Indium Tin Oxide Nanoparticle-Coated Silica Microsphere with Large Optical Nonlinearity and High Quality Factor

High nonlinearity optical devices are of interest for compact, low power devices. Whispering gallery mode (WGM) microresonators offer strong nonlinear optical responses due to high quality factors and the small mode volume. To achieve high nonlinearity with these WGM devices, both a material with a high nonlinear index as well as a high quality factor is required. Indium tin oxide (ITO) is an excellent nonlinear material due to an exceptionally high nonlinear refractive index found at the epsilon-near-zero wavelength. However, ITO's enormous absorption at this point prohibits having a resonator with a high quality factor. Here, we present a novel ITO nanoparticle-coated silica microsphere with significantly enhanced nonlinearity while maintaining high quality factors. Nonlinear refractive index and quality factor of the ITO nanoparticle-coated silica microsphere are obtained by fitting the measured transmission spectra with a theoretical model that includes thermal and Kerr effects. By controlling the number of particles on a silica surface, we achieve 39-187 times higher nonlinear indices compared with a pure silica microsphere and quality factors between 10(6) and 10(7). The study establishes a new avenue toward novel nonlinear optical devices based on ITO nanoparticles, which can be readily incorporated in a variety of geometries.