Fabrication and characterization of a composite TiO2-polypropylene high-refractive-index solid immersion lens for super-resolution THz imaging

Abstract

Terahertz (THz) near-field imaging is attracting a lot of attention for its potential applications in medical diagnosis and material characterization. However, the spatial resolution of the recorded THz image was mainly limited by the diffraction limit of the commonly used lens- and mirror-based THz optical systems. Alternatively, a solid immersion lens (SIL) can be a promising approach for achieving super-resolution imaging as it reduces the spot size of the focused THz beam 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 SIL using powder mixes of titanium dioxide (TiO2) and polypropylene (PP) whose RIs are ≈10 and ≈1.51, respectively, at 1.0 THz. In particular, we present two different lens fabrication strategies that are simple and cost-effective solutions. The first strategy 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 design consists in pressing the TiO2 powder in a hollow hemisphere that is 3D printed using PP. The fabricated lenses are then characterized physically and optically, and their RIs are compared to the theoretical estimates using the Bruggeman model of the effective media. From the experimental measurements of the proposed SIL, a resolution limit as low as 0.2λ was achieved at 0.09 THz (λ ≈ 3.3 mm), which is comparable to the best resolutions reported in the literature.