Nanoporous SiO₂ based on annealed artificial opals as a favorable material platform of terahertz optics

In this paper, artificial opals, made of 300-nm-diameter nanoporous SiO₂ globules by sedimentation of a colloidal suspension and annealing at different temperatures in the range of 200–1500 ^°C, are studied as a promising material platform for terahertz (THz) optics. Our findings reveal that THz optical properties of such materials can be predictably varied in a wide range by annealing, while being a deterministic function of the material porosity. Thus, when increasing annealing temperature, the resultant material refractive index increases from 1.65 to 1.95 at 1.0 THz, while the material absorption coefficient (by field) reduces from 10 to 1 cm⁻¹. The Bruggeman effective medium theory was then successfully applied to model optical properties of the nanoporous SiO₂ at THz frequencies as a function of the material porosity and the annealing temperature. Finally, bulk nanoporous SiO₂ were shaped using conventional grinding techniques into plates and cylindrical lenses to demonstrate robustness of the novel THz optical materials. A wide range of the nanoporous SiO₂ refractive indices, their low-to-moderate THz-wave absorption, as well as their mechanical robustness make such materials a promising platform for THz optics.