Simultaneously achieving narrowband and broadband light absorption enhancement in monolayer graphene

We study theoretically how to simultaneously achieve narrowband and broadband light absorption enhancement of monolayer graphene, which is placed on a metallic substrate surface with a periodic array of small holes. By a couple model of double resonances, the narrowband and broadband absorption peaks of monolayer graphene are proven to stem from the hybrid modes of delocalized surface plasmon polartions propagating on the substrate surface and localized magnetic plasmons confined within individual holes. The positions, maxima, and bandwidths of the absorption peaks can be tuned conveniently through changing the hole array period. For an optimized period, the maximum absorption values at the narrowband and broadband peaks are able to reach up to 65% and 85%, respectively. The bandwidth of the narrowband absorption peak can be reduced to only several nanometers, and the bandwidth of the broadband absorption peak can be expanded to tens of nanometers. Our work will be helpful to design graphene-based photoelectric nanodevices whose optical responses are in the visible and near-infrared regions.