PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 791 September 6, 2006 by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi www.aip.org/pnu
LASER OPTICAL ANTENNAS represent a relatively new approach to getting around the old diffraction limit characterizing conventional optics, namely the inability of a lens to focus light for imaging purposes to any better than about half the wavelength of the light being used. Like a rooftop antenna which grabs meter-sized radio waves and turns them (courtesy of a tuned circuit) into signals far smaller in physical extent, so the optical antenna converts visible light into an illuminating beam of much higher resolving power. For example, 800-nm light can produce images with a spatial resolution of no better than about 400 nm. A new device, built by the groups of Ken Crozier and Federico Capasso at Harvard, producing spot sizes as small as 40 nm using 800-nm light, is the first optical antenna to be fully integrated (laser and focusing apparatus on one platform) and the first to prove (by directly measuring light intensities) the narrowness of the focused spot of light. Their method combines two proven techniques---plasmonics, in which light waves, striking a metal surface, can create plasmons, which are a sort of electromagnetic disturbance (see http://www.aip.org/pnu/2006/split/770-1.html for background) with a wavelength less than that of the incoming light; and near-field microscopy, in which the diffraction limit is avoided by placing the specimen very close to the imaging device. In the Harvard setup the antenna consists of two gold patches (130 nm long by 50 nm wide) separated by a 30 nm gap. Light falling on the gold strips (which sit right on the facet of an ordinary commercial laser diode) excites a huge electric field in the gap. A specimen located beneath this gap sees it as a 30-nm wide burst of light (although at this stage in the work the spot size is more like a 40 nm x 100 nm rectangle). In many forms of subtle microscopy, power is sometimes feeble, but here, in pulsed operation, the antenna can generate a robust peak intensity of more than a gigawatt/cm^2. (For comparison images recorded with a force microscope, an electron microscope, and the new laser antenna, see http://www.aip.org/png/2006/266.htm ). Crozier ([EMAIL PROTECTED], 617-496-1441) says that spot sizes of 20 nm should be possible and that likely applications for their laser antenna will be found in the areas of optical data storage (where 3 terabytes of data could be stored on a CD), spatially-resolved chemical imaging, and near-field scanning optical microscopy (NSOM). (Cubukcu et al., Applied Physics Letters, August 28, 2006; lab website at www.deas.harvard.edu/crozier ; see also http://www.aip.org/pnu/2004/split/701-1.html) ARTIFICIAL MUSCLES FOR LIFELIKE COLOR DISPLAYS. Adjustable diffraction gratings made of tiny artificial muscles could bring more lifelike colors to TVs and computer displays, physicists at ETH Zurich show in the September 1 issue of Optics Letters. In ordinary displays such as TV tubes, flat-screen LCDs, or plasma screens, each pixel is composed of three light-emitting elements, one for each of the fundamental colors red, green, and blue. The fundamental colors in each pixel are fixed, and only their amounts can change--by adjusting the brightness of the color elements---to create different composite colors. That way, existing displays can reproduce most visible colors, but not all. For example, current displays do not faithfully reproduce the hues of blue one can see in the sky or in the sea, says Manuel Aschwanden ([EMAIL PROTECTED], +41-44-632-08-04). Aschwanden and his colleague Andreas Stemmer figured that one can overcome such limitations by changing the fundamental colors themselves, not just their brightness, using a tunable diffraction grating. In their setup, white light hits a 100-micron wide, gold-coated artificial muscle membrane that's been molded into a shape that resembles microscopic pleated window shades. The artificial muscle is made of a polymer that contracts when voltage is applied. When white light hits a diffraction grating, different wavelengths fan out at different angles. "It's like when you hold a CD in direct sunlight, and you rotate it," Aschwanden says. Like the microscopic tracks on a CD surface, the grooves on the artificial muscle split white light into a rainbow of colors. But instead of rotating the surface to obtain different colors, the ETH team adjusts the diffraction angle by applying different voltages to the artificial muscle. As the membrane stretches or relaxes, the incoming light "sees" the grooves spaced closer or tighter. All the angles of reflection change, so the entire fan of wavelengths turns as a whole. The desired color can then be isolated by passing the light through a hole: As the hole stays fixed, different parts of the spectrum will hit it and go through it. To obtain composite colors, every pixel would use two or more diffraction gratings. By this method, a display could produce the full range of colors that the human eye can perceive, Aschwanden says. Tunable diffraction gratings are routinely used in applications such as fiberoptic telecommunications and video projectors, but existing technologies are based on hard, piezoelectric materials rather than artificial muscles, limiting their stretchability to less than a percentage point. By contrast, artificial muscles can change their length by large amounts. Getting a full range of colors requires a source of "true" white light to begin with -- rather than a mere combination of red, green and blue that looks like white light to the human eye. For that purpose, the technology could exploit a new generation of white LED lights that have recently been developed, Aschwanden says (see PNU 772, http://www.aip.org/pnu/2006/split/772-3.html). ) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. It is provided free of charge as a way of broadly disseminating information about physics and physicists. For that reason, you are free to post it, if you like, where others can read it, providing only that you credit AIP. Physics News Update appears approximately once a week. 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