Ultra-Narrow Dual-Band Anisotropic Perfect Absorbers Based on α-MoO ₃ Nanodisc Metamaterials for Refractive Index Sensing

The intrinsic in-plane anisotropy of α-MoO 3 offers untapped capabilities for polarization-dependent precise manipulation of electromagnetic waves; however, its potential in high- Q near-infrared perfect absorbers with ultra-narrow bandwidth and sensing applications remains largely unexplored. In this work, an anisotropic metamaterial perfect absorber (MPA) consisting of α-MoO 3 nanodisc arrays deposited on a SiO 2 spacer with a gold film underneath is demonstrated and investigated numerically. The proposed MPA exhibits narrow dual-band perfect absorption for polarization along the [100] direction in the near-infrared range with an extremely narrow band of 5.4 nm, while for polarization along the [001] direction, a line width of 9.2 nm is observed, which are distinct from previously reported α-MoO 3 devices operating in the visible-to-infrared region. The corresponding electric field distributions and energy absorption reveal that the resonance mechanism arises from the unique hybrid coupling between the strongly localized field confinement stemming from the α-MoO 3 anisotropy-driven localized multipolar (dipole and quadrupole) resonances, Rayleigh anomalies of the α-MoO 3 nanodisc arrays, and Fabry–Pérot cavity resonances formed in the SiO 2 layer. Influences of various parameters on MPA’s optical properties are interrogated in detail, unveiling a high geometrical tolerance (>95% stability under ±15% dimensional deviations). More importantly, such a unique anisotropy-driven hybrid coupling mechanism enables a high sensitivity of 338.5 nm RIU –1 and a figure of merit of 65.15 RIU –1, outperforming previously reported α-MoO 3 -based absorbers. The demonstrated concept establishes a design paradigm for ultra-narrow, polarization-dependent, anisotropic metamaterials and paves the way toward high-performance on-chip photonic and sensing technologies based on anisotropic materials.