Shining a bright light on quantum darkness.

Shining a bright light on quantum darkness

In their quest for true darkness in the twilight zone of quantum mechanics, researchers have been exploring the possibility of machining materials or arranging microscopic, electrically insulating spheres into geometric forms that would completely exclude photons of certain wavelengths from a region of space. Such a structure would prevent an excited atom embedded within it from spontaneously emitting a photon, in effect greatly prolonging the time an atom could spend in an excited state.

As a first step toward that goal, physicists Nabil M. Lawandy and Jordi Martorell of Brown University in Providence, R.I., have now demonstrated that an orderly arrangement of tiny polystyrene spheres suspended in water can delay photon emission from laser-excited dye molecules trapped among the spheres. The researchers will report their findings in the Oct. 8 PHYSICAL REVIEW LETTERS.

Spontaneous emission of photons by atoms is such a fundamental, ubiquitous phenomenon that it's easy to forget that an excited atom will emit a photon only if the surrounding vacuum (the space between atoms) can receive it. The explanation for this effect hinges on the peculiar quantum-mechanical notion that the vacuum itself consists of a seething sea of electromagnetic fields that interact with photons and allow their passage. This vacuum field normally acts as a giant reservoir into which excited atoms can deposit photons.

To inhibit spontaneous emission, researchers have tried various strategies for modifying the vacuum to cancel out or suppress -- in a well-defined region of space -- the quantized electromagnetic fields required for carrying photons of a certain wavelength. One strategy involves creating an orderly array of identical spheres of just the right size and spacing to inhibit the emission or transmission of certain photons.

Lawandy and Martorell relied on nature to create the desired periodic structure. Uniformly sized, negatively charged polystyrene spheres immersed in pure water settle into an orderly pattern resembling the arrangement of atoms in a crystal. These colloidal crystals display a variety of optical effects, becoming transparent at some wavelengths and beautifully iridescent at others (see photo).

The Brown team used short pulses of yellow laser light to excite dye molecules distributed among the colloidal crystal's spheres. Ordinarily, the laser-excited dye molecules would spontaneously emit red light. But measurements of the number of molecules still excited after a given time period suggest that the presence of the polystyrene-sphere lattice delays photon emission in certain directions for this particular wavelength of light.

"The dye molecules are not giving off their photons as quickly," Lawandy says. "The [vacuum field] isn't there to tickle the dye molecules to radiate."

Although the results look promising, "it's a weak effect," says theorist K. Ming Leung of Polytechnic University in Brooklyn, N.Y. "One would like to achieve complete suppression regardless of the direction of travel of the photon."

That goal may now lie within reach. In the last few months, Leung and other theorists have developed ways to extend to photons the kind of calculations researchers conventionally use for modeling the behavior of electrons in materials.

"This field has really heated up in the past couple of months," says Eli Yablonovitch of Bell Communications Research in Red Banl, N.J. "The theorists can now tell us which structures to make, and we're making those structures. We already have evidence that they're going to work out beautifully."

Yablonovitch and his colleagues create these structures by drilling patterns of spherical hollows on the surfaces of flat, electrically insulating plates, which the researchers stack and bolt together to produce an array of air-filled spheres. They then study what happens to microwaves traveling through the arrays.

"Right now we are just trying to test the theory to see if it is sound," Leung says. "It's all done in microwaves because the longer wavelength allows one to fabricate the crystal in the lab just by drilling. But everything's to scale, and if it works, it should also work for visible light."

Achieving control of spontaneous emission in atoms holds the promise of improved laser performance and greater control of certain types of chemical reactions. Physicists would also have at their disposal a volume of space quite unlike any they have yet probed.

"This would actually be a volume of space that is quieter, or in a sense emptier, than the vacuum," Yablonovitch says. "If you were an atom inside this structure, you would be living in an unfamiliar world."