  In this work, we report numerical simulations and experiments of the optical
response of a gold nanostrip embedded in a silicon strip waveguide gap at
telecom wavelengths. We show that the spectral features observed in
transmission and reflection when the metallic nanostructure is inserted in the
gap are extremely different to those observed in free-space excitation. First,
we find that interference between the guided field and the electric dipolar
resonance of the metallic nanostructure results in high-contrast (> 10)
spectral features showing an asymmetric Fano spectral profile. Secondly, we
reveal a crossing in the transmission and reflection responses close to the
nanostructure resonance wavelength as a key feature of our system. This
approach, which can be realized using standard semiconductor nanofabrication
tools, could lead to fully exploit the extreme properties of subwavelength
metallic nanostructures in an on-chip configuration, with special relevance in
fields such as biosensing or optical switching.
