Electromagnetically induced transparency in a three-level lambda system with permanent dipole moments

Electromagnetically induced transparency in a three-level Lambda-type molecular system with nonzero permanent dipole moments is investigated. It is shown that in the (2+2)-transition processes, when the sign of d(21), the difference in permanent dipole moments of the probe transition, is positive, perfect electromagnetically induced transparency with steep normal dispersion could be obtained under specific conditions. In contrast, when the sign of d(21) is negative, surprisingly gain without inversion with steep anomalous dispersion could be attained.