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Nanohole arrays patterned into aluminum (inset) can be employed as ultraviolet filtering elements as illustrated by the calculated electric field resonance shown in cross-section for a single hole when illuminated by a particular wavelength. Concepts in plasmonics and metamaterials can potentially be exploited to yield optical components and integrated detector systems that are difficult to accomplish with classical devices and conventional optical materials.
Above: Nanohole arrays patterned into aluminum (inset) can be employed as ultraviolet filtering elements as illustrated by the calculated electric field resonance shown in cross-section for a single hole when illuminated by a particular wavelength. Concepts in plasmonics and metamaterials can potentially be exploited to yield optical components and integrated detector systems that are difficult to accomplish with classical devices and conventional optical materials.

Novel Materials & Detectors

GaN Family of Materials: Intrinsic Solar Blindness, Environmental Robustness

Wide bandgap semiconductors such as gallium nitride and its alloys are excellent candidate materials for fabricating solar-blind UV detectors. One promising class of detectors is the avalanche photodiode (APD) which can provide high UV gains while rejecting longer wavelength light. This rejection results from the wide bandgap of the material. Because this is a solid-state device, it avoids the reactive free surfaces of photocathode-based detectors, and it will not require the high voltages needed for those tube-based detectors. In addition, due to the wide bandgap, the array will be capable of operating at higher temperatures and radiation levels than are possible with silicon arrays. This detector array will be capable of counting individual photons with high efficiency, high gain, and low noise. We have demonstrated interface-engineered p-i-n GaN detector designs with high quantum efficiency and low leakage.

We are developing solar-blind photocathodes for ultraviolet detector applications, using intrinsically solar-blind gallium-nitride-based materials. Photocathodes absorb incoming light and emit photo-excited electrons from the device surface. Because UV detection must often be performed in the presence of a large visible background, rejection of those longer wavelengths is critical. The use of filters to accomplish this rejection results in significant attenuation of the desired UV signal, but wide bandgap detector materials provide high rejection without sacrificing UV response. When paired with microchannel plates or electron bombarded CCDs, they enable solar-blind UV imaging and photon counting detection.

UV photocathodes using the gallium nitride family of materials provide high quantum efficiency, low dark count rate, and high out-of-band rejection. The performance of these detectors critically depends on the efficiency and stability of their photocathodes. While conventional photocathodes must be protected from air exposure in sealed vacuum tubes, we are developing air-exposable devices that will dramatically increase versatility and decrease cost. We use epitaxial techniques to exploit the natural high polarization and piezoelectricity of the gallium nitride semiconductor family, producing a band profile that provides high quantum efficiency without the need for reactive materials.
<strong>Top left:</strong> External quantum efficiency (electrons out / photons incident) as a function of photon wavelength for a GaN p-i-n APD with zero applied voltage. <strong>Top right:</strong> UV response for an non-cesiated GaN photocathode. Values of QE > 40% in the UV have now been demonstrated, while providing many orders of magnitude of visible rejection. <strong>Bottom:</strong> Dark current (blue) and photocurrent at 363 nm (red) for a GaN p-i-n APD. Calculated gain is shown in green. Avalanche gains of >105 have been measured on these devices.
Top left: External quantum efficiency (electrons out / photons incident) as a function of photon wavelength for a GaN p-i-n APD with zero applied voltage. Top right: UV response for an non-cesiated GaN photocathode. Values of QE > 40% in the UV have now been demonstrated, while providing many orders of magnitude of visible rejection. Bottom: Dark current (blue) and photocurrent at 363 nm (red) for a GaN p-i-n APD. Calculated gain is shown in green. Avalanche gains of >105 have been measured on these devices.
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Metamaterials: Moving Beyond Limitation of Conventional Material

Metamaterials are structures with arrays of periodic features, with dimensions that are much smaller than the wavelength of the light they are designed to interact with. By taking advantage of optoelectronic resonances with light that occur because of their small feature sizes, they can produce effective optical properties that are not found in natural bulk materials. Frequency response of phase and amplitude, in transmission or reflection, can be varied by variation of shape, size, and material composition of these arrays. Polarization sensitivity can also be achieved by introducing asymmetry into the feature shape. Although fabrication of 3D metamaterials can be challenging, 2D metasurfaces (in which engineered features are placed only on the surface) provide many of the breakthrough capabilities of 3D structures while substantially reducing fabrication difficulties.

At the MDL we are developing several types of metasurfaces for enhanced detector performance. There is a wide range of active research and development efforts worldwide to develop metamaterial components. Because of tailored wavelength response possible with metasurfaces, we are focusing on developing filter elements for wavelength ranges (such as ultraviolet) where traditional filter performance is poor. Frequency-selective UV anti-reflection coatings are being developed by using thin subwavelength multilayers, fabricated by atomic-layer deposition, that provide tailored frequency response. We are also developing 2D patterned metasurfaces that function as polarization-sensitive filters with engineered response over a particular range of wavelengths (see figure). These consist of 2D arrays of apertures in a thin metallic film that provide different transmission characteristics for the two polarization directions. These types of structures will provide enhanced detector response in a specific frequency band while providing rejection of unwanted frequencies.
<strong>Left:</strong> Doug Bell preparing samples. <br/> <strong>Right:</strong> Array of rectangular holes in a 50 nm aluminum layer. The holes are 150 x 50 nm in size and are fabricated by electron-beam lithography.
Left: Doug Bell preparing samples.
Right: Array of rectangular holes in a 50 nm aluminum layer. The holes are 150 x 50 nm in size and are fabricated by electron-beam lithography.
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