Abstract

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 $1064\mathrm{nm}$ pump field with a ${\mathrm{LiNbO}}_3$ disk produced a weak but measurable non-phase-matched $532\mathrm{nm}$ second-harmonic field at room temperature ($>100°\mathrm{C}$ 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 $587\mathrm{nm}$. The relative spectral shift between two consecutive fields corresponds to a $455{\mathrm{cm}}^{-1}$ vibrational mode in ${\mathrm{LiNbO}}_3$ 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.

© 2011 Optical Society of America

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