Meta-optics and bound states in the continuum (1810.08698v1)

Abstract: We discuss the recent advances in meta-optics and nanophotonics associated with the physics of bound states in the continuum (BICs). Such resonant states appear due to a strong coupling between leaky modes in optical guiding structures being supported by subwavelength high-index dielectric Mie-resonant nanoantennas or all-dielectric metasurfaces. First, we review briefly very recent developments in the BIC physics in application to isolated subwavelength particles. We pay a special attention to novel opportunities for nonlinear nanophotonics due to the large field enhancement inside the particle volume creating the resonant states with high-quality (high-$Q$) factors, the so-called quasi-BIC, that can be supported by the subwavelength particles. Second, we discuss novel applications of the BIC physics to all-dielectric optical metasurfaces with broken-symmetry meta-atoms when tuning to the BIC conditions allows to enhance substantially the $Q$ factor of the flat-optics dielectric structures. We also present the original results on nonlinear high-$Q$ metasurfaces and predict that the frequency conversion efficiency can be boosted dramatically by smart engineering of the asymmetry parameter of dielectric metasurfaces in the vicinity of the quasi-BIC regime.

• The paper demonstrates that fine-tuning dielectric resonators into a quasi-BIC regime can boost Q factors to values as high as 120 for efficient nanoscale light confinement.
• It reveals that enhanced resonances can increase nonlinear processes, such as second-harmonic generation, by two orders of magnitude in dielectric nanoantennas.
• The study suggests that dielectric metasurfaces with broken symmetry effectively harness BICs to significantly improve performance in sensing and photonic applications.

Meta-optics and Bound States in the Continuum: An Insightful Exploration of Advanced Nanophotonic Concepts

The paper by Kirill Koshelev, Andrey Bogdanov, and Yuri Kivshar offers a comprehensive examination of recent advancements in the field of meta-optics and the intriguing domain of bound states in the continuum (BICs). The focus is on how these phenomena can be exploited within meta-optics and nanophotonic structures, specifically using high-index dielectric metasurfaces and nanoantennas. This paper offers critical insights into enhancing resonant states in optical structures by leveraging BICs to achieve high quality (high-$Q$) factors, thereby opening new avenues in nonlinear nanophotonics.

Advances in BIC Physics for Isolated Nanoparticles

The primary theme of the paper revolves around the recent theoretical and empirical insights into BICs as applied to isolated subwavelength dielectric resonators. The authors present the fascinating prospect of quasi-BICs forming in isolated nanoparticles by fine-tuning the nanoparticle's dimensions to reach the so-called "supercavity" regime. This regime intriguingly results in substantial $Q$ factor enhancements through the interference and coupling of leaky modes, a phenomenon elegantly described by the Friedrich-Wintgen scenario. Such a leap in $Q$ factors, reaching values as high as 120 in certain configurations, signifies a compelling stride towards energy-efficient light confinement at the nanoscale.

Nonlinear Nanophotonics and Quasi-BICs

Exploring the application of these high-$Q$ resonances, the paper posits a significant amplification of nonlinear optical processes, such as second-harmonic generation (SHG). It provides compelling evidence that the SHG efficiency in dielectric nanoantennas, specifically those fashioned from materials like AlGaAs, can be dramatically increased near the quasi-BIC regime, exhibiting two orders of magnitude improvement over conventional dipolar resonances. This connection underscores the potential for quasi-BICs to serve as a cornerstone in the development of advanced nonlinear optical devices, including compact laser sources and quantum optical systems.

Metasurfaces Enabling Enhanced Nonlinear Interactions

The discussion extends further into the domain of dielectric metasurfaces, particularly those with broken symmetries. These metasurfaces, designed with dissimilar meta-atoms to leverage the formation of quasi-BICs, exhibit sharp resonant features pivotal for nonlinear interactions. The authors emphasize the relation between BIC-driven high-$Q$ resonances and their ability to significantly boost nonlinear processes on metasurfaces, with particular focus on SHG efficiency. Such enhancements are crucial in fabricating devices for applications spanning sensing, imaging, and information processing.

Broader Implications and Future Directions

The practical implications of these findings are profound, suggesting that dielectric nanostructures might serve as functional replacements or enhancements for metallic components in various photonic applications. By minimizing energy dissipation and exploiting strong resonance coupling, these structures hold promise for widespread utility across photonics. The exploration of BICs and their quasi-doublet states in these settings opens numerous pathways for innovation in designing ultracompact and highly efficient optical devices.

In conclusion, the paper presents a compelling argument for the strategic implementation of BICs in nanophotonic structures, offering a blueprint for future research directions within meta-optics. While the current advances elucidate a path toward high-efficiency optical components, the potential integration of these concepts into commercial and practical applications will be a stimulating evolution to observe in the ongoing development of nanophotonics and related fields.