Enhanced infrared photoresponse induced by symmetry breaking in a hybrid structure of graphene and plasmonic nanocavities

The infrared photoresponse of a hybrid structure of graphene and plasmonic nanocavities is enhanced by plasmonic symmetry breaking. The hybrid structure overcomes two bottleneck problems of metal-graphene-metal photodetectors, i.e. photoresponse cancellation due to contact symmetry and limited light absorption of graphene. In the hybrid structure, the graphene with two contacts is supported by a metal plane with a dielectric spacer, and covered by metal patches. When the metal patches fuse with one contact only, plasmonic nanocavities are formed at this specific contact, giving rise to locally enhanced absorption of graphene and a prolonged junction borderline. At the other contact, the absorption of graphene is suppressed by the bottom metal plane. As a result, a contrast as high as 105 times between the photoresponses at the two contact-graphene junctions is achieved. Due to the superior capability to couple the incident light into a localized mode, the plasmonic nanocavity enhances the responsivity of graphene one order of magnitude more effectively than a subwavelength metal grating. Further characterizations reveal that the resonant behavior of the hybrid structure is well controllable, the photoresponse time is shorter than several microseconds, and the photoresponse mechanism is attributed to photothermoelectric effect of photoexcited hot carriers. (C) 2020 Elsevier Ltd. All rights reserved.