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Abstract

A major challenge in third harmonic generation and its converse, parametric down-conversion, is how to arrange phase matching between signals at omega and 3 omega while maintaining a high nonlinear overlap. In this Letter, we present a design consisting of a nanostrand of glass with two hollow channels. The fundamental and third harmonic modal fields, enhanced in the region between the channels, have high nonlinear overlap, while the phase-matching wavelength can be coarse-tuned by gas pressure and fine-tuned by axial strain and mechanical twist, which, remarkably, have opposite effects. The ability to adjust the phase-matching condition may facilitate efficient generation of entangled photon triplets. (C) 2021 Optical Society of America.