Scientists have created a crystal that acts like a semiconductor for light: It reflects wavelengths essential for optical communications but allows other wavelengths to pass through, akin to the way semiconductors harbor electrons at certain energies while excluding others. The new work, described in the 1 January Optics Letters, opens the door to a wide variety of new devices for optical communications and eventually perhaps even light-based computing.
The new device is the latest to emerge in the red-hot research field of photonic crystals. These crystals are made from a repeating pattern of reflecting elements, spaced at about twice the wavelength of light or other electromagnetic waves to be manipulated. Those waves, reflected by the many elements of the crystal, interfere with each other, which keeps them from being transmitted. Yet most photonic crystals have problems: Some work only with long wavelength radiation, such as microwaves, and others require exotic techniques to build.
Last July a team at Sandia National Laboratory in Albuquerque, New Mexico, reported using standard computer chip technology to build a three-dimensional (3D) lattice of silicon rods, each just 1.2 micrometers across, that acts like a 3D mirror for infrared light--albeit only light with a relatively long wavelength. For their current study, the Sandia team--led by physicist Shawn-Yu Lin and materials scientist Jim Fleming--decided to try to make a similar structure capable of manipulating infrared light at shorter wavelengths of around 1.5 micrometers, the kind most commonly used for optical communications. But to do so they had to shrink their silicon rods 10-fold to a mere 0.18 micrometers in width.
To accomplish this, they took advantage of recent improvements in computer chip manufacturing. By combining cutting-edge silicon deposition, masking, and etching methods, the researchers created layers of silicon rods stacked atop one another like a child's toy Lincoln Logs. When they beamed light of different frequencies at the assembly, it reflected light with a wavelength near 1.5 micrometers, while transmitting all others.
The new work will likely make a quick impact, says physicist Daniel Tsui of Princeton University. "Its applications are imminent in the world of photonics," says Tsui. Initially such applications are likely to include filters, light guides, and mirrors for conventional optical communications equipment. But Fleming adds that down the road, such devices could play a role in new kinds of photonic circuitry for ultrafast optical computers.
--ROBERT F. SERVICE