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

TL;DR

This paper demonstrates nonlinear frequency conversion in a LiNbO3 whispering-gallery mode resonator, revealing hyper-Raman scattering and multi-phonon processes at room temperature with low pump power.

Contribution

First experimental observation of second-order hyper-Raman generation in LiNbO3 WGMR, expanding understanding of nonlinear optical interactions in resonators.

Findings

Generated multiple new optical fields at different wavelengths

Observed spectral shifts corresponding to vibrational modes in LiNbO3

Indicated multi-phonon hyper-Raman scattering as the underlying mechanism

Abstract

In this manuscript we report experimental demonstration of nonlinear frequency conversion at several optical frequencies in a whispering-gallery mode resonator (WGMR). Due to the enhancement of nonlinear interactions inside a WGMR, interaction of a 1064 nm pump field with a LiNbO3 disk produced a weak, but measurable non-phase matched 532 nm second harmonic field at room temperature (>100C below the phase-matching temperature), for pump powers of a few tens of mW. For higher pump powers, we observed the generation of four additional fields at 545 nm, 559 nm, 573 nm, and 587 nm. The relative spectral shift between two consecutive fields corresponds to a 455 cm^-1 vibrational mode in LiNbO3 crystal. Our preliminary analysis indicates that these fields are the result of a multi-phonon 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.