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

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) slab 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 × 10 −2 W −1 intrinsic conversion efficiency under continuouswave 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.

Doubly-Resonant Photonic Crystal Cavities for Efficient Second-Harmonic Generation in III–V Semiconductors

“…a BIC [6]. This approach has also been recently shown to experimentally enhance SHG efficiency in GaN PhC cavities [7]. Here we show that the above strategy can be further generalized and automated, and it can be applied to an arbitrary material platform.…”

“…Recently, a novel approach based on the so-called bound-states in the continuum (BIC) has been proposed [6], overcoming the aforementioned obstacle. This strategy has later been shown to be practically effective by realizing doubly resonant SH generation in a III-V semiconductor PhC cavity [7]. The BICs are particular states that stay confined despite the fact they lie above the light line, i.e., their dispersion falls within the continuum of radiative modes [9].…”

Photonic Bound States in the Continuum in Si Structures with the Self‐Assembled Ge Nanoislands

“…Such states are now called bound states in the continuum (BICs) and they have recently attracted enormous attention in photonics. [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63] BICs represent spatially localized states with vanishing radiation despite their energy embedded in the continuum spectrum of the environment. Fundamentally, BICs originate from destructive interference, when two or more waves superpose to completely suppress radiative losses.…”

Topologically protected second-harmonic generation via doubly resonant high-order photonic modes

“…It has been demonstrated that the presence of topological edge states could enhance the harmonic generation based on robustness of the local field enhancement [32][33][34][35][36][37]. One of the main limiting factors for high conversion efficiencies is the Q factors of the resonant modes of the devices [38][39][40][41][42]. Essentially, SPTIs with tightly localized corner states could be deemed as topology-protected resonant cavities with high Q factors [21], thus providing a promising platform for improving the harmonic generation efficiency.…”