Resonances in Ge disk lattices on Si at telecommunication wavelengths

Metasurfaces consisting of subwavelength dielectric particles can change the propagation direction of incident light due to the excitation of Mie resonances. Here we investigate the interaction of light with square lattices of subwavelength and wavelength-sized Ge disks formed on a Si substrate. The optical response is strongly dependent on the disk diameter. For lattices with disk diameters of 370--580 nm, pronounced minima are observed in the transmittance and reflectance spectra within the 1000--2000 nm range. In contrast, lattices with larger disk diameters (1160--1490 nm) exhibit spectral modulation across the same wavelength region. The calculations carried out show that for small disks, the effect is associated with Mie resonances, while for large disks, the effect is provided by the excitation of circular Fabry-Perot resonances. The excitation of collective Fabry-Perot modes occurs in the directions along the rows and diagonals of the disk lattice. The simulations of far-field scattering patterns and radiation patterns for a single large Ge disk reveal that circular Fabry-Perot modes promote the redirection of incident light along the substrate surface. The ability of Ge disk lattices on Si substrates to alter the light propagation direction could become an effective tool for coupling light into an optical waveguide.