Light turns a sharp corner with photonic bandgap crystals
By Yvonne Carts-Powell
Light spreads, light scatters, and in a waveguide, light rounds smooth bends. But light cannot turn sharp corners the way electrical current can. That inability is one of the barriers to creating all-optical communications and computing systems: The area required to turn light 90 without excessive scattering losses is about a square millimeter. That limits the ability to miniaturize optical or optoelectronic circuits.
However, two recent developments in photonic bandgap crystals may enable much more compact optoelectronics.
Last fall, researchers at Sandia National Laboratory and Massachusetts Institute of Technology reported in the journal Science turning microwave radiation 90 in a two-dimensional photonic bandgap crystal with effectively no loss. In early January, the same researchers reported in Optronic Letters the fabrication of a photonic bandgap crystal that operates in the infrared. The material is the smallest device with a complete three-dimensional photonic bandgap reported thus far, effective from 1.35 to 1.95 microns.
What is photonic bandgap material?
The device is called a photonic crystal because its regularly repeating internal structure and its dimensions create a bandgap--frequencies at which light cannot propagate. Unlike semiconductors, which have inherent bandgaps, photonic bandgap crystals are designed to block specific wavelengths. The device is made of slivers of silicon arranged in a grid--like the structure of a log cabin (see photo). The width of the "logs" and the distances between them are designed so that light within the bandgap reflects and interferes destructively.
The three-dimensional crystal is the latest development in a series of crystals created by Shawn-Yu Lin`s group at Sandia National Laboratories (Albuquerque, NM). In this case, the polycrystalline silicon rods are designed to be 0.18 micron wide and 0.22 micron tall, and have a distance between them (the pitch, or the lattice constant of the crystal) of 0.65 micron.
The rods were made using photolithographic and microelectromechanical engineering techniques. To create rods that are so much thinner than the typical minimum feature size for I-line photolithographic steppers, Lin and co-worker Jim Fleming used fillet processing, which takes advantage of a peculiarity of reactive ion etching (RIE). When RIE is used to remove a thin film, a tiny piece of the film remains at the edge of the step after the rest is gone. The sliver`s width is roughly equal to the thickness of the film. Because it is relatively easy to deposit thin films with thicknesses of only 180 nm, the researchers were able to make arrays of the rods with these very narrow widths.
How to turn light
Although the fabrication method is different from previous microwave photonic bandgap crystals, the design is the same. When the design was scaled down (a ratio of 6.6:1), the bandgap shifts by the same ratio. The previous two-dimensional microwave version, created by researchers at Sandia and the Massachusetts Institute of Technology (Cambridge, MA), used a square array of circular alumina rods with a dielectric constant of 8.9. The researchers created a defect in the crystal simply by removing a row of rods, allowing an optical mode to propagate within the crystal.
A distinctive feature of the photonic bandgap crystal with the defect is that the losses due to absorption are small. "Compared with a ridge dielectric waveguide," report the researchers, "signal loss is reduced by a factor of 10." The group estimates that the guiding loss of the straight waveguide is 0.3 dB/cm over the entire bandgap.
The researchers then created a 90 turn in the waveguide by replacing some rods and removing others to create a curve from three straight lines. The middle line can be considered to act as a small 45 mirror. Some of the light was lost, but at two wavelengths there was no loss at all. The bend radius was smaller than either wavelength, at about 1 micron.
For a sharp curve (consisting of two, rather than three, lines) the overall transmission efficiency for all the wavelengths was more than 80%. As a comparison, the transmission efficiency of a similarly shaped high-index dielectric waveguide is about 30%.
When taken together, the implications of these two developments are important for developers of lightwave devices. With these photonic bandgap crystals designed to operate at near-infrared wavelengths, waveguides with low-loss sharp bends, high-Q resonant cavities (resulting in lasers with much lower threshold currents), and singlemode light-emitting diodes are possible. The researchers have not yet made a device that has defects in the near-infrared range, although this is clearly the next step. q