Optical Bound States in the Continuum Enabled by Magnetic Resonances Coupled to a Mirror

Abstract

Dielectric metasurfaces made of high refractive index and low optical loss materials have emerged as promising platforms to achieve high-quality factor modes enabling strong light–matter interaction. Bound states in the continuum have shown potential to demonstrate narrow spectral resonances but often require asymmetric geometry and typically feature strong polarization dependence, complicating fabrication and limiting practical applications. We introduce a novel approach for designing high-quality bound states in the continuum using magnetic dipole resonances coupled to a mirror. The resulting metasurface has simple geometric parameters requiring no broken symmetry. To demonstrate the unique features of our photonic platform we show a record-breaking third harmonic generation efficiency from the metasurface benefiting from the strongly enhanced electric field at high-quality resonances. Our approach mitigates the shortcomings of previous platforms with simple geometry enabling facile and large-area fabrication of metasurfaces paving the way for applications in optical sensing, detection, quantum photonics, and nonlinear devices.

Lighting Up Si Nanoparticle Arrays by Exploiting the Bound States in the Continuum Formed in a Si/Au Hybrid Nanostructure

“…45−47 In comparison, less attention has been paid to dielectric-metal hybrid nanostructures owing to the Ohmic loss of metals, which may influence the quality factors of BICs. 33,48 Recent studies reveal that two types of BICs with quality factors as large as ∼800 can also be realized in dielectric-metal hybrid nanostructures by exploiting the surface plasmon polaritons (SPPs) generated on the surfaces of metal films. 49 Considering the spectrum match between the optical mode and the excitation light (e.g., femtosecond laser pulses), BICs with quality factors of ∼100 are sufficient to dramatically enhance the nonlinear optical absorption of Si nanoparticles.…”

“…The Auger effect, which is proportional to the cubic of the carrier density, plays a crucial role in this scenario . By placing Si nanoparticles on a thin metal film, it was demonstrated very recently that the quantum efficiency can be further improved to ∼6.0–8.0% by exploiting the strongly localized electric field and dramatically reduced linewidth provided by the so-called mirror-image-induced magnetic dipole (MMD) resonance. , In addition, the good thermal conductivity of the metal film enables the stable emission of white light for a long time. It was shown that surface lattice resonances with quality factors larger than those of the MMD resonance can be achieved in a regular array of Si nanopillars, which offers a larger nonlinear optical absorption .…”

“…Basically, BICs can be classified into symmetry-protected BICs and accidental (Friedrich–Wintgen or FW) BICs. , While the former originates from the mismatch in symmetry between the optical mode and the incident wave, the latter arises from the interference of two optical modes . So far, both BICs have been exploited to enhance light-matter interaction, such as the realization of lasers , and achievement of highly efficient harmonic generation. , In most cases, all-dielectric nanostructures with extremely low optical losses are employed because the high quality factors of BICs are generally pursued. − In comparison, less attention has been paid to dielectric-metal hybrid nanostructures owing to the Ohmic loss of metals, which may influence the quality factors of BICs. , Recent studies reveal that two types of BICs with quality factors as large as ∼800 can also be realized in dielectric-metal hybrid nanostructures by exploiting the surface plasmon polaritons (SPPs) generated on the surfaces of metal films . Considering the spectrum match between the optical mode and the excitation light (e.g., femtosecond laser pulses), BICs with quality factors of ∼100 are sufficient to dramatically enhance the nonlinear optical absorption of Si nanoparticles …”

Ultrasensitive Frequency Shifting of Dielectric Mie Resonance near Metallic Substrate

“…However, the Mie resonant frequencies in these cases are influenced by various factors including material property, structure, and resonance mode and exhibit a complex variation with the gap size between dielectric particle and metallic substrate, which still needs a comprehensive mechanism interpretation. For example, the hybrid MD resonant wavelength of Si nanorod on Ag substrate demonstrates a blue shift as the gap size increases [ 21 ], while for the case of Si nanorod on Au substrate, the resonant wavelength is blue-shifted with the decrease of thin film thickness [ 23 ]. In addition, some researches demonstrate that the MD resonant wavelength is hardly changed or slightly red-shifted when Si nanosphere is away from metallic substrate [ 24 , 25 ].…”

“…It is important to figure out the dependency relationship between resonant frequency and their gap size. Such a frequency shift will sharply degrade the performance of many optical devices designed at specific wavelength, such as nonlinear optics [ 23 , 24 , 26 ] and reflection-type metasurfaces [ 27 – 30 ].…”