Observation of second-order hyper-Raman generation in LiNbO3 whispering-gallery mode disk resonators

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 nm pump field with a LiNbO(3) disk produced a weak but measurable non-phase-matched 532 nm second-harmonic field at room temperature (>100 °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 nm. The relative spectral shift between two consecutive fields corresponds to a 455 cm(-1) vibrational mode in 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.