级联三能级介质中非线性光学信号的增强

Enhancement of nonlinear-optical signals in a cascade three-level system

揭示非最大相干可能比最大相干产生更有效的非线性频率转换.为此以三能级级联系统(没有最大相干)为例计算其非线性光学信号的强度,并与三能级Λ型系统(具有最大相干)进行比较.利用非微扰方法计算表明,在双光子共振和相同参数的条件下,级联系统产生的非线性光学信号的最大强度约是Λ系统的两倍.利用缀饰态表象分析表明,两个系统中耦合跃迁的自发辐射及其介入的不可逆三光子过程具有相反的作用.它们在级联系统中加强非线性转换,而在Λ系统中削弱非线性转换.

In this paper we show that non-maximal coherence can lead to more efficient nonlinear frequency conversion than maximal coherence does. For this purpose we exemplify with the three-level cascade system （with no maximal coherence） to calculate the intensity of nonlinear-optical signal. A comparison is made between the Ξ system and the A system （with maximal coherence）. The generated intensity is calculated by employing a nonperturbative approach. It is shown that under the conditions of the twophoton resonance and the same parameters, the largest intensity from the Ξ system is about twice as large as that from A system. The difference is analyzed in terms of dressed states. It is shown that both the spontaneous emission on the coupling transition and the spontaneous emission mediated three photon process play a contrary role for the two systems. These two processes enhance the generation of nonlinear signals for Ξ system but act contrarily for the A systems.