Effects of amplitude and phase of microwave field on single and two-photon absorption properties in a four-level Λ-type atom

When a microwave field is used to drive two lower lying levels of the four-level Λ-type atom, the absorption peaks in the single- and two-photon absorption profile become asymmetric and their magnitudes strongly depend on the amplitude of the microwave field. In particular, one of the peaks in the case of single-photon turns into a gain dip if the microwave field amplitude exceeds a certain value. On the other hand these absorption spectra profiles can be modulated by the phase due to the closed-loop structure connected by the microwave field.