Integrated Plasmonic Lens Photodetector

James A. Shackleford1, Richard Grote1, Mark Currie2, Jonathan Spanier3, and Bahram Nabet1. (1) Electrical Engineering / Electrophysics, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, (2) Optical Sciences Division, Naval Research Laboratory, 4555 Overlook Ave., S.W, Washington, DC 20375, (3) Materials Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104

The metal-semiconductor-metal (MSM) photodetector is an attractive alternative to other types of photodetectors due to its ease of fabrication. Typical MSM photodetectors are characterized not only by their low dark current but also by their high speed, which is a result of their small device capacitance. The biggest barrier preventing the proliferation of MSM photodetectors in commercial applications is their relatively poor responsivity, which is a result of several factors including reflection of incident light at the metal surfaces. It has been theoretically and experimentally shown, however, that it is possible to harness and guide such incident light by implementing a properly designed metal nanostructure at a metal/dielectric interface. Such light guiding systems are designed for very specific wavelengths and are capable of sustaining resonant collective electron oscillations at their design frequencies. These collective electron oscillations, or surface plasmon polaritons (SPPs), are a manifestation of incident light coupling to free electrons in a metal. It has been shown through FDTD simulation that the implementation of such a periodic corrugation nanostructure is capable of guiding light so as to concentrate the incident field about an aperture.

By implementing a properly designed plasmonic lens, it is possible to guide light that is normally reflected at the gold interface into the active area of the MSM photodetector. In this fashion, the responsivity of the MSM photodetector is increased without sacrificing the speed of the device. Here we present a fabricated MSM photodetector that exhibits an enhanced photocurrent by integrating a properly tuned nanoscale grating structure that serves to guide light into the active area of the device by coupling incident photons to surface plasmon polaritons (SPP). Through localized laser excitation and time response data we show that the responsivity of such an MSM device may be increased without sacrificing speed.