Asymmetric metasurfaces with high-$Q$ resonances governed by bound states in the continuum (1809.00330v1)

Abstract: We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-$Q$ resonances that originate from the physics of bound states in the continuum. We prove rigorously a direct link between the bound states in the continuum and the Fano resonances, and develop a general theory of such metasurfaces, suggesting the way for smart engineering of resonances for many applications in nanophotonics and meta-optics.

• The paper establishes a theoretical framework linking bound states in the continuum to high-Q Fano resonances in asymmetry-broken metasurfaces.
• It derives an analytical expression showing an inverse quadratic relationship between the radiative quality factor and the asymmetry parameter.
• The study validates the model via simulations on dielectric and plasmonic designs, enabling tunable optical devices across visible to THz regimes.

Asymmetric Metasurfaces and High-$Q$ Resonances: Insights from Bound States in the Continuum

The paper under review presents a detailed investigation into the behavior of asymmetric metasurfaces, proposing a novel framework that connects the high-$Q$ resonances exhibited by these structures to the phenomenon of bound states in the continuum (BICs). The primary objective is to establish a theoretical foundation that explains the occurrence of sharp spectral features in metasurfaces with broken in-plane symmetry and to leverage this understanding for the engineering of optical devices with enhanced functionalities.

Key Findings and Methodological Rigor

The authors develop a comprehensive theory that rigorously links the bound states in the continuum with Fano resonances, characterizing the transmission spectra of asymmetric metasurfaces. By employing a perturbative approach, they derive an analytical expression for the radiative quality factor $Q_{\text{rad}}$, which manifests an inverse quadratic relationship with the structure's asymmetry parameter. Such a relationship is indicative of the transformation of true BICs into quasi-BICs when symmetry is broken, resulting in high-$Q$ resonances.

The paper demonstrates that the transmission spectra of these metasurfaces obey the classical Fano lineshape, providing an explicit correspondence between the Fano parameters and eigenmode spectra. The paper effectively utilizes eigenmode expansions and the Green’s function formalism to characterize the interaction of metasurfaces with incident fields, thereby elucidating the transition from ideal BICs to their practical, finite-$Q$ counterparts.

Experimental and Computational Validation

The research includes a series of numerical simulations and spectral analyses covering a spectrum of metasurface designs encompassing dielectric and plasmonic elements. These designs vary significantly in terms of geometric configurations and elemental materials yet conform to the quadratic dependency of $Q_{\text{rad}}$ on the asymmetry parameter, underscoring the universality of the proposed theoretical model. By adjusting parameters like geometric scaling and asymmetry, the authors show that these metasurfaces can be fine-tuned to operate over wide spectral ranges, from visible to THz regimes, without compromising the quality factor, which can reach values as high as $10^5$.

Implications and Future Directions

This paper opens promising avenues in meta-optics and nanophotonics, providing a robust theoretical basis for designing metasurfaces with customizable optical properties. Practical implications include the potential for developing advanced optical sensors, light-emitting devices, and ultrafast photonic components that exploit high-$Q$ modes for improved performance and efficiency. Furthermore, the analytical tools and insights from this work could be instrumental in exploring other photonic structures, including photonic crystal slabs and metamaterials with engineered dark states.

In summary, the insights offered by this paper enrich our understanding of the interplay between BICs and Fano resonances in metasurfaces, presenting a powerful framework for designing next-generation photonic devices. The demonstrated ability to predict and control high-$Q$ resonances through symmetry manipulation represents a valuable advancement in the field, with wide-ranging applications across optical technologies.