Doubly resonant second-harmonic generation of a vortex beam from a bound state in the continuum

TL;DR

This paper demonstrates a novel photonic crystal cavity design that achieves doubly-resonant second-harmonic generation with high efficiency, enabling enhanced nonlinear optical processes and vortex beam generation.

Contribution

It introduces the first doubly-resonant PhC slab cavity using a bound state in the continuum, achieving high conversion efficiency and specific far-field emission patterns.

Findings

Achieved 2.4×10^{-2} W^{-1} conversion efficiency.

Confirmed highly normal-direction concentrated far-field emission.

Demonstrated vortex beam generation at second harmonic frequency.

Abstract

Second harmonic generation in nonlinear materials can be greatly enhanced by realizing doubly-resonant cavities with high quality factors. However, fulfilling such doubly resonant condition in photonic crystal (PhC) cavities is a long-standing challenge, because of the difficulty in engineering photonic bandgaps around both frequencies. Here, by implementing a second-harmonic bound state in the continuum (BIC) and confining it with a heterostructure design, we show the first doubly-resonant PhC slab cavity with $2.4\times10^{-2}$ W$^{-1}$ conversion efficiency under continuous wave excitation. We also report the confirmation of highly normal-direction concentrated far-field emission pattern with radial polarization at the second harmonic frequency. These results represent a solid verification of previous theoretical predictions and a cornerstone achievement, not only for nonlinear frequency conversion but also for vortex beam generation and prospective nonclassical sources of radiation.