Elastic light scattering from a germanium microsphere in the far-IR region

Abstract

Far-infrared (far-IR) elastic light scattering from a germanium microsphere excited by a Gaussian beam is numerically simulated using generalized Lorenz-Mie theory (GLMT) [1]. Microspheres possess optical resonances [2,3], known as morphology dependent resonances (MDRs) [4] or whispering gallery modes (WGMs) [5]. WGMs can also exist in optical resonators of various other shapes such as microdisks, and microtoroids [6]. WGMs of microspheres of different materials result in interesting elastic scattering spectra [7]. The semiconductor material, germanium has a refractive index of 4 in the far-IR [8], which is higher than the refractive index of the more common semiconductor materials, such as silicon. The high refractive index of germanium results in high quality factor WGMs [9]. The germanium microsphere simulated in this study has a radius of 120 µm. The far-IR spectral simulation region ranges from 19.8 µm to 20.2 µm. The light can be coupled to the germanium microsphere by using a tapered fiber coupler, an optical fiber half coupler (OFHC), [10] or an optical waveguide [11]. The 0° transmission and 90° elastic scattering from the germanium microsphere is studied for both the transverse electric (TE) and transverse magnetic (TM) polarizations. A spectral mode spacing of approximately 180 nm is observed in the numerical results, which correlates well with the theoretically calculated mode spacing [12]. Among the possible applications of the WGM excitation in germanium microspheres are optical modulators [13], mechanical, and thermal sensing [14].