Dual‐Resonant Binary Metasurfaces Tailored for Synergistic Excitation/Emission Enhancement of Upconversion Nanoparticles

ABSTRACT Upconversion nanoparticles (UCNPs) have garnered significant attention across diverse fields owing to their outstanding luminescent properties. However, their practical applications are still hampered by intrinsically weak luminescence, which results from small absorption cross‐sections and low quantum yields. Plasmonic near‐field enhancement offers a promising strategy to address this limitation. Yet, the large spectral separation between both the excitation and emission wavelengths of UCNPs makes it challenging for a single resonant mode to cover these two wavelengths. This study proposes a plasmonic binary metasurface (BM) that simultaneously supports the quasi‐bound states in the continuum (qBIC) and broadband surface lattice resonance (SLR) modes. The BM enables a 402‐fold enhancement of upconversion luminescence (UCL) by precisely aligning these two modes with both the excitation and emission wavelengths of UCNPs. Wavelength optimization of these two modes is achieved via a deep learning‐assisted inverse design approach. Furthermore, we elucidate the excitation mechanisms and near‐field distributions of these two modes, as well as their respective contributions to the UCNPs enhancement. Specifically, the qBIC mode significantly increases the absorption cross‐section of sensitizer ions by enhancing the excitation field, and the SLR mode accelerates the radiative decay rate by increasing the photonic density of state via the Purcell effect with a slight lifetime reduction. This study leverages the flexible structural design of the metasurface with multiple resonance modes to achieve simultaneous enhancement of both the excitation and emission of UCNPs, establishing a new paradigm for high‐efficiency upconversion photonics.