  The influences of optical fields on the tunneling time in graphene are
investigated in real time using the finite-difference time-domain method. The
tunneling time of electrons irradiated by an optical field is significantly
different from that observed in traditional quantum tunneling. We found that
when the barrier width increases, the group delay becomes constant for the
reflected wave packet, but increases linearly for the transmitted wave packet.
This peculiar tunneling effect can be attributed to current leakage in a
time-dependent barrier generated via the optical Stark effect.
