Light cannot be focused on anything smaller than its wavelength—or so says more than a century of physics wisdom. But a new study now shows that it is possible, if light is focused extremely close to a very special kind of lens.
The traditional limit on the resolution of light microscopes, which depends on the sharpness of focusing, is a typical wavelength of visible light (around 500 nanometers). This limitation inspired the invention of the electron microscope for viewing smaller objects like viruses that are only 10 to 300 nanometers in size. But scientists have discovered that placing a special pattern of circles in front of a laser enables them to focus its beam down to 50 nanometers, tiny enough to illuminate viruses and nanoparticles.
To achieve this, scientists draw opaque concentric circles on a transparent plate with much shorter spacing than the wavelength of light, and vary the line thickness so that the circles are far apart at the center but practically overlap near the edge. This design ensures that light transmitted through the plate is brightest at the core and dimmer around the edges.
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"This construction is a way to convert traveling waves into evanescent waves," says Roberto Merlin, a physicist at the University of Michigan at Ann Arbor and author of the study published today in the online journal Science Express. Unlike ordinary light waves (such as sunlight), which can travel forever, evanescent waves traverse only very short distances before dying out. Whereas most of the light shining on such a plate is reflected back, a portion of the light leaks out the other side in the form of evanescent waves. If these waves, which have slipped through the different slits between the circles, can blend before disappearing, they form a single bright spot much smaller than the wavelength. The plate effectively acts like a "superlens", and the focal length or distance between the lens and the spot is nearly the same as that between the plate's bright center and dim edge; the size of the spot is fixed by the spacing between the circles.
With current nanofabrication technology, scientists say, it is not unreasonable to imagine circles spaced 50 nanometers apart giving a comparable spot size, which is about 10 times smaller than what conventional lenses can achieve. The rub, however, is that the smaller the spot, the faster it fades away from the plate. For instance, the intensity of the spot from a circle spacing of 50 nanometers would halve every 5.5 nanometers away from the plate, so anything that needs lighting would have to be extremely close to it. Positioning with such nanometer-scale precision is well within present technological acumen, and is routinely used in other microscopy techniques like scanning tunneling microscopy.
There are currently other ongoing investigations of so-called superlensing schemes, but researchers say this technique may be more tolerant to variations in the color of light used. This is important because scientists would want to use the brightest lasers available (usually pulsed lasers spanning a range of colors) due to the attenuation in intensity of the focused spot.
Merlin and his U.M. collaborator Anthony Grbic, an electrical engineer, are wrapping up the construction of a focusing system for microwaves based on the above theory; they say they're confident it will focus 30-centimeter wavelength microwaves onto a spot 1.5 centimeters wide. If a similar system could be built for light, it would enable the study of viral and nanoparticle structures by focusing light on them and detecting their scattered light. Other potential applications include larger capacity CDs and DVDs, which are currently limited by the size of the laser dot used to encode individual bits.