  The rising of quantum spin Hall insulators (QSHI) in two-dimensional (2D)
systems has been attracting significant interest in current research, for which
the 1D helical edge states, a hallmark of QSHI, are widely expected to be a
promising platform for next-generation optoelectronics. However, the dynamics
of the 1D edge states has not yet been experimentally addressed. Here, we
report the observation of optical response of the topological helical edge
states in a-Bi4Br4, using the infrared-pump infrared-probe microscopic
spectroscopy. Remarkably, we observe that the carrier lifetime of the helical
edge states reaches nanosecond-scale at room temperature, which is about 2 - 3
orders longer than that of most 2D topological surface states and is even
comparable with that of the well developed optoelectronics semiconductors used
in modern industry. The ultralong carrier lifetime of the topological edge
states may be attributed to their helical and 1D nature. Our findings not only
provide an ideal material for further investigations of the carrier dynamics of
1D helical edge states but also pave the way for its application in
optoelectronics.
