Giant nonlinear response at the nanoscale driven by bound states in the continuum (1804.02947v1)

Abstract: Being motivated by the recent prediction of high-$Q$ supercavity modes in subwavelength dielectric resonators, we study the second-harmonic generation from isolated subwavelength AlGaAs nanoantennas pumped by a structured light. We reveal that nonlinear effects at the nanoscale can be enhanced dramatically provided the resonator parameters are tuned to the regime of the bound state in the continuum. We predict a record-high conversion efficiency for nanoscale resonators that exceeds by two orders of magnitude the conversion efficiency observed at the conditions of magnetic dipole Mie resonance, thus opening the way for highly-efficient nonlinear metadevices.

• The paper reports that SHG efficiency surpasses previous benchmarks by two orders of magnitude using BIC-enabled resonances.
• It employs eigenmode analysis and COMSOL simulations to optimize AlGaAs nanoantenna designs for enhanced nonlinear responses.
• The study highlights the practical relevance of meta-optics in developing next-generation frequency conversion and quantum photonics devices.

Giant Nonlinear Response at the Nanoscale Driven by Bound States in the Continuum

The research paper explores the enhancement of second-harmonic generation (SHG) in subwavelength AlGaAs nanoantennas, highlighting the substantial potential of metastructures in nonlinear photonics. The authors delve into the emerging field of meta-optics, specifically focusing on utilizing high-quality factor (Q-factor) modes known as bound states in the continuum (BIC) to significantly boost the nonlinear optical response at the nanoscale.

The paper is motivated by recent predictions in the domain of high-Q supercavity modes in subwavelength dielectric resonators, which demonstrate an increased potential for nonlinear optical processes. Traditional Mie-resonant nanoparticles are recognized for their low Q-factor in low-order resonances; however, when leveraging BIC, the situation alters drastically. A BIC is typically considered a mathematical construct, often requiring infinite structures or permittivity extremes. Nevertheless, the authors adeptly harness the principle in realistic settings using high-index dielectric nanorods to achieve resonances with high Q-factors.

A paramount aspect of this paper is the detailed eigenmode analysis performed using resonant-state expansion for AlGaAs resonators, which reveals the coupling between different modes as a function of nanoparticle aspect ratios. Notably, this approach allows the realization of high-Q supercavity modes associated with BIC conditions, distinctive for their symmetry and enhanced resonance attributes.

Central to the paper's assertions is the dramatic enhancement of SHG efficiency facilitated by the BIC phenomenon. The authors successfully predict a conversion efficiency exceeding previous benchmarks by two orders of magnitude. This impressive increase, verified through numerical simulations and analytical models, arises from the increased Q-factor relevant to BIC, enabling substantial volumetric confinement and efficient mode coupling.

The research encapsulates intrinsic conversion efficiency $\rho_{\rm SH}$ by deriving a rigorous expression and corroborating it through COMSOL simulations under different excitation and pumping scenarios. The paper elaborates on the substantial impact of mode spectral and spatial overlaps on SH frequency, highlighting how specific geometric and material parameters can be fine-tuned to achieve peak efficiency.

The implications of these findings are significant, paving the path for the development of highly efficient nonlinear metadevices. From a practical standpoint, the enhancements in nonlinear effects at the nanoscale can revolutionize frequency conversion applications and advance quantum nanophotonics by providing a new paradigm for device engineering that optimizes nonlinear optical interactions.

Looking forward, this paper sets a pivotal precedent in leveraging bound states in the continuum within nanoresonators for nonlinear optical advances. Future explorations could explore multiplexed and multi-frequency generation, energy-efficient photonics devices, and integration with quantum optical systems. The intersection of high-Q resonances and tailored photonic structures presents astounding possibilities in both theoretical research and technological innovation in the domain of optical nanomaterials.