Putting a far finer point on visible light.

Putting a far finer point on visible light

By repackaging light as molecular disturbances known as excitons, researchers have accomplished the equivalent of passing a camel through the eye of a needle. In this remarkable scheme for "slimming down" light to get it through a tiny opening, an incoming beam strikes a microscopic crystal wedged in the narrow end of a tapered, open-ended tube to generate excitons. The light-generated excitons, effectively only one-billionth the volume of the corresponding light beam, roam through the crystal, readily passing through the narrow aperture, and then decay into visible light.

"It's very hard to force light through a very small opening," says chemist Raoul Kopelman of the University of Michigan in Ann Arbor. "Photons don't want to go through, but if you repackage the light as excitons, it has no problem getting through."

By channelling the incoming light's energy in this way, the crystal in effect acts as a tiny light source much smaller than the light's wavelength. Such a source may permit researchers to develop improved visible-light microscopes that would have the same high resolution as electron microscopes but likely would do much less damage to delicate biological samples. Exciton light sources may also prove useful for molecular sensing and as inexpensive alternatives to light-emitting diodes and diode lasers in optical integrated circuits.

Kopelman and his colleagues at Hebrew University in Jerusalem grow tiny crystals of anthracene at the tips of metal-coated glass micropipettes. The tips' inner diameters measure 100 nanometers or less. Intense ultraviolet light from an argon ion laser, directed through the pipette, illuminates the anthracene crystal, generating exitons that diffuse through the material. These excitons, which transport energy on a molecular or atomic scale, collect at certain spots on the anthracene crystal's surface, where they combine and decay into blue light. That light emerges from a point no larger than the crystal itself.

"With this approach, energy can be guided directly to the aperture at the pipette tip instead of being allowed to propagate freely in the form of an electromagnetic wave. . . ," the researchers report in the Jan. 5 SCIENCE. Energy-confining materials such as anthracene "allow light to be effectively transmitted through the 'bottleneck' created by the subwavelength dimensions of the tip near the aperture."

Such a transport process is also reminiscent of a key stage in photosynthesis. "Green plants have learned the same trick of collecting light over a large area and then channeling it to a very small place," Kopelman says.

He and his colleagues are now studying ways of using their light source for "near-field" optical microscopy. The idea is to bring a sample so close to the microscope that light emerging from an aperture doesn't have enough room to spread out into the wavelength-dependent diffraction pattern that normally limits a microscope's resolution. In near-field imaging, because the size and shape of the aperture rather than the wavelength determine the resolution, the new light source provides a solution to the problem of getting enough light through a sufficiently small aperture to create a clear image.