  Topological systems are inherently robust to disorder and continuous
perturbations, resulting in dissipation-free edge transport of electrons in
quantum solids, or reflectionless guiding of photons and phonons in classical
wave systems characterized by topological invariants. Despite considerable
efforts, direct experimental demonstration of theoretically predicted robust,
lossless energy transport in topological insulators operating at terahertz
frequencies is needed further investigations to shed affirmative light on the
unique properties enabled by topological protection. Here, we introduce Kagome
lattice that exhibits a new class of symmetry-protected topological phases with
very low Berry curvature but nontrivial bulk polarization, and fabricate an
optical topological insulator that provide the valley hall effect. Theoretical
analysis show that four type edge states can be obtained. Measurements of
THz-TDs with high time-resolution demonstrate that terahertz wave propagating
along the straight topological edge and Z-shape edge with sharp turns have
almost same high transmission in 0.440 THz to 0.457 THz domain range. Those
results quantitatively illustrate the suppression of backscattering due to the
non-trivial topology of the structure. The THz-TDs measurement yields amplitude
and phase information, showing significant advantage compared to general
broadband infrared, single wavelength continuous-wave THz measurements and
visible spectroscopy. It allows further exploration of the effective refractive
index, group velocity and dispersion relations of edge states. Our work offers
possibilities for advanced control of the propagation and manipulation of THz
waves, and facilitates the applications including sixth-generation (6G)
wireless communication, terahertz integrated circuits, and interconnects for
intrachip and interchip communication.
