  Stable transport of laser beams in highly over-dense plasmas is of
significance in fast ignition of inertial confinement fusion, relativistic
electrons generation, and powerful electromagnetic emission, but hard to
realize. Early in 1996, Harris proposed an electromagnetically induced
transparency (EIT) mechanism, analogous to the concept in atom physics, to
transport a low-frequency (LF) laser in over-dense plasmas aided by a
high-frequency pump laser. However, subsequent investigations show that EIT
cannot occur in real plasmas with boundaries. Here, our particle-in-cell
simulations show that EIT can occur in strongly-relativistic regime and results
in stable propagation of a LF laser in bounded plasmas with tens of its
critical density. A relativistic three-wave coupling model is developed, and
the criteria and frequency passband for EIT occurrence are presented. The
passband is sufficiently wide in strongly-relativistic regime, allowing EIT to
work sustainably. Nevertheless, it is narrowed to nearly an isolated point in
weakly-relativistic regime, which can explain the quenching of EIT in bounded
plasmas found in previous investigations.
