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A three level ladder system is analyzed and the coherence of initiallyelectric-dipole forbidden transition is calculated. Due to the presence of twolaser fields the initially dipole forbidden transition becomes dynamicallypermitted due to ac Stark effect. It is shown that such transitions exhibitquantum-interference-related phenomena, such as electromagnetically inducedtransparency, gain without inversion and enhanced refractive index. Gain anddispersion characteristics of such transitions strongly depend upon therelative phase between the driving and the probe fields. Unlike allowedtransitions, gain/absorption behavior of ac-Stark allowed transitions exhibitantisymmetric feature on the Rabi sidebands. It is found that absorption/gainspectra possess extremely narrow sub-natural resonances on these ac Starkallowed forbidden transitions. An interesting finding is simultaneous existenceof gain and negative dispersion at Autler-Townes transition which may lead toboth reduction of the group velocity and amplification of light.

Phase sensitive quantum interference on forbidden transition in ladder scheme