Ultra-low energy switches based on silicon photonic crystals for on- chip optical interconnects Sean P. Anderson* a , Philippe M. Fauchet a,b a The Institute of Optics, b Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY USA 14627 ABSTRACT Although switching techniques based on charge injection in silicon have progressed greatly in recent years, switching energies are still above 10 fJ/bit, which is considered the threshold for practical implementation in on-chip optical interconnects 1 . This is due primarily to silicon’s relatively weak electro-optic response 2 , as well as the large physical extent of existing switching geometries, both of which increase the energy required to achieve switching. By using a resonant approach in which the optical mode is spatially tightly confined, however, the volume of active material is decreased, resulting in reduced switching energy. In this paper we report on the use of a thin MOS capacitor to inject charge into a resonator based on a photonic crystal microcavity. By injecting charge only into the volume in which the optical mode is localized, switching energy can be reduced below 1 fJ/bit. The index shift available (n ~ 0.001) allows the use of a relatively low-Q resonator (Q ~ 550), enabling high optical bandwidth of 100 Gbps with a device footprint below 25 m 2 . Keywords: photonic crystal, electro-optic switch, modulator, low-energy, optical interconnect, charge injection, SOI, silicon 1. INTRODUCTION Optical interconnects have the potential to vastly increase the computational power of future microprocessors by replacing electrical interconnects at the system level. It has already been shown that they can exhibit lower latency (including lower timing jitter) and higher bandwidth density (if WDM is used) than electrical interconnects 3,4 . In order to constitute a truly viable alternative to electrical interconnects, however, optical interconnects must also be competitive in terms of power consumption. In state of the art approaches to optical interconnects, the modulator, which converts electrical signals into the optical domain, is the primary limitation in terms of power consumption. Reducing the power consumption of modulators becomes a particular challenge given the requirement of CMOS compatibility, which restricts the choice of active material to silicon only. D.A.B. Miller has suggested that power consumption is more readily compared on the basis of energy per transmitted bit, and has identified a target of ~10 fJ/bit as the threshold for practical implementation in on-chip optical interconnects 1 . Attempts at creating a low-energy modulator for optical interconnects, such as those based on microrings 5 or microdisks 6 , have been limited by the size of the modulated volumes, which is determined in large part by the circumference of the ring or disk. A photonic crystal microcavity, however, can confine the optical mode to a much smaller volume, on the order of a cubic wavelength. Switching can then be realized by moving electrical charge into or out of only the volume occupied by the mode, which can theoretically reduce switching energy to below 1 fJ/bit using existing microcavity geometries. In this paper we explain the physical operation of such a modulator, and show how it can be realized using a MOS capacitor structure to inject charge. We also provide preliminary details on the development of a higher-Q microcavity- based resonator that can be used to reduce switching energy by an additional order of magnitude. Modulators based on this approach exhibit high bandwidth and small footprint (< 25 m 2 ) using only CMOS-compatible materials, and can thus help enable optical interconnects as a viable alternative to their electrical counterparts. *sanderso@optics.rochester.edu; phone 1 585 275-1252 Silicon Photonics V, edited by Joel A. Kubby, Graham T. Reed, Proc. of SPIE Vol. 7606, 76060R © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.843592 Proc. of SPIE Vol. 7606 76060R-1 Downloaded from SPIE Digital Library on 26 Aug 2010 to 128.151.160.118. Terms of Use: http://spiedl.org/terms