Intermediate state absorption enhancement in resonance-mediated (2+1) three-photon excitation process

In this paper, we theoretically study the control of intermediate state absorption in resonance-mediated (2+1) three-photon excitation process by shaping the femtosecond pulse with a π phase step modulation under weak laser field. Our results show that the intermediate state absorption can be enhanced, and the coherent enhancement increases with the increase of the pulse intensity and pulse duration. Our analysis indicates that the absorption enhancement results from the absorption reduction of the final state from the intermediate state in the shaped laser field. Furthermore, the effects of the population difference of the final and intermediate states in the unshaped laser field and the transition dipole moment from the intermediate state to the final state on the absorption enhancement are discussed and analyzed.