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We have proposed an approach to improve the resolution of terahertz (THz) imaging using the effect of solid immersion – i.e. a reduction in the dimensions of the electromagnetic beam caustic by its formation in a free space, behind the medium possessing high refractive index. We have designed a THz solid immersion lens (SIL) comprised of a polymer wide-aperture aspherical singlet and a crystalline truncated sphere mounted in front of the image plane. We have proposed an approach for the object handling at the focal plane of the THz SIL. We have combined numerical simulations and experimental studies to demonstrate the advanced 0.2λ spatial resolution of the proposed THz SIL – it is beyond the 0.5λ Abbe limit. Finally, we have assembled the continuous-wave THz SIL imaging system and applied it for studying various objects with sub-wavelength variations of structure and dielectric properties.

We have proposed a method of THz solid immersion microscopy, which yields imaging soft biological tissues with the sub-wavelength resolution up to 0.2-wavelengths. To achieve this advanced resolution, it employs a solid immersion phenomenon – i.e. a reduction in the dimensions of the THz beam caustic by its formation on a small distance behind the medium featuring high refractive index. We have assembled an experimental setup, which realizes the principles of the THz solid immersion microscopy, and proposed an approach for handling the soft tissue at the object plane. This setup uses a backwardwave oscillator, as a source of continuous-wave THz radiation, and a Golay cell, as a detector of the THz field intensity. We have examined the resolution of the THz solid immersion microscopy using both numerical simulations and experimental studies. Finally, in order to highlight the prospective of the proposed THz imaging modality, we have applied the experimental setup for imaging of representative examples of biological tissues.

We have developed a method of terahertz (THz) solid immersion (SI) microscopy for continuous-wave reflection-mode imaging of soft biological tissues with a sub-wavelength spatial resolution. In order to achieve strong reduction in the dimensions of the THz beam caustic, an electromagnetic wave is focused into the evanescent field volume behind a medium with a high refractive index. We have experimentally demonstrated a 0.15λ-resolution of the proposed imaging modality at λ = 500 μm, which is beyond the Abbe diffraction limit and represents a considerable improvement over the previously-reported arrangements of SI imaging setups. The proposed technique does not involve any sub-wavelength near-field probes and diaphragms, thus, avoiding the THz beam attenuation due to such elements. We have applied the developed method for THz imaging of various soft tissues: a plant leaf blade, cell spheroids, and tissues of the breast ex vivo. Our THz images clearly reveal sub-wavelength features in tissues, therefore, promising applications of THz SI microscopy in biology and medicine.