz sensing in harsh environments, particularly at high temperatures and pressures, remains acute due to the lack of THz materials and optical components capable for operation under the extreme conditions. To address this problem, the THz hollow‐core photonic crystal sapphire waveguides that are fabricated using shaped crystal growth technique are developed. Numerical analysis and experimental study show that the proposed waveguides operate in a few‐mode regime and allow for the broadband transmission of THz pulses with small dispersions and low propagation losses. Thanks to the unique physical properties of sapphire, the proposed waveguides are capable of operating in a variety of aggressive environments. As an example, the developed waveguides are used to conduct the intra‐waveguide interferometric sensing of phase transitions in sodium nitrite films at high temperatures. It is believed that the proposed sapphire‐based material's platform has strong potential for developing THz guided optics for applications in intra‐waveguide spectroscopy, interferometry, and remote sensing in aggressive environments. In order to address the problem of terahertz (THz) sensing in harsh environments, the hollow‐core photonic crystal sapphire waveguides are developed and fabricated using shaped crystal growth technique. Thanks to the unique physical properties of sapphire, the developed THz waveguides can be used in intra‐waveguide spectroscopy, interferometry, and remote sensing in aggressive environments, at high temperatures and pressures. [ABSTRACT FROM AUTHOR]