Selectable Narrowband Anisotropic Perfect Absorbers Based on α-MoO₃ Metamaterials for Refractive Index Sensing

Despite metamaterial-based absorbers enabling flexibly manipulating electromagnetic waves, achieving highly anisotropic and tunable absorption still remains a challenge. The discovery of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-phase molybdenum trioxide (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-MoO3), a 2-D van der Waals material with strong crystal anisotropy, has aroused significant interest in developing exotic polarization-dependent optoelectronic devices. However, effectively leveraging its anisotropic properties for perfect absorption requires careful structural design and optimization. In this work, we theoretically propose an anisotropic metamaterial perfect absorber (MPA) consisting of a square array of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-MoO3 nanostructures. The meta-atom composed of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-MoO3 ring intersected by a central cross deploying on a gold mirror is designed to realize narrowband perfect absorption for polarization along both [100] and [001] crystalline directions in the visible to the near-infrared region. Our analysis shows that the perfect absorption results from the interactions of the strongly localized electromagnetic field confinement induced by the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-MoO3’s crystal anisotropy, the magnetic dipole mode, and the Rayleigh anomalies (RAs) of the meta-atoms. Investigation of varying geometric parameters of the MPA demonstrates that the narrow perfect absorption bands can be precisely tunable. Moreover, the MPA exhibits a high bulk sensitivity of 593.88 nm RIU<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula> and a figure of merit of 19.94 RIU<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula>, which indicates strong potential for bulk sensing applications. Surface sensing characterization reveals that the surface sensitivity of the MPA is different over different absorbate layer thickness ranges under both y- and x-polarized excitations, highlighting the complexity of designing efficient biosensing platforms. These findings not only point out the challenges and opportunities for developing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-MoO3-based MPAs but also suggest great potential applications in integrated bulk sensing and biosensing technologies.