Fluorescence whispering gallery modes in Tm3+-doped Ge-Ga-S chalcogenide glasses microsphere-silica fiber taper coupling system

Abstract

Microsphere resonators based on chalcogenide glasses combine the superior optical properties of microsphere resonators (such as high Q-factors and small mode volumes) and excellent material properties of chalcogenide glasses in the infrared spectrum (such as good transmissivities, high refractive indices, and low phonon energies), and thus have promising applications in the fields of low-threshold infrared lasers, nonlinear Raman amplifiers/lasers, and narrow bandwidth infrared filters.In this work, the infrared microsphere resonators are built by using a novel chalcogenide glass composition of 75 GeS2-15 Ga2S3-10 CsI (Ge-Ga-S), doped with 1.3 wt% Tm. Compared with previously reported chalcogenide microsphere resonators fabricated with As2S3 and gallium lanthanum sulfide (Ga-La-S) glasses, the proposed Ge-Ga-S glass does not contain the toxic element of As nor the expensive rare earth element of La, and thus is more environmentally friendly and cost-effective for fabricators and users. We first fabricate bulk Ge-Ga-S glasses by using the facility in our laboratory. After measuring the absorption and fluorescence spectra of bulk glasses, they are crushed into powders and the powders are blown downwards through an inert-gas-filled vertical furnace (temperature set at 1000 ℃). Molten glass powders are transformed into high-quality microspheres in the furnace due to surface tension. Thousands of microspheres with diameters ranging from 50 to 200 m can be made in one fabrication process. By using optical microscopy and scanning electron microscopy, a microsphere with high surface quality is selected for further optical characterization. The selected microsphere has a diameter of 72.84 m, an eccentricity less than 1% (about 80 nm), and a Q-factor of 1.296104. A silica fiber taper with a waist-diameter of 1.93 m is fabricated as the coupling mechanism for the microsphere resonator. The coupling between the microsphere and the micro fiber taper is realized with the aid of nano-positioning stages. An 808 nm laser diode is used as a pump light source, which is sent into one end of the fiber taper and is evanescently coupled into the microsphere. Spontaneous emissions of fluorescent light are then generated in the microsphere, whose spectral characteristics are measured by using an optical spectrum analyzer. It can be clearly noted from the measurement results that the typical fluorescence spectrum of the Tm3+-doped Ge-Ga-S glass is modified by whispering gallery mode (WGM) patterns as periodic intensity peaks/valleys are apparently present in the measured spectral curves. The locations of those experimentally measured spectral peaks/valleys are in good agreement with WGM mode calculated results through using the Mie scattering theory, which verifies that the proposed Ge-Ga-S glass can be used to build high-quality infrared microsphere resonators. The largest deviation between the experimentally measured spectral peaks/valleys and theoretically calculated WGM modes is about 0.047%. Minor deviation is present because the experimentally fabricated microsphere has a small difference from an ideal sphere (with an eccentricity of about 1% in this work). Longer processing time of glass powders in the vertical furnace or a post-thermal treatment could help improve the sphericity of microspheres.