Effective refractive index tuning in the terahertz range using silicon microparticle-dispersed lenses

• Tunable terahertz refractive indices achieved using Si microparticle dispersions. • Si-MPDLs with 0–25 vol% Si show clear shifts in refractive index and focal length. • Higher Si concentration shortens focal length at both 0.10 and 0.15 THz. • Si dispersion exhibits minimal polarization dependence in terahertz operation. • Refractive-index tunability supports optics for Beyond 5G/6G communication bands. In this study, a silicon microparticle-dispersed lens (Si-MPDL) was developed for controlling terahertz waves. The lens material was a dispersion of Si microparticles that we had previously created and used to achieve refractive-index control in the terahertz region. By adjusting the Si concentration in the dispersion, we were able to tune the effective refractive index between those of silicon and cyclo -olefin polymer (COP). Since the material also exhibits minimal polarization dependence, it is a promising optical material for the terahertz region, where suitable natural refractive-index materials are scarce. In this study, we fabricated Si-MPDLs of identical shape but with different Si concentrations and measured their focal lengths at 0.10 and 0.15 THz. The measured focal lengths agreed with the theoretical expectation that higher Si concentrations should yield shorter focal lengths, confirming that the effective refractive index is controllable. These results demonstrate that Si microparticle dispersions have strong potential for use in lenses and other terahertz optical elements.