Observation of the Topologically Originated Edge States in large-gap Quasi-One-Dimensional a-Bi$_4$Br$_4$

Two-dimensional topological insulator features time-reversal-invariant spin-momentum-locked one-dimensional (1D) edge states with a linear energy dispersion. However, experimental access to 1D edge states is still of great challenge and only limited to few techniques to date. Here, by using infrared absorption spectroscopy, we observed robust topologically originated edge states in a-Bi4Br4 belts with definitive signature of strong infrared absorption at belt sides and distinct anisotropy with respect to light polarizations, which is further supported by first-principles calculations. Our work demonstrates for the first time that the infrared spectroscopy can offer a power-efficient approach in experimentally probing 1D edge states of topological materials.