Simulation of elastic scattering from a germanium microsphere in the near-ir

Abstract

Optical resonators made of dielectric and semiconductor microsphere cavities are of interest due to their whispering gallery modes (WGMs). Initially the WGMs of the microspheres have been excited with plane waves. Later on, single mode optical fiber excitation has been used to couple to the WGMs of the microspheres. Optical waveguides can also be used for the excitation of microresonator WGMs. WGMs have already been observed in the near-infrared (near-IR) transmission of semiconductor resonators such as silicon microspheres and germanium microspheres; since germanium in particular, has certain advantages in photonic applications due to its large nonlinearity, high optical damage threshold and most importantly, its wide transparency range (2–15 µm). Moreover, germanium has a refractive index (4), which is more than twice the refractive index of silica (1.5), giving rise to higher quality factor modes. Here, we performed numerical simulation of the 90° elastic scattering in addition to the 0° transmission from a germanium microsphere in the near-IR. We simulated the elastic scattering spectrum from 1.9 µm to 2.1 µm of the germanium microsphere with a 10 µm radius using the generalized Lorenz-Mie theory (GLMT) for both transverse electric (TE) and transverse magnetic (TM) polarization excitation. The numerical simulations resulted in a spectral mode spacing of 12 nm, which is in accordance with the theoretical estimations. The WGMs excited in germanium microspheres can be used in optical applications such as mechanical or thermal sensing.