Electromagnetically induced transparency in a Λ-type molecular system with permanent dipole moments revisited

Electromagnetically induced transparency (EIT) in a molecular three-level Lambda system with permanent dipole moments and undergoing m- and n-photon transitions by pump and probe lasers is investigated. Analytical expressions are derived for probe absorption spectrum and dispersion for a medium of stationary as well as thermal molecules. Contrary to the earlier study by Zhou et al. [J. Chem. Phys. 131, 034105 (2009)], we observe no amplification in 2 + 2 photon process when the sign of the difference of the permanent moments of the excited and the ground levels is reversed. Reasons for these contrasting observations are discussed. Our study shows that the permanent moments essentially damp the laser-molecule Rabi frequency to result in narrower EIT line width and larger group velocity index. These effects are further enhanced when the order of the multi-photon process is increased. The importance of the virtual mechanism is discussed by considering the special case of 2 + 1 photon EIT. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4751445]