Three‐wave mixing in whispering gallery resonators

Abstract

We review the progress in the development of frequency converters based on three-wave mixing in whispering gallery resonators (WGRs). The theoretical description, given in a unified notation for all basic processes, reveals that the phase-matching condition known from conventional devices is replaced by several selection rules and, furthermore, the fact that conversion efficiencies of more than 25% can be reached in the overcoupled regime only. Experimentally, the conversion efficiencies exceed 50% already at milliwatt input powers. This is achieved, however, so far in bulk resonators only since today the on-chip devices have two orders of magnitude lower quality factors. Regarding the stability of the conversion process, one has to consider impurities left from the crystal growth and material specific effects like photoconductivity, photorefractivity, and pyroelectricity. The impressive experimental progress paves the way that micrometer-sized frequency converters based on WGRs will find the way out of the lab into real-world applications.

Periodically poled thin-film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/W

“…A SHG conversion efficiency of 250,000 %/W is extracted. (b) Absolute conversion efficiency as a function of pump power, including the experimental data and theoretical fit[51,52].…”

“…b) presents the pump power dependence of the absolute conversion efficiency, showing a measured saturation value of 15% when the on-chip pump power reaches 115 µW. Based on the theoretical modelling considering the pump depletion[51,52], the maximum absolute conversion efficiency is b . On condition that both pump and SH modes are strongly over-coupled (Q 0,a(b)…”

Experimental observation of internally pumped parametric oscillation and quadratic comb generation in a χ(2) whispering-gallery-mode microresonator

“…Indeed, lithium niobate exhibits very large normal dispersion around 1060 nm, which is prohibitive for the phase-matching of degenerate fourwave mixing. The fact that our observations of sideband generation is always correlated with strong SHG provides further evidence of the sidebands' χ (2) us to reliably generate sidebands around the pump and the second-harmonic, we are unable to maintain stable operation for more than some tens of seconds at a time. In addition to sub-optimal thermal isolation, the linewidth of our pump laser (∼ 200 kHz) necessitates sacrifices with regards to the resonator Q-factor, which in turn leads to increased pump power requirements that can impede system stability.…”

“…To conclude, we have reported on the experimental observation of internally-pumped parametric oscillation and initial stages of quadratic comb formation in a χ (2) WGM microresonator. Whilst our current experimental configuration has enabled the proof-of-principle demonstrations reported in this work, there are a number of improvements that are left for future work.…”

Selective Coupling Enhances Harmonic Generation of Whispering-Gallery Modes

“…WGM resonators are natural vessels to study nonlinear effects due to an unparalleled combination of favorable characteristics [6,7]. The method of entrapment is total internal reflection, which allows for very good spatial confinement along the rim of the resonator.…”