rove the performance of VCSEL by drilling holes in their surfaces. Faster and cheaper long-haul optical communication systems, as well as photonic inte- grated circuits, could be the result. Low-cost VCSELs are currently used in data communication applications where beam quality is of little importance, such as in short fiber runs. To operate at higher speeds and over longer distances, however, the devices must function in a single transverse mode with a carefully controlled beam. "These characteristics are normally found only in very expensive lasers, not in mass-produced VCSELs," says Kent D. Choquette, a professor of electrical and computer engineering and a researcher at the university's Micro and Nanotechnology Laboratory. "By embedding a two-dimensional photonic crystal into the top face of a VCSEL, however, we can accurately design and control the device's mode characteristics." The two-dimensional photonic crystal, created by drilling holes in the semi- conductor surface, introduces a periodic change in the index of refraction, Choquette says. The holes represent regions of low refractive index, sur- rounded by semiconductor material where the index is higher. A particular combination of refractive indices will produce a single-mode waveguide that permits only one transverse wave of the laser beam to propagate. "Our pho- tonic crystal consists of a triangular array of circular holes that have been etched into the top of a VCSEL," Choquette says. "Because the index varia- tion has to be on the length scale of light, the periodicity of the holes must be on the order of several hundred nanometers." To create such a precise array of holes, the researchers first lithographically define the desired pattern into a silicon dioxide mask layer on the semicon- ductor surface using focused-ion beam etching. Researchers then bore holes into the semiconductor material using inductively coupled plasma etching. "By selectively varying parameters such as depth, diameter, and spacing of the holes, we can control the modal 