Terahertz (THz) solid immersion microscopy is a modality of THz imaging, which allows one to overcome the Abbe diffraction limit and provides high energy efficiency due to the absence of subwavelength apertures and probes in an optical scheme. It exploits the effect of a reduction in dimensions of electromagnetic-wave caustic, when it is formed in free space, at a small distance (<λ, where λ is an electromagnetic wavelength) behind a material with high refractive index. In our previous study, we introduced an original arrangement of the THz solid immersion lens (SIL), which provides superior spatial resolution of 0.15λ and is capable of imaging soft biological tissues. We applied the finite-difference time-domain technique for solving Maxwell’s equations in order to estimate the resolution limit and the depth of field for the proposed SIL arrangement as well as to define the confidence intervals for the alignment of optical elements. Next, we described the continuous-wave THz solid immersion microscope, which relies on the proposed SIL and exploits a backward-wave oscillator and a Golay cell as an emitter and a detector, respectively. Finally, we studied experimentally the spatial resolution of this microscope and visualized several representative objects featuring subwavelength structural inhomogeneities. The observed results revealed potential of the THz solid immersion microscopy in nondestructive testing and biophotonics.

Sapphire capillary needles fabricated by edge-defined film-fed growth (EFG) technique hold strong potential in laser thermotherapy and photodynamic therapy, thanks to the advanced physical properties of sapphire. These needles feature an as-grown optical quality, their length is tens of centimeters, and they contain internal capillary channels, with open or closed ends. They can serve as optically transparent bearing elements with optical fibers introduced into their capillary channels in order to deliver laser radiation to biological tissues for therapeutic and, in some cases, diagnostic purposes. A potential advantage of the EFG-grown sapphire needles is associated with an ability to form the tip of a needle with complex geometry, either as-grown or mechanically treated, aimed at controlling the output radiation pattern. In order to examine a potential of the radiation pattern shaping, we present a set of fabricated sapphire needles with different tips. We studied the radiation patterns formed at the output of these needles using a He–Ne laser as a light source, and used intralipid-based tissue phantoms to proof the concept experimentally and the Monte-Carlo modeling to proof it numerically. The observed results demonstrate a good agreement between the numerical and experimental data and reveal an ability to control within wide limits the direction of tissue exposure to light and the amount of exposed tissue by managing the sapphire needle tip geometry.