Composite TiO2/polypropylene solid immersion lenses for super-resolution terahertz imaging

Abstract

Terahertz (THz) near-field imaging promises new advances in medical diagnostics and material characterization. However, its spatial resolution is limited by the light diffraction on lenses/mirrors, which limits the optical resolution of a standard free-space imaging system to ~λ/2 (Abbe limit). Alternatively, super-resolution imaging can be achieved by employing a solid immersion lens (SIL) as the spot size of the focused THz beam is further reduced by a factor of 1/n, where n is the refractive index (RI) of the lens material. In this work, we present the design and fabrication of hemispherical THz SILs using powder mixes of titanium dioxide (TiO₂) and polypropylene (PP) having n_Tio2≈10 and n_pp≈1.51 at 1.0 THz. We present two different lens fabrication strategies that are simple and cost-effective. The first one uses pressing the TiO₂ powder with a PP powder at the Vicat temperature of PP while controlling the concentration of TiO₂ and the resultant lens porosity. The second one uses pressing the TiO₂ powder in a hollow hemisphere that is 3D printed using PP. The fabricated lenses are then characterized optically, and their RIs are compared to predictions of the Bruggeman model of the effective media. From the experimental characterization of the composite SILs, a resolution λ/5 was achieved at 0.09 THz (λ≈3.3 mm), which is one of the best resolutions for THz SILs reported in the literature. We believe that further improvements in material processing can reduce the resolution of the TiO₂-based THz SILs to their fundamental λ/20 limit .