Near-Unity and Narrowband Light Absorption in Mid-Infrared α-MoO3-Based Metasurfaces

Accessing mid-infrared narrowband absorption is crucial for applications such as spectroscopic analysis, infrared imaging and optical sensing. Here, we investigate the interaction between light and α-phase molybdenum trioxide (α-MoO3) nanocavities, uncovering pronounced and highly tunable resonant responses across the hyperbolic transition regime. Moving beyond standard phonon polariton excitations, our results reveal that the elevated refractive index of α-MoO3 beyond the Reststrahlen band promotes strong electromagnetic confinement and enhanced light-matter coupling in subwavelength nanostructures across the 12–13 μm spectral window. Analysis of the electric-field distributions reveals a hybrid collective mode arising from the interaction between Fabry–Perot cavity modes and geometry-dependent Mie-type resonances, leading to absorption approaching unity together with spectrally sharp resonances exhibiting quality factors exceeding 100 near the transverse-optical phonon frequency of α-MoO3. These findings provide novel insights for the development of compact mid-infrared photonic structures for tailoring thermal emission and enhancing electromagnetic coupling in α-MoO3-based nanophotonic platforms.