Navy

Metamaterial photocathode for infrared image intensifier

Detection of light from visible through long-wave infrared (LWIR) wavelengths (0.4 to 15 μm) while meeting size, weight and power constraints of a handheld, or helmet-mounted unit

Photonics Military Technology Electronics

DoD photo

One of the issues with broadening the applicable spectrum for infrared sensors is their inability to achieve a high refresh rate and still minimize size, weight, and power consumption (SWaP) of the overall payload. The night vision goggle meets these criteria for high refresh rate and low SWaP, but it is limited to operation only in the visible and near infrared. The current state-of-the-art in mid-wave infrared (MWIR) imagers is a solid state sensor using Indium Antimonide (InSb) or type-II super-lattice semiconductor structures as the absorbing elements. Both of these material technologies require cryogenic cooling between 77 and 150 K. This deep cooling requirement is almost an immediate disqualification for handheld use because of the power and size needed for the cold Dewar and pumping components, but some MWIR systems do exist in a handheld form factor at a high cost.

Current uncooled LWIR imagers operate with an optimal SNR at a maximum of around 30 frames per second. Any further decrease in the response time of the pixel reduces the SNR below a usable limit. Thus current LWIR imagers are also not viable candidates as an out-of-band solution because they are not capable of operating high paced, high frame rate environment. For the short-wave infrared (SWIR), the dominant imaging technology is a solid state sensor with Indium Gallium Arsenide (InGaAs) pixels. The primary factor in its determination of use is the fact that it has a relatively high sensitivity and can operate at room temperature. Some tradeoffs for consideration for this technology are the large costs associated with manufacturing focal plane architectures and the power requirements associated with digital imaging arrays when operated at high frame rates.

A Navy scientist has developed a metamaterial photocathode to enable detection of light from visible through LWIR wavelengths. A metal-semiconductor with a negative electron affinity layer (MSNEA) is used to enhance electron emission. In this metamaterial stack which additionally includes a metal absorber layer and a silicon substrate layer, plasmonic decay creates a hot electron on the surface of the metal. Excess majority carriers (holes) in the p-type semiconductor diffuse into the metal across the Ohmic contact to balance out the charge. Electron-hole pairs are thermally created within the semiconductor as holes leave allowing extraction of electrons into the vacuum of the MSNEA surface. By incorporating an MSNEA structure into the metamaterial absorber, the structure is impedance matched for high absorption and optimized for electron emission from the surface of the metal. Because the core technology behind the absorption mechanism can be scaled to other absorption bands by size and shape, the only upper limit is shorter wavelengths and the ability to fabricate smaller structures.

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