Substrate-engineered tunable bound states in the continuum and directional radiation in dielectric metasurfaces

TL;DR

This paper demonstrates how out-of-plane symmetry breaking in dielectric metasurfaces enables tunable bound states in the continuum with high Q-factors and directional radiation, advancing control over light-matter interactions.

Contribution

It reveals a general mechanism for tuning BICs through out-of-plane symmetry breaking in all-dielectric metasurfaces, supported by a versatile multilayer substrate platform.

Findings

High-Q BICs are maintained when guided-mode wavelength matches BIC and coupling is suppressed.

Layer number increases Q factor without shifting BIC wavelength.

Spectral detuning causes blueshift and Q degradation.

Abstract

Tunable bound states in the continuum (BICs) in metasurfaces offer powerful opportunities to control light-matter interactions, yet the role of out-of-plane symmetry breaking remains poorly understood. Here, we reveal a mechanism that enables tunable high-Q BICs and directional radiation through out-of-plane symmetry breaking in all-dielectric metasurfaces. A substrate-free metasurface composed of periodically arranged multilayer cylinders that support overlapping magnetic dipole and electric quadrupole resonances, yielding electric mirror and symmetry-protected BIC responses at 1550 nm. Introducing multilayer substrates breaks out-of-plane symmetry and excites guided modes. When the guided-mode wavelength matches that of the BIC and coupling to the substrate is suppressed, the BIC wavelength remains nearly invariant, while the Q factor increases with layer number. In contrast, spectral detuning and enhanced coupling lead to pronounced blueshifts and rapid Q degradation. The interplay between guided-mode matching and coupling strength thus governs whether a BIC remains robust or becomes tunable. These findings establish a general framework for BIC engineering via out-of-plane symmetry breaking and provide a versatile platform for tunable metasurfaces with potential applications in integrated optics.