Parametric design study for waveguide-based graphene MIR photodetectors

Abstract

MIR sensing technology has the potential capacity to dramatically improve aspects of real-world problems - from planet formation to molecular species identification – once integrated “system on a chip” sensing devices are available at a palatable cost. A key limiter in achieving a full integrated on a chip design is the lack of broadband waveguide-based detectors, with no designs or experimental broadband devices having been demonstrated to date. Simulation and analysis conducted in this work has produced for the first time, practical graphene-based waveguide detector designs that can be made with existing and improved MIR waveguide technology. Using Rsoft FemSIM with high resolution graded gridding to accurately capture the effects in single layer graphene; extensive, and time consuming, parametrised simulations have been carried out for different waveguide designs to understand the design space available and optimise the absorption of light in graphene coated chalcogenide waveguides. The accuracy of the optical modelling and the values for the graphene material model were verified by modelling experimentally reported devices in the NIR region to ensure there is a close match to the measured points. Geometry (height, width), wavelength, waveguide type (Channel, Rib), Monolayer, bilayer and multilayer graphene layouts were the core variables investigated as functions of the waveguide core refractive index and top and bottom cladding indices. Fig. 1 give the examples of fully-etched waveguide design. Additionally, different materials as buffer/spacer layers to compress the field or to try and locate the graphene in a higher field region were also trialled. The results were contour graphs of absorption versus width and height for the fundamental mode at 10 different wavelengths with different Top/Bottom cladding index contrasts, from which it is possible to design wideband detectors.