Fabrication of silicon microparticle dispersion as terahertz wave refractive index control material

Optical systems utilizing terahertz waves hold significant potential across various fields; however, the materials for the optical elements that constitute such systems are currently scarce and the range of refractive indices that can be handled is limited. In the visible light region, there are refractive index control materials made of nanocomposites with dispersed dielectric nanoparticles. However, few technologies for refractive index control materials exist in the terahertz wave region. In this study, we developed an Si microparticle dispersion in which Si microparticles are dispersed in a cycloolefin polymer (COP) matrix as a high refractive index material for terahertz waves. Results showed that a material with a refractive index between that of Si and COP can be designed by changing the volume fraction of the Si cube, a subwavelength structural particle, in the dispersion. A method in which Si cubes were fabricated by dicing with COP and using a mold was implemented to produce four types of Si microparticle dispersions with Si concentrations of 1 vol%, 5 vol%, 15 vol%, and 25 vol%. The effective refractive index was evaluated, and results demonstrated that in the 0.1–0.2 THz range, including the D band of the Beyond 5G/6G communication band, the effective refractive index values were according to the concentration, achieving control in the range of 1.53–1.96. Additionally, the Si cubes were uniformly dispersed in the dispersion with random orientations in three dimensions, making them an optical material with minimal polarization dependence. The proposed Si microparticle dispersion can be used as a new refractive index control material in the terahertz band.