Combination of hollow-core sapphire waveguide and immersion lens for ultra-high resolution THz endoscopy

The development of terahertz (THz) imaging methods is hampered by the low spatial resolution of traditional diffraction-limited imaging systems, mainly due to the large wavelength of used radiation (from a few of mm to tens of $\mu$m). To solve this problem, we have proposed a new method of THz endoscopy with subwavelength spatial resolution, which is designed to study hard-to-reach areas of living organisms in vivo. A hollow-core sapphire tube with polytetrafluoroethylene outer coating is used as a waveguide, in which the antiresonant principle of radiation transmission is implemented. The waveguide and the immersion lens are optimized to provide high optical characteristics in a given wavelength range to ensure the best focusing. Two immersion lenses made of sapphire and silicon were developed and fabricated, which were then mounted on plane-parallel windows fixed on the rear end of the waveguide. The study of the field intensity distribution on the shadow side of the glqq “waveguide–lens”system revealed a focal spot diameter of $\simeq$0.2$\lambda$ in the case of a lens made of sapphire and $\simeq$0.3$\lambda$ in the case of a lens made of crystal silicon at a wavelength $\lambda$ = 500 $\lambda$m, which significantly exceeds the Abbe diffraction limit. This agrees with our numerical predictions and demonstrates the promise of using the proposed endoscope for measurements with subwavelength resolution.