A Tale of Two Technology Disruptions

h OVER THE PAST decade we have experienced what might be called a ‘‘perfect storm’’ in technological disruptions. The two seemingly separate communities of photonics and microprocessors have each undergone paradigm shifting developments that emerged intertwined and are driving the future of computing. With power dissipation severely limiting the frequency scaling of microprocessors, the emergence of chip multiprocessors (CMPs) had shifted computing to embrace highly parallel multicore architectures. Realizing performance in these new generations of chips is becoming increasingly dependent on how efficiently applications can exploit the growing number of parallel resources. Whereas computation power as measured in FLOPs was the key metric of past microprocessors, performance in today’s CMPs is dominated by data movement challenges. In this so-called ‘‘communications bound’’ era of computing, new technologies are sought that can deliver energy-efficient high-bandwidth interconnectivity. While optics is a natural communications technology with broad success in long-haul fiber optic telecommunications, integration at the chip scale, particularly with silicon, had been challenging. Silicon is perhaps one of the worst optical materials. With no direct bandgap it is extremely challenging to make active components such as modulators, receivers, and especially lasers. In addition, unlike fibers, the optical losses in silicon were substantial. The timing for a photonics integration breakthrough could not have been more perfect! Almost simultaneously to the multicore evolution in microprocessors, the breakthrough for silicon photonics occurred with the introduction of the CMOS 90-nm processing node. At this nanoscale fabrication capability, an optical waveguide could be formed with smooth enough features such that the optical mode could propagate with minimal loss. Almost instantly silicon transformed to a low-loss photonic substrate enabling the creation of nanophotonic waveguides that propagated light over several centimeters with minimal losses. Furthermore, the silicon material was friendly to the 1.5mwavelength range that is widely used in telecommunications. The next challenge centered on the development of active components, particularly optical modulators. The key invention arrived with the silicon photonic ring resonator. Essentially, although the electro-optic coefficient of silicon is weak, because of the low losses enabled by nanofabrication, researchers led by Lipson’s group at Cornell could make resonators with very high quality factor (Q)Vso that the optical mode circulated many times accumulating enough of a phase changeVor index change. The Q could reach 10 000 or more thus creating an effective index change that was sufficient to modulate the light. With these developments, and the visionary support