Graphene based plasmonic photodetection

Abstract

Graphene is a promising material for novel photonic devices due to its broadband optical absorption, ultrafast carrier dynamics and electrical tunability. However, the quantum efficiency of graphene devices is intrinsically limited by low absorption of graphene (2.3% of normal incident light). To enhance light-matter interaction, optical focusing elements such as plasmonic metal nanoparticles (NPs) can be utilized. This thesis examines the interaction between graphene and plasmonic NPs with special emphasis on mechanisms of enhanced photocarrier generation, which is further applied to improve photodection efficiency of graphene plasmonic hybrid devices. In order to study graphene based devices, CVD graphene must first be transferred from the Cu growth substrate to a target substrate. To this end, various parameters of a transfer technique were studied focusing on the source of extrinsic doping. Next, a novel transfer process based on an hBN spacer layer between graphene and a polymethyl-methacryalate film was developed to facilitate large scale CVD graphene transfer and encapsulation. The demonstrated devices showed improved electrical and aging properties. Following successful demonstration of the transfer technique, graphene was combined with gold NPs to study structural and optical properties of these complexes. Using ultrafast measurements, carrier dynamics of the hybrid structures were analysed focusing on hot carrier generation under plasmon excitation. Based on the observed results, hot carrier generation in the graphene was attributed to direct photoexcitation through the intense plasmonic electromagnetic fields with a minimal contribution from charge transfer over the graphene/NP interface. Furthermore, highly localised carrier heating in graphene is anticipated to result in strong electronic temperature gradients. Asymmetric plasmon-nanobar electrical contacts were designed with a view to exploit plasmon-induced electronic temperature gradients via the thermoelectric effect. The findings of this work lead to improved knowledge of the internal graphene photodetection mechanisms as well as contributing to the understanding of graphene/plasmonic hybrids and their interactions.