  We use computational approaches to explore the role of a
high-refractive-index dielectric TiO2 grating with deep subwavelength thickness
on InSb as a tunable coupler for THz surface plasmons. We find a series of
resonances as the grating couples a normally-incident THz wave to standing
surface plasmon waves on both thin and thick InSb layers. In a marked contrast
with previously-explored metallic gratings, we observe the emergence of a much
stronger additional resonance. The mechanism of this giant plasmonic resonance
is well interpreted by the dispersion of surface plasmon excited in the
air\TiO2\InSb trilayer system. We demonstrate that both the frequency and the
intensity of the giant resonance can be tuned by varying dielectric grating
parameters, providing more flexible tunability than metallic gratings. The
phase and amplitude of the normally-incident THz wave are spatially modulated
by the dielectric grating to optimize the surface plasmon excitation. The giant
surface plasmon resonance gives rise to strong enhancement of the electric
field above the grating structure, which can be useful in sensing and
spectroscopy applications.
