Elastic scattering from germanium microspheres in the terahertz region

Abstract

Elastic light scattering from a germanium microsphere with a radius of 2000 µm is analyzed numerically in the terahertz region from 297 µm to 303 µm. Germanium has a high refractive index (n = 4) in the terahertz range, which makes it possible to trap light effectively and achieve high quality factor whispering gallery modes (WGMs). Microsphere WGMs have high quality factors and germanium microsphere WGMs can find many applications in the terahertz range, which can be exploited for astronomical and security applications. We studied 90° elastic scattering and 0° transmission spectra from the germanium microsphere using the generalized Lorenz-Mie theory (GLMT). The excitation terahertz source can be coupled to the germanium microsphere through single mode terahertz waveguides. The numerical simulations are performed for both transverse electric (TE) and transverse magnetic (TM) polarization of the terahertz excitation source. The WGM mode spacing, i.e., the spectral separation of the modes with the same radial mode number and consecutive polar mode orders, can be used for the analysis of the microsphere WGM spectra. We observed a mode spacing of 2.4 µm, which agrees well with the theoretically estimated value of the WGM mode spacing. All in all, germanium microspheres with their high quality factor WGMs might find applications in terahertz photonics.