Second-harmonic generation using [Image: see text]-quasi-phasematching in a GaAs whispering-gallery-mode microcavity

The [Image: see text] crystal symmetry in materials such as GaAs can enable quasi-phasematching for efficient optical frequency conversion without poling, twinning or other engineered domain inversions. [Image: see text] symmetry means that a 90° rotation is equivalent to a crystallographic inversion. Therefore, when light circulates about the [Image: see text] axis, as in GaAs whispering-gallery-mode microdisks, it encounters effective domain inversions that can produce quasi-phasematching. Microdisk resonators also offer resonant field enhancement, resulting in highly efficient frequency conversion in micrometre-scale volumes. These devices can be integrated in photonic circuits as compact frequency convertors, sources of radiation or entangled photons. Here we present the first experimental observation of second-harmonic generation in a whispering-gallery-mode microcavity utilizing [Image: see text]-quasi-phasematching. We use a tapered fibre to couple into the 5-μm diameter microdisk resonator, resulting in a normalized conversion efficiency η≈5 × 10(−5 )mW(−1). Simulations indicate that when accounting for fibre-cavity scattering, the normalized conversion efficiency is η≈3 × 10(−3 )mW(−1).