双光子过程中的跃迁截面

重新检验了双光子过程中跃迁截面的概念：当密度矩阵中对角元随时间的变化缓慢时，速率方程和相关的跃迁截面是很好的近似。导出速率系数的表达式：在强光场中描述单光子过程中的系数要作很大的修改，而描述双光子过程的系数可从单光子过程的系数导出。激光场的功率分配的最佳化随总功率的强度而激烈变化。当有多个能级或多个激光模式的干涉效应时，利用跃迁截面的计算比直接利用布洛赫方程的计算还要复杂。

Transition Cross Sections in Two-Photon Process

The concept of transition cross section is re-examined in two-photon process. The rate equations and the relevant transition cross sections are good approximation in the region where the diagonal elements of the density matrix change slowly with time. The rate coefficients are derived explicitly:those describing formally the single photon process will be subject to a significant modification for strong light field compared with that of the weak laser field and those describing formally the multi-photon process can also be constructed from the parameters relevant to the real single photon process. The optimization of power distribution of laser fields changes dramatically from weak to strong total laser intensity available. To describe the interference effect originated either from the multi-mode laser field or from a group of energy levels located closely needs more complicated computation of the transition cross sections than using the Bloch equations directly.