Individual nanoantennas empowered by bound states in the continuum for nonlinear photonics (1908.09790v1)

Abstract: Bound states in the continuum (BICs) represent localized modes with energies embedded in the continuous spectrum of radiating waves. BICs were discovered initially as a mathematical curiosity in quantum mechanics, and more recently were employed in photonics. Pure mathematical bound states have infinitely-large quality factors (Q factors) and zero resonant linewidth. In optics, BICs are physically limited by a finite size, material absorption, structural disorder, and surface scattering, and they manifest themselves as the resonant states with large Q factors, also known as supercavity modes or quasi-BICs. Optical BIC resonances have been demonstrated only in extended 2D and 1D systems and have been employed for distinct applications including lasing and sensing. Optical quasi-BIC modes in individual nanoresonators have been discovered recently but they were never observed in experiment. Here, we demonstrate experimentally an isolated subwavelength nanoresonator hosting a quasi-BIC resonance. We fabricate the resonator from AlGaAs material on an engineered substrate, and couple to the quasi-BIC mode using structured light. We employ the resonator as a nonlinear nanoantenna and demonstrate record-high efficiency of second-harmonic generation. Our study brings a novel platform to resonant subwavelength photonics.

• The paper demonstrates that quasi-bound states in isolated AlGaAs nanoresonators significantly enhance second-harmonic generation efficiency.
• The study uses engineered ITO-layer substrates to achieve high-Q factors (~180) by suppressing radiation losses through destructive interference of leaky modes.
• Its findings pave the way for compact photonic devices by optimizing nanoscale light-matter interactions for advanced nonlinear applications.

An Analysis of Bound States in the Continuum for Nonlinear Photonics

This paper presents a significant advancement in the field of nonlinear photonics through the experimental demonstration of quasi-bound states in the continuum (quasi-BICs) in isolated nanoresonators. These findings pave the way for efficient nonlinear processes at the nanoscale, showcasing the potential of subwavelength resonators in enhancing second-harmonic generation (SHG).

The research identifies quasi-BICs in individual cylindrical nanoantennas made from aluminum gallium arsenide (AlGaAs), a material known for its nonlinear optical properties. The resonators are mounted on a carefully engineered substrate that includes layers of ITO, functioning as a conductor and insulator at specific wavelengths, thereby enhancing the resonator's Q factor. The interplay between different leaky modes and their destructive interference results in the suppression of radiation losses, achieving a high-Q factor for these optical states.

Experimentally, a resonant Q factor of approximately 180 was observed, confirming the theory-derived value. Notably, the paper achieved record-high efficiencies in SHG, facilitated by the strong confinement of light within these quasi-BIC modes. This efficiency represents a marked improvement over previous nonlinear processes in individual nanoresonators, highlighting the paper's contribution to nonlinear nanophotonics.

The implications of these findings are manifold. In practical terms, the results suggest significant improvements in designing compact photonic devices such as lasers, sensors, and quantum sources, where high-Q resonant modes are crucial. Additionally, the theoretical insights provided by the exploration of quasi-BICs enhance our understanding of light-matter interactions at the nanoscale.

Future developments in the field could focus on optimizing the spatial matching of pump light to the mode distribution, thereby maximizing the coupling efficiency and further enhancing nonlinear conversion processes. There is potential for exploring other material systems beyond AlGaAs to extend these high-Q modes' operational wavelengths, further broadening the application spectrum.

In summary, this paper provides a comprehensive experimental and theoretical framework demonstrating the uses of quasi-BIC modes in nonlinear photonics, opening up new avenues for resonant photonic applications. With advancements like these, significant progress in the miniaturization and efficiency of optical devices within the field of nanophotonics is anticipated.