Application of a hemispherical rutile lens in THz solid immersion microscopy to achieve super-resolution

Solid immersion (SI) microscopy is a promising approach to overcome the diffraction limit by focusing light beam at a short distance behind a medium with a high refractive index. Due to the absence of subwavelength probes and apertures in the optical system, this method allows achieving high-energy efficiency. The super-resolution and high optical throughput of SI microscopy together open up possibilities for its application in various fields of science and technology. Spatial resolution of SI microscope is mainly limited by the refractive index of the SI lens, so that denser lenses allow higher resolution to be achieved. In this work, a bulk crystal of rutile (TiO$_2$), characterized by an extremely high refractive index of $\sim$10 in the terahertz range, is used for the first time to produce SI lens. This is the highest refractive index value ever used in SI microscopy. We assembled an original SI microscope utilizing a IMPATT diode as a continuous-wave terahertz source at a frequency of 0.2 THz (wavelength $\lambda$ = 1.5 mm) and a Golay detector. The spatial resolution of our microscope is in the range of 0.06–0.11$\lambda$ was obtained by experimental investigations. This is the highest resolution ever recorded for any SI optical system.