We demonstrate gallium phosphide (GaP) microdisk optical cavities with
intrinsic quality factors >2.8×105 and mode volumes <10(λ/n)3, and study their nonlinear and optomechanical properties. For
optical intensities up to 8.0×104 intracavity photons, we observe
optical loss in the microcavity to decrease with increasing intensity,
indicating that saturable absorption sites are present in the GaP material, and
that two-photon absorption is not significant. We observe optomechanical
coupling between optical modes of the microdisk around 1.5 μm and several
mechanical resonances, and measure an optical spring effect consistent with a
theoretically predicted optomechanical coupling rate g0​/2π∼30 kHz for
the fundamental mechanical radial breathing mode at 488 MHz.Comment: Published Versio