PRINCETON (US)—In photography, there are always trade-offs—zoom in for detail or go wide to capture the scene. An imaging method developed by Princeton University researchers could lead to lenses that reveal all parts of a panorama at once in high detail. The technique is designed to enhance resolution using normal light, allowing scientists to build microscopes and devices capable of so-called super-resolution.
Jason Fleischer, an assistant professor of electrical engineering at Princeton who led the research, says the method “allows you to take a closer look at an object without narrowing your field of view.”
Cameras and other optical devices—including the human eye—are limited by the amount of light that they can collect through their lens openings, or apertures. In order for a light ray to be recorded, it has to pass through the lens and reach the device’s “detector”—such as the eye’s retina or a digital camera’s detector. But many light rays never make it to the detector, either because they are too weak or because they are deflected.
This problem is particularly acute with details that are smaller than the wavelength of light. Light rays from such tiny features fade before they reach the lens. To capture these rays, devices have to probe very near the surface of the object and scan it point-by-point, stitching together a full image.
“In effect, these devices suffer from ‘tunnel vision,'” explains Fleischer.
The new method addresses the shortcomings of small apertures by taking advantage of the unusual properties of substances called nonlinear optical materials. In conventional lens materials such as glass or plastic, rays of light pass through without interacting with one another. In nonlinear materials, light rays mix with each other in complex ways. Rays that don’t reach the camera may pass along some of their information to rays that do get recorded by it. Thanks to the mixing of rays, information that would otherwise be lost manages to reach the camera.
The image from a nonlinear lens would therefore be rich in detail. Unfortunately, it would also be distorted—and useless for conventional optics. But if the information could be unscrambled, a computer could reconstruct a high-resolution undistorted image of the entire scene.
“In such an image all parts of the scene will be ‘zoomed in’ at the same time,” says Fleischer.
Until now, scientists have achieved this unscrambling and reconstruction only in highly constrained settings such as fiber-optic cables. To capture the visual information given by their nonlinear material, the Princeton researchers used equipment to take a special type of photograph, called a hologram. They also combined data from a normal camera. As the first step in processing all this information, they created a simplified model of the flow of light through a nonlinear material. Next they developed a mathematical technique that takes the distorted image and works backward to calculate the visual information at every point in space between the image and the object. This method makes it possible to create high-resolution images at any chosen point—at the camera, at the location of the object itself, or somewhere in between.
Armed with these techniques, the Princeton team set up its imaging system. The core component, a nonlinear wave mixer, is a rectangular pill-sized crystal of a material called strontium barium niobate. The researchers placed the object to be imaged on one side of the crystal and image-capturing equipment on the other. They tested the system by obtaining images of various objects, including a chart developed by the Air Force that is widely used to calibrate optical devices. In each case the system could image the objects with high resolution.
In addition to enhancing optical devices, the new technique could be applied to etch surfaces of computer chips and biomedical components, and could advance tomography, a technology often used to get 3-D images of body parts for medical diagnostics.
This research was supported by the National Science Foundation, the Department of Energy, and the Air Force Office of Scientific Research.
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