Dual Flat Bands Enable Synergistic Excitation–Emission Enhancement in Rare-Earth Ions

Abstract Rare-earth ions are pivotal for integrated photonic light sources, but their low optical transition cross sections inherently limit pump absorption efficiency and spontaneous emission rates. Here, we overcome these drawbacks by designing a dielectric metasurface that supports dual flat bands, which synergistically enhance both excitation and emission processes. Strong momentum–space anticrossings arise from hybridization between transverse-electric-like and transverse-magnetic-like surface lattice resonances hosted by a TiO2 nanodisk array integrated on a Yb3+-doped Ta2O5 slab waveguide, giving rise to two spectrally separated flat bands. The shorter-wavelength flat band resonantly boosts pump absorption, whereas the longer-wavelength counterpart elevates the radiative decay rate of emitters through the Purcell effect. Angle-resolved measurements corroborate the dispersion characteristics of these flat bands. Under resonant excitation, we demonstrate a photoluminescence enhancement of over 250-fold, alongside a marked reduction in emission lifetime from 0.47 to 0.1 ms. Back-focal-plane imaging further confirms that the boosted luminescence aligns with the pre-engineered flat-band dispersion relation. This scheme offers a general route to tailor pump absorption efficiency, radiative decay rate, and emission directivity toward high-performance rare-earth-ion-based on-chip light sources.