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26 Mar 2009
US researchers are seeking to patent a simple photodetector design that incorporates a plasmonic lens that doubles the efficiency of the device.
A team of scientists has doubled the efficiency of a photodetector by incorporating a plasmonic lens that encourages surface plasmons to couple to incident photons. The photodetector demonstrates up to 90% increase in sensitivity at a wavelength of 830 nm, which the researchers say could benefit imaging and optical communication applications (Applied Physics Letters 94 083501).
"What is new is the integration of a plasmonic lens and a metal-semiconductor-metal (MSM) photodetector," Bahram Nabet, a professor at Drexel University, told optics.org. "This device is simpler than previous attempts to couple plasmonic lenses with photodetectors. In addition, the device is monolithically integrable with optoelectronic circuitry and operates over a wide wavelength range."
Increasing the efficiency and sensitivity of the photodetector means that it can be used for imaging in low light conditions and it would reduce the number of repeaters needed in optical communication networks.
In the design, the metallic cathode and anode of the photodetector are modified by ten parallel linear corrugations. Plasmons on the surface of the metal couple with incident photons to form surface plasmons, which allow light energy to be carried along the surface to a narrow central slit aperture measuring 1 µm.
"We have used the collective response of electrons in a specifically designed metallic structure to direct and collect light - that is we have used metal that reflects light as a lens," explained Nabet. "As a result the efficiency of the detector is doubled and importantly, the response speed of the device remains the same."
While Nabet's plasmonic lens has been designed to couple with 830 nm light, such lenses can be fabricated to operate over a broad range of wavelengths. Nabet and colleagues also believe that with careful lens design, device performance could be further improved.
"The lens can be designed to operate at visible wavelengths as well as at 1310 and 1550 nm," commented Nabet. "Our simulations show that proper design of aperture width, corrugation pitch, duty cycle and height could result in an increase in responsivity by a factor of 10.7 at 855 nm incident light."
Nabet and his colleagues (Professor Jonathan Spanier and graduate students James Shackleford and Richard Grote and Dr Marc Currie of the Office of Naval Research) will continue to work with two-dimensional and one-dimensional (nanowire) devices for detection of various stimuli such as light, charged particles or terahertz radiation. "Our research is supported by two grants from the National Science Foundation and by the US Army Research Office," concluded Nabet. "We also have commercialization activity supported by the university's technology transfer office."
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