  SmB6 is a promising candidate for topological Kondo insulator. In this
letter, we report ultrafast carrier dynamics of SmB6. Two characteristic
temperatures: T1=100 K and T2= 20 K are observed. T1 corresponds to the opening
of the f-d hybridization gap revealed by an abrupt disappearance of terahertz
f-band plasmon oscillations. Between T1 and T2, a phonon bottleneck effect
dominates the photocarrier relaxation processes. Below T2, we observe the
formation of in-gap states, which are strongly affected by optically injected
hot electrons and the transient electron temperature change.
