  We present a computational study of terahertz optical properties of a
grating-coupled plasmonic structure based on micrometer-thin InSb layers. We
find two strong absorption resonances that we interpret as standing surface
plasmon modes and investigate their dispersion relations, dependence on InSb
thickness, and the spatial distribution of the electric field. The observed
surface plasmon modes are well described by a simple theory of the air/InSb/air
trilayer. The plasmonic response of the grating/InSb structure is highly
sensitive to the dielectric environment and the presence of an analyte (e.g.,
lactose) at the InSb interface, which is promising for terahertz plasmonic
sensor applications. We determine the sensor sensitivity to be 7200 nm per
refractive index unit (or 0.06 THz per refractive index unit). The lower
surface plasmon mode also exhibits a splitting when tuned in resonance with the
vibrational mode of lactose at 1.37 THz. We propose that such interaction
between surface plasmon and vibrational modes can be used as the basis for a
new sensing modality that allows the detection of terahertz vibrational
fingerprints of an analyte.
