  The recent development of the terahertz waveguide makes it an excellent
platform for integrating many intriguing functionalities, which offers
tremendous potential to build compact and robust terahertz systems. In the
context of next-generation high-speed communication links at the terahertz
band, engineering the dispersion and birefringence of terahertz waves is
essential. Here, we experimentally demonstrate subwavelength birefringent
waveguide gratings based on the low-loss cyclic olefin copolymer exploiting
micro-machining fabrication techniques. Asymmetric cross-section and
periodic-structural modulation along propagation direction are introduced to
achieve birefringent THz grating for filtering and dispersion compensation.
Because of strong index modulation in the subwavelength fiber, a high negative
group velocity dispersion of -188 (-88) ps/mm/THz is achieved at 0.15 THz for
x-polarization (y-polarization), i.e., 7.5 times increase compared to the
state-of-the-art reported to date. Such high negative dispersion is realized in
a 43 mm grating length, which is less than half of the length reported until
now (e.g., 100 mm). Further, the subwavelength fiber grating filters two
orthogonal polarization states and exhibits transmission dips with 8.5-dB and
7.5-dB extinction ratios for x and y polarization, respectively. Our experiment
demonstrates the feasibility of using polymer-based terahertz gratings as a
dispersion compensator in terahertz communications and steering polarized
terahertz radiations for polarization-sensitive THz systems.
