  Modern computing and data storage systems increasingly rely on parallel
architectures where processing and storage load is distributed within a cluster
of nodes. The necessity for high-bandwidth data links has made optical
communication a critical constituent of modern information systems and silicon
the leading platform for creating the necessary optical components. While
silicon is arguably the most extensively studied material in history, one of
its most important attributes, an analysis of its capacity to carry optical
information, has not been reported. The calculation of the information capacity
of silicon is complicated by nonlinear losses, phenomena that emerge in optical
nanowires as a result of the concentration of optical power in a small
geometry. Nonlinear losses are absent in silica glass optical fiber and other
common communication channels. While nonlinear loss in silicon is well known,
noise and fluctuations that arise from it have never been considered. Here we
report sources of fluctuations that arise from two-photon absorption and
free-carrier plasma effects and use these results to investigate the
theoretical limit of the information capacity of silicon. Our results show that
noise and fluctuations due to nonlinear absorption become significant and limit
the information capacity well before nonlinear loss itself becomes dominant. We
present closed-form analytical expressions that quantify the capacity and
provide an intuitive understanding of the underlying interactions. Our results
provide the capacity limit and its origin, and suggest solutions for extending
it via coding and coherent signaling. The amount of information that can be
transmitted by light through silicon is the key element in future information
systems. Results presented here are not only applicable to silicon but also to
other semiconductor optical channels.
