rk could be the beginning of an era in which photons are directed around a chip in much the same fashion as electrons have been maneuvered on conventional chips. "In retrospect, we could say the groundwork for this was laid more than 56 years ago with John Bardeen and Walter Brattain and their first germanium transistor," says Holonyak, who was Bardeen's first graduate student. "But the direct recombination involving a photon is weak in germanium materials, and John and Walter just wouldn't have seen the light-even if they had looked. If John were alive and we showed him this device, he would have to have a big grin." 4.6.3 VCSEL A VCSEL (pronounced "vixel") is a tiny laser that emits a beam vertically from a chip's surface. PARC has developed the world's most densely packed VCSEL array, with independently addressable laser elements placed on a linear pitch as small as 3 um. Silicon CMOS-based driver-integrated circuits can be combined with the laser array to form an ultradense optoelectronic multi-chip module using PARC's StressedMetal MEMS high-density interconnect technology. Such an imager-on-a-chip allows thousands of independently addressable lasers to be packaged in a compact module. The imagers could eliminate optical systems with mechanical moving parts, replacing them with a more compact, all solid- state light source. VCSELs could be used for building optical interconnects that transfer data between computers via lasers. One application would replace copper cables LIGHT EMITTERS with bundles of optical fiber ribbons to connect electronic boards in computer back-planes, enabling much faster data transfer rates. 4.6.4 Improved VCSEL With optical communications becoming increasingly important to telecom manufacturers, service providers, and users, scientists are pushing hard to create better performing lasers that promise to increase distance, boost effi- ciency, and reduce the cost of optical connections. At the University of Illinois at Urbana-Champaign, researchers have found a way to significantly imp