Waveguide-based graphene mid-infrared photodetection and a pathway towards chemical-free device fabrication

Abstract

The mid-infrared (MIR) spectral region is of tremendous importance in both science and real world applications from planet formation to molecular species identification. To detect and extract information in this wavelength region on a widespread scale and in handheld devices, a planar integrated low cost sensing device is needed. A key missing element in this picture is waveguide detectors that can cover the necessary wavelength range and that can be wafer scale processed. Graphene based detectors could fill this void. A parametric study is presented on broadband light absorption in graphene on waveguide devices of varied designs, index contrasts and dimensions. Generic design information is provided, and Genetic Annealing algorithms combined with Eigenmode expansion analysis provide a shortest design of 121 micro long that absorbs >90% of light from 1 to 10 micro, and a wide range of designs under 500 micro long. This shows for the first time that 2-D material based broadband waveguide MIR photodetectors could be viably integrated in MIR planar optics devices.

In addition, a detailed study of graphene interdigitated electrode photodetectors fabrication and characterisation is presented with an original discussion and investigation of the experimental results. The findings indicate graphene devices fabricated by the conventional lithophilic process are suffering from undesired doping and contamination, thus degrading the device performance. To address these issues a chemical-free and lithography-free manufacturing route for graphene devices fabrication was developed by implementing femtosecond laser ablation and laser-induced forward transfer (LIFT) techniques, along with proposing a liquid metal based graphene growth method with the aim to provide 3-D conformal coating of lithographically defined areas of high quality single crystal graphene onto MIR waveguide photodetector arrays. An advanced femtosecond laser processing setup was developed and successfully employed to demonstrate the laser ablation of large area graphene on different substrates and LIFT metal droplets on graphene. An ion beam sputtering (IBS) has been fully assembled and the substrate preparation methodology has been unusual circumstances developed.