Terahertz microscope based on solid immersion effect for imaging of biological tissues

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 0.15$\lambda$ was demonstrated for the developed microscope, while the measurements were carried out at the wavelength of $\lambda$=500 $\mu$m, 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.