Decoupled spectral tuning for multi-band perfect absorption via Brillouin zone-folding induced quasi-BICs

Monolayer graphene-based perfect absorber plays a critical role in advanced sensing, photodetection, and optical communications. Significant effort has been devoted to realizing an ideal absorber at a single wavelength by integrating graphene onto a high-quality resonant metasurface. Here, we present what we believe to be a novel strategy for achieving independent, multi-band perfect absorption in graphene by harnessing Brillouin zone-folding induced quasi-bound states in the continuum (quasi-BICs). Our dual-grating design allows for folding two distinct guided modes to the Γ point, where controlled structural perturbations transform them into quasi-BICs with unique symmetry properties. We show that resonance wavelengths and quality factors of quasi-BICs can be independently tuned so that the critical coupling condition can be simultaneously satisfied, giving rise to perfect absorption at two distinct wavelengths. Moreover, such a strategy can be readily extended to a four-band configuration by incorporating a partially etched silicon layer with monolayer graphene. This versatile platform not only achieves perfect absorption at multiple wavelengths but also offers arbitrary spectral control, paving the way for next-generation multi-channel photonic devices with enhanced performance.