Very High- <i>Q</i> Whispering-Gallery Mode Resonances Observed on Fused Silica Microspheres

Abstract

We have studied by phase modulation spectroscopy the whispering-gallery modes (Mie resonances) of 60 to 200 μm diameter microspheres obtained by fusing with a CO2 laser the end of a high-transmission silica fibre. An evanescent wave at 780 nm was produced by total internal reflection of a phase-modulated semiconductor diode laser beam in a glass prism. It was coupled into the Mie mode of the microsphere positioned at a fixed distance from the prism face. The spectrum was obtained by measuring the phase-to-amplitude conversion of the laser beam modulation as its carrier frequency was scanned. Record Q-factors ⩾ 2 · 109 were observed, corresponding to photon storage times longer than 1 μs. Applications of these resonances to fundamental and applied projects are discussed.