Gavrilyuk Sergey, Liu Ji-Cai, Kamada Kenji, Agren Hans, Gel'mukhanov Faris
Theoretical Chemistry, Royal Institute of Technology, Roslagstullsbacken 15, S-106 91 Stockholm, Sweden.
J Chem Phys. 2009 Feb 7;130(5):054114. doi: 10.1063/1.3072560.
We present a dynamical theory of nonlinear absorption and propagation of laser pulses with duration in the microsecond time domain. The general theory is applied to fullerene C(60) because of its good optical limiting properties, namely, a rather low ground state absorption and a strong triplet-triplet absorption. It is shown that sequential absorption involving strong triplet-triplet transitions is the major mechanism of nonlinear absorption. The intrinsic hierarchy of time scales makes an adiabatic solution of the coupled rate equations valid, which therefore can be reduced to a single dynamical equation for the ground state population. The slow evolution of this population is defined by an effective rate of population transfer to the triplet state and by the pulse duration. The propagation effect plays an important role in the optical power limiting performance. The intensity of the field as well as the population of the triplet state decreases during the pulse propagation, and a weakened nonlinear sequential two-photon absorption is followed by a linear one-photon absorption which gradually becomes the dominating process. The competition between these qualitatively different processes depends on the field intensity, the length of the absorber, and the concentration. The pulse propagation is studied by solving numerically the two-dimensional paraxial field equation together with the effective rate equation for the ground state population.
我们提出了一种关于微秒时域内激光脉冲非线性吸收和传播的动力学理论。由于富勒烯C(60)具有良好的光学限幅特性，即基态吸收较低且三重态-三重态吸收较强，因此该通用理论被应用于富勒烯C(60)。结果表明，涉及强三重态-三重态跃迁的顺序吸收是非线性吸收的主要机制。时间尺度的固有层次结构使得耦合速率方程的绝热解有效，因此可以简化为基态粒子数的单个动力学方程。该粒子数的缓慢演化由粒子转移到三重态的有效速率和脉冲持续时间决定。传播效应在光功率限幅性能中起着重要作用。在脉冲传播过程中，场强以及三重态的粒子数都会降低，随后非线性顺序双光子吸收减弱，线性单光子吸收逐渐成为主导过程。这些性质不同的过程之间的竞争取决于场强、吸收体长度和浓度。通过数值求解二维傍轴场方程以及基态粒子数的有效速率方程来研究脉冲传播。
