  On-chip photonic networks have the potential to transmit and route
information more efficiently than electronic circuits. Recently, a number of
silicon-based optical devices including modulators, buffers, and wavelength
converts have been reported. However, a number of technical challenges need to
be overcome before these devices can be combined into network-level
architectures. In particular, due to the high refractive index contrast between
the core and cladding of semiconductor waveguides, nanoscale defects along the
waveguide often scatter light into the backward-propagating mode. These
reflections could result in unwanted feedback to optical sources or crosstalk
in bidirectional interconnects such as those employed in fiber-optic networks.
It is often assumed that these reflected waves spatially overlap the
forward-propagating waves making it difficult to implement optical circulators
or isolators which separate or attenuate light based on its propagation
direction. Here, we individually identify and map the near-field mode profiles
of forward-propagating and reflected light in a single-mode silicon waveguide
using Transmission-based near-field scanning optical microscopy (TraNSOM). We
show that unlike fiber-optic waveguides, the high-index-contrast and nanoscale
dimensions of semiconductor waveguides create counter propagating waves with
distinct spatial near-field profiles. These near-field differences are a
previously-unobserved consequence of nanoscale light confinement and could
provide a basis for novel elements to filter forward-propagating from reflected
light.
