1Jiangsu Key Laboratory on Opto-Electronic Technology and Department of Physics, Nanjing Normal University,Nanjing 210046, China.
Opt Lett. 2010 Feb 1;35(3):417-9. doi: 10.1364/OL.35.000417.
Based on five-level rate-equation theory, we develop the laser-pulse-duration dependence of two-photon-absorption-induced singlet and triplet excited-state absorptions (ESAs). We present analytical expressions for the effective three-photon absorption coefficients caused by both singlet and triplet ESAs under the pulsed excitation on time scales from femtoseconds to microseconds. We demonstrate that the triplet ESA is predominant with longer laser pulses (microseconds to tens of nanoseconds) and that the resultant nonlinear absorption (NLA) can be adequately interpreted by a simplified four-level model. Under the excitation of picosecond laser pulses, generally speaking, the competition between singlet and triplet ESAs is observable. In this instance, the photodynamics of the system can be understood by a five-level model. In the femtosecond regime, however, a three-level model is validated in the prediction of NLA, because the triplet ESA becomes negligible.
基于五能级速率方程理论，我们研究了双光子吸收诱导的单重态和三重态激发态吸收（ESA）对激光脉冲持续时间的依赖性。我们给出了在飞秒到微秒时间尺度上，由单重态和三重态 ESA 引起的有效三光子吸收系数的解析表达式。我们证明，在较长的激光脉冲（微秒到数十纳秒）下，三重态 ESA 占主导地位，并且可以通过简化的四能级模型充分解释由此产生的非线性吸收（NLA）。在皮秒激光脉冲的激发下，一般来说，单重态和三重态 ESA 之间的竞争是可以观察到的。在这种情况下，可以通过五能级模型来理解系统的光动力学。然而，在飞秒范围内，三能级模型在预测 NLA 时是有效的，因为三重态 ESA 可以忽略不计。
