Designing absorbers for graphene based mid-infrared wide band waveguide photodetectors.

The mid-infrared (MIR) spectral region is of great importance in scientific and real-world applications ranging from detecting forming planets to identifying molecular species for industrial process control. Existing instrumentation to perform analyses is neither low cost nor compact, robust, or low power consumption, presenting opportunities for a planar integrated MIR sensing device to cost effectively detect and extract information on a widespread scale and in handheld devices. A key missing element in this vision is low cost waveguide photodetectors, which can cover the necessary wavelength range and are made with a wafer scale process. 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 Finite Element modal analysis provide a shortest design of 121 µm long that absorbs >90% of light from 1 to 10 µm, and a wide range of designs under 500 µm 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.