Photon sieves enhance weapons vision
Researchers at the University of Florida (UF; Gainesville, FL) are combining several well-known optical techniques and phenomena—including diffractive optics, Fresnel zone plates, photon sieves, and surface plasmons...
Researchers at the University of Florida (UF; Gainesville, FL) are combining several well-known optical techniques and phenomena—including diffractive optics, Fresnel zone plates, photon sieves, and surface plasmons—to create what they believe will be a better vision system for smart weapons. The goal is to give users the ability to focus in one direction with high resolution without sacrificing peripheral vision—much like the human eye sees and perceives images naturally.
In order to achieve this, the UF system actually mimics the vision system of insects rather than that of humans. According to Paul Holloway, professor of materials science and engineering at UF and the project's lead researcher, current smart weapons rely on refractive optics to produce a focused view of the target. The resulting image is like that seen through a telescope—the view of the target is good, but the surroundings are completely lost. This limits a weapon's accuracy on moving targets and its ability to overcome flares or other counter measures designed to confuse the weapon. In addition, the mechanical systems required to move the lens and keep the target in view make refractive systems relatively heavy, increasing both the size and the cost of these systems.
Holloway and his colleagues have opted instead to use diffractive optics, in which interference effects redirect the light rather than bend it. In particular, they have modified the zones in Fresnel zone plates, replacing the transparent rings used in the lenses to diffract light into a single, marginally focused beam with photon sieves that sharpen the focus quality of the beam (see Fig. 1). If light is passed through a mask made up of a pattern of concentric rings of quadratically decreasing width, a focus is formed. These zone plates are used to focus soft x-rays, which conventional optics cannot focus because of strong absorption of all materials in this spectral region. The ultimate resolution of a Fresnel zone plate is determined by the width of the outermost zone. The focal spot is surrounded by rings of light that blur the images obtained in x-ray microscopy and scanning spectroscopy.1
These limitations can be overcome by using a large number of appropriately distributed pinholes (photon sieves) instead of rings as the diffracting elements. To obtain a distinct first-order focus, the pinholes have to be positioned such that the optical path length from the source via the center of the pinholes to the focal point is an integral number of wavelengths. While photon sieves are typically used for x-rays or other electromagnetic radiation outside the visible light spectrum, Holloway and his group are the first to develop them specifically for use with visible and infrared energy.
"We think we can use this concept to make smart weapons smarter," Holloway said. "In a weapons application, you would like to have foviated vision, with the ability to look in one direction with high resolution but with peripheral vision also. With refractive optics, this currently requires a mechanical system to allow the lens system to scan and look for other objects. With our design, we believe we could design both high- and low-resolution lenses on the same platform."
Critical to the UF design is the creation of surface plasmons. According to Art Hebard, a UF physicist and member of the project team, although the holes help sharpen the focus of the light they also significantly reduce the amount of light that gets through the metal plate. When light strikes a metal surface, such as silver, it generates electrical charge oscillations, called surface plasmons. Hebard said the UF team has made progress in "reconverting" these plasmons into light by using microlithography to alter the surface characteristics of the metal.
"If you can corrugate or structure the metal properly, you can reconvert plasmons back into light," he said. "That way, you get increased transmission of light because some of the light that is hitting the opaque part of the lens is transmitted rather than absorbed."
The team has made and tested small prototypes of the lenses. Once perfected, the next step could be to put many such lenses together—some designed for high resolution, others for lower resolution—onto a surface to produce a multiple-eye effect, Holloway said. The result would be a lightweight panoramic vision device with no moving parts.
Smart weapons aren't the only potential application. Robots designed to operate autonomously, such as those used to transport nuclear materials, fight oil-well fires, or do other tasks too dangerous for people could also benefit from improved vision systems, he said. Eventually, such lenses may even replace refractive lenses in consumer products, such as cameras, making them lighter and potentially reducing their costs.
Other researchers working on the UF project, which has so far received more than $400,000 in phase I funding from the U.S. Defense Advanced Research Projects Agency, include David Tanner, a UF professor of physics; Mark Davidson, a UF research scientist; and Gary McGuire, Olga Shenderova, and Alex Shenderov, researchers at the International Technology Center (Research Triangle Park, NC). Raytheon Missile Systems (Tucson, AZ) is also involved in the research and development, according to Holloway.
- L. Kipp et al., Nature 414, 184 (Nov 8, 2001).