Schematic diagram of a five-level atomic system interacting with four classical optical fields. A weak probe field with Rabi frequency Ω _p drives the | 1 〉 ↔ 〈 2 | transition, at the same time, three strong control, driving and signal fields with Rabi frequencies Ω _c , Ω _d and Ω _s couple the transitions | 5 〉 ↔ | 2 〉 , | 2 〉 ↔ | 3 〉 , | 2 〉 ↔ | 4 〉 , respectively.