Plasmonic lens increases photodetection capability

Researchers at Drexel University (Philadelphia, PA) and the Naval Research Laboratory (Washington, D.C.) have increased the photocurrent reaching a metal-semiconductor-metal (MSM) photodiode by applying corrugations to the metal contacts of the device without paying a penalty for its speed of operation.

Researchers at Drexel University (Philadelphia, PA) and the Naval Research Laboratory (Washington, D.C.) have increased the photocurrent reaching a metal-semiconductor-metal (MSM) photodiode by applying corrugations to the metal contacts of the device without paying a penalty for its speed of operation. This grating structure essentially creates a “plasmonic lens” that increases photon collection at the metal/dielectric interface through the creation of surface-plasmon polaritons (SPPs).

To fabricate the lens, grating parameters were optimized according to plasmonic coupling equations, resulting in a corrugation groove-to-pitch ratio of 1/2 and a height of 25 ± 5 nm for gratings fabricated in the gold metal layer using electron-beam lithography. These structures enable photons at the gallium arsenide (GaAs) edge of detection at 830 nm that would otherwise be reflected off of the surface metal of an unpatterned MSM photodetector to be guided as SPPs, increasing the electron-hole-pair generation rate and causing an increase in the GaAs detector responsivity. The MSM photodiode includes a 1 × 30 µm2 GaAs substrate that acts as the aperture of the plasmonic lens. Time-response experiments using an 830 nm, 100 fs Ti:sapphire laser source showed a factor-of-two increase in detector response using the plasmonic lens as compared to an unpatterned MSM photodetector, while maintaining the same speed of response. Simulations predict a factor-of-ten increase for an 855 nm photodiode with optimized grating structures. Contact Bahram Nabet at bnabet@coe.drexel.edu.

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