Turning Opaque Materials into Lenses
Phase modulation allows nontransparent samples to focus laser light.
Michael J. Lander
Materials with a milky-white appearance hardly seem a likely choice for making lenses. Any light that manages to penetrate such materials comes out the other side in a faint, random speckle pattern. By shaping the wavefront of laser light entering opaque samples, however, researchers have found a way to give them focusing abilities. The method may permit improved imaging in situations where scattering is high.
Plane-wave light focused onto an opaque sample creates a random speckle pattern on the other side (left). When a specially programmed modulator is used to alter the wave, however, the sample acts as a lens rather than as a diffuser (right). Courtesy of Ivo M. Vellekoop, University of Twente.
Ivo M. Vellekoop and Allard P. Mosk at the University of Twente in Enschede, the Netherlands, conducted their tests with a 632.8-nm HeNe laser from JDSU of Milpitas, Calif., projected onto a spatial light phase modulator from Holoeye Photonics AG of Berlin with a Sony liquid crystal display. A beam condenser focused the modulated light onto the samples, which included a 10.1-μm-thick piece of titanium dioxide, desiccated and fresh daisy petals, an eggshell and a human tooth. Beyond each sample, the researchers placed a CCD camera to image the transmitted intensity pattern. The camera also provided feedback to an algorithm that programmed the phase modulator.
By setting the phase delay of the modulator to zero, the researchers first shot a plane wave at the titanium-dioxide sample, which produced a diffuse pattern of weak speckles. But under appropriate modulation, light directed at the sample produced a single spot with approximately 1000 times greater intensity than an individual speckle’s. Dried and fresh petals, the eggshell and the tooth yielded comparatively lower enhancements of 630×, 64×, 250× and 70×, respectively.
The basic principle behind the enhancement lies in the setup’s ability to control interference through modulation, which causes rays that normally would spread out to instead interfere constructively and converge on a single point. Traditional adaptive optics, in contrast, cannot correct the wavefront of the completely scrambled light that opaque substances let through. Efficiency of the investigators’ nontransparent lenses is low, however, because only about 1 percent of incoming light is transmitted to the focus.
The scientists are still researching the method. Systems utilizing modulated light with opaque lenses could offer advantages over crystal-clear optics in surgical or semiconductor applications, where the lenses would protect surfaces beyond the focus from radiation exposure. Inexpensive, they could find use in settings where damage or fouling is likely. Using substances such as metamaterials, the group hopes to synthesize opaque lenses with resolutions that match or exceed those of their transparent counterparts.
Optics Letters, Aug. 15, 2007, pp. 2309-2311.
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