  After the classification of topological states of matter has been clarified
for non-interacting electron systems, the theoretical connection between
gapless boundary modes and nontrivial bulk topological structures, and their
evolutions as a function of dimensions are now well understood. However, such
dimensional hierarchy has not been well established experimentally although
some indirect evidences were reported, for example, such as the half-quantized
Hall conductance via quantum Hall effect and extrapolation in the
quantum-oscillation measurement. In this paper, we report the appearance of the
possible chiral edge mode from the surface state of topological insulators
under magnetic fields, confirming the dimensional hierarchy in three
dimensional topological insulators. Applying laser pulses to the surface state
of Bi1-xSbx, we find that the sign of voltage relaxation in one edge becomes
opposite to that in the other edge only when magnetic fields are applied to the
topological insulating phase. We show that this sign difference originates from
the chirality of edge states, based on coupled time-dependent Poisson and
Boltzmann equations.
