  The electromagnetically induced transparency (EIT) in an $N$ configuration is
studied under both resonant and off-resonant conditions. In a certain
off-resonant condition the dark state of the four level system, which is almost
the same as the resonant dark state in $\Lambda$ configuration, is rebuilt. The
actual system with damping is examined using optical Bloch equation, both
numerically and analytically. Based on this detuned dark state, some new
applications with frequency shifts can be realized.

Quantum Physics