Evanescently coupled mid-infrared photodetector for integrated sensing applications: Theory and design

We present theory and simulation of a mid-infrared (λ = 3.2 μm) chalcogenide waveguide monolithically integrated with an evanescently coupled PbTe photodetector for lab-on-a-chip sensing applications. A spacer layer is used to modify effective index of the structure, enabling a waveguide-detector mode to propagate in the chalcogenide waveguide and be absorbed in the PbTe detector. The relation between quantum efficiency and detector dimensions is analyzed showing that the design geometry can be optimized to maximize signal to noise ratio. In addition, the location of metal contacts is optimized to minimize loss while maintaining good device performance. The design is compatible with standard planar lithographic processing and its flexibility suggests multiple applications in the fields of biological and chemical sensing.