In this Letter we report experimental demonstration of nonlinear frequency conversion at several optical frequencies in a whispering-gallery mode resonator (WGMR). Because of the enhancement of nonlinear interactions inside a WGMR, interaction of a pump field with a disk produced a weak but measurable non-phase-matched second-harmonic field at room temperature ( below the phase-matching temperature) for pump powers of a few tens of milliwatts. For higher pump powers, we observed the generation of four additional fields at 545, 559, 573, and . The relative spectral shift between two consecutive fields corresponds to a vibrational mode in crystal. Our preliminary analysis indicates that these fields are the result of a multiphonon hyper-Raman scattering in which two photons of the pump field are converted into one photon of a higher-frequency field and one or several optical phonons.
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