Numerical Simulation And Analysis Of Four-level System Population Transfer

The technique of stimulated-Raman-adiabatic-passage(STIRAP) is a method of using partially overlapping pulses(from pump and Stokes lasers) to produce complete population transfer between two quantum states of an atom or molecule. This method has been successfully used for efficient and selective population transfer in ideal three-level system and extended to multilevel system. In this paper, the population transfer in six different four-level systems through STIRAP process is considered using numerical method, the transfer efficiency on two-photon resonance and off two-photon resonance is calculated and the influence of the fourth level on the transfer efficiency under different conditions is analyzed. The calculation results show the applicability of the STIRAP technique in four-level system. It appearsthat, for many atomic or molecular systems, the criteria which must be met in order to achieve high transfer efficiency are very stringent and for certain four-level systems, if particular conditions can not be satisfied, even the system is on two-photon resonance, the failure of complete or nearly complete population transfer will occur. In the hind part of this paper, a brief introduction of density matrix method is presented and its equivalence to Schroinger equation in multilevel system is described.