Merging bound states in the continuum for ultrahigh-Q near-perfect absorption in all-dielectric gratings

Quasibound states in the continuum (QBICs) in all-dielectric nanostructures provide a promising framework for narrow band perfect absorption due to their high-quality ($Q$) factors and strong field confinement. However, structures supporting isolated bound states in the continuum (BICs) face $Q$-factor limitations due to quadratic scaling with asymmetry parameters. This work introduces a subwavelength silicon grating that supports one symmetry-protected BIC (SP-BIC) at the $\mathrm{\ensuremath{\Gamma}}$ point and two Friedrich-Wintgen BICs (FW-BICs) at off-$\mathrm{\ensuremath{\Gamma}}$ points. By tuning the filling factor, these BICs merge topologically, producing a QBIC with enhanced $Q$ factors and stable resonant wavelengths over a broad range of incident angles. When integrated with monolayer graphene, this structure achieves 94.2% absorption at 1311.06 nm with a $Q$ factor of $1.05\ifmmode\times\else\texttimes\fi{}{10}^{5}$, despite graphenes inherent 2.3% absorption. The high-reflection background reduces radiative losses, surpassing the 50% absorption limit. This study offers a pathway for ultrahigh-$Q$ perfect absorption in dielectric nanostructures, paving the way for advancements in ultrafast optoelectronics.