Date: Dec 15, 2009 Author: Joan M. Zimmermann Source: MDA (
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by Joan M. Zimmermann/jzimmermann@nttc.edu
A new anti-reflective coating has two faces: it can sequester light more efficiently in optical sensors or solar cells, or it can keep light from being detected by other sensors.
Magnolia Optical Technologies, Inc. (Woburn, MA), has developed an anti-reflective optical coating that can both reduce the cost and improve the performance of optical sensors and has potential applications for solar cells and medical optics. The coating, developed with help from several supporting MDA Phase I SBIRs to optimize the multicolor sensor performance of MDA's next-generation focal plane arrays, function to capture as much incident light as possible, from ultraviolet (UV) to infrared (IR) with visible light in between.
Magnolia's anti-reflective (A/R) coating reduces the amount of light leaving the sensor, thus it has the potential to increase the efficiency of solar cells, for example. By testing and combining materials such as cadmium telluride, silicon nitride, and diamond-like carbon (DLC), which possess the desired spectral range (8-14 microns) and excellent bond strength, the company produced anti-reflective coatings with a high optical quality and controlled physical properties. The low-cost coating can be used over the entire spectrum of incident light, including further into the infrared, an area where solar sensors, particularly the newer inorganic ones, have been traditionally deficient in light utilization. Thus if a solar cell technology manages to push further into the infrared end of the spectrum, the coating will increase the amount of radiation that can be collected, increasing the efficiency and power-generating capability of the sensor. Efficient light capture also means increased efficiency for other optical sensors, such as those used in infrared imaging.
Low-cost, high-efficiency optical sensors have their place in dozens of applications, from security cameras that detect body heat, to optical coherence tomography, an emerging form of biological imaging. The coatings also have some uses in military situations where reflective gear is a hazard. In combat situations, rifle scopes can provide visual clues to a sniper, such as when the glass on a scope reflects sunlight. Infrared sensors also can be used to pick up the heat of a firearm that has just been discharged, pinpointing a sniper's location. Depending on which side you're on, the ability to either detect or prevent detection of this light has its obvious advantages. When used in a light sensor, the A/R coating can improve the sensor's ability to pick up the glinting of reflective material, or conversely, the A/R coating can reduce the reflectivity of the weapon's features.
In related work stemming from MDA SBIRs to develop gallium nitride (GaN) and zinc oxide (ZnO) UV arrays, Magnolia is also developing ZnO nanowires (under a Phase II STTR sponsored by another agency) that have potential as both UV sensors and solar materials. The use of nanowires as solar collectors has the potential to reduce the cost of solar power roughly four-fold, to less than $1 per watt, making them a powerful competitor in the growing alternative energy market.
The ZnO nanowires are hollow tubes about one one-thousandth the width of a human hair. Magnolia's co-founder Ashok Sood says the nanowires have some intellectual grounding in an MDA Phase I SBIR for the development of single-photon-detecting focal plane arrays. These nanowires can convert light into an electric signal, translate motion into an electrical current, and detect and identify gases. They are extremely sensitive to the UV flash that is given off when a firearm is discharged, potentially thousands of times more sensitive than other sensors. In fact, other Federal agencies are now funding Magnolia to further develop its nanowires for a variety of sensor applications. The ZnO nanowires directly convert photons into signals that can be processed electronically and fed into a computer program that locates the source of gunfire. Naturally, their sensitivity to UV and ability to convert light to electricity also make the nanowires prime candidates for solar cells. The nanowire shape confers more surface area for gathering sunlight, creating a more efficient conduit for the electrons being transferred to the solar cell's electrodes.
The nanowires can also convert electricity to light. In this application, they can be used as light-emitting diodes (LEDs) to generate intense UV light. Just as the sun's rays have disinfecting properties, a ZnO nanowire-based device could render bacteria-laden water potable with five minutes of exposure. In the same vein, researchers think the nanowires could be used to produce UV lasers.
The nanowires also have piezoelectric properties—creating an electrical current when bent. Developers foresee the nanowires embedded in "power-generating" clothing. In a rare case of engineers turning to fashion, such outfits could convert the wearer's motion into current for recharging batteries or powering small electronic devices. Dr. Sood is eager to bring his technologies to fruition. Magnolia is seeking partnerships and venture capital in order to mature and refine the company's promising capabilities.
