Magnetic mirrors and plasmonic metasurfaces for mid-infrared graphene photodetectors and biosensors

Abstract

Graphene is the name given to a monolayer of carbon atoms arranged in a two-dimensional honeycomb lattice. Recently, there has been much interest concerning the use of graphene in photodetectors and biosensors due to its unique electronic and optical properties. Specifically, graphene is an attractive material for developing broadband and high-speed photodetectors because of its gapless band structure and ultrafast carrier dynamics. The high spatial confinement and electrical tunability of mid-infrared (MIR) graphene plasmon have also been used for biosensors which permit the quantification and identification of biomolecule monolayers. However, the realisation of high-performance graphene photodetectors operating in the MIR is hindered by the intrinsically low optical absorption (< 2.3 %) and short carrier lifetime (sub-picosecond) of this material. In addition, the sensitivity of graphene biosensors based on plasmons is limited by the relatively small field enhancement of graphene plasmons compared to that of conventional metal plasmons.

In this thesis, we present nano-optical approaches to enhance the performance of graphene-based photodetectors and biosensors operating in the MIR by employing magnetic mirrors and/or plasmonic metasurfaces.

First, we propose and experimentally demonstrate a long-wave infrared device that we termed a magnetic mirror, which consists of an array of amorphous silicon cuboids on a gold film. The device is demonstrated to reflect light with high reflectance and zero phase shift. A modified multipole analysis method is devised and employed to interpret the magnetic mirror behaviour. We investigate the use of this device in a graphene photodetector application and show that the light absorption by graphene placed on top can be boosted by more than three orders of magnitude compared to the absorption that would occur were the graphene instead placed on a gold mirror. This is achieved by producing a field distribution with enhanced intensity at the device surface.

Second, we design and experimentally demonstrate a mid-wave infrared polarization-independent graphene photodetector via the integration of plasmonic nanoantennas that we term Jerusalem-cross antennas (JC-antennas). The JC-antennas serve to concentrate the incident light onto graphene for strongly enhanced optical absorption, as well as to collect the photocarriers. We demonstrate mid-wave infrared detection both at room temperature and at cryogenic temperatures. Our device also shows a fast and broadband photoresponse that extends to visible and near-infrared wavelengths, thanks to the carrier collection by the JC-antennas.

Last, we propose and investigate a biosensor device that combines the strong field confinement and electrical tunability of graphene plasmons with the large field enhancement of metallic nanoantennas. The device consists of an array of plasmonic nanoantennas and graphene nanoslits on a resonant substrate. Systematic electromagnetic simulations are performed to quantify the sensing performance of the proposed device. Our simulations show that the proposed device outperforms designs in which only plasmons from metallic nanoantennas or plasmons from graphene are utilized.