Theoretical Research of Optical Pulse Response of Quantum Confined Excitons

我们发展了超短脉冲光响应的理论，其中正确地考虑了介质中响应场的纳米尺度空间结构。该理论具有以下特点：考虑了非局部响应，正确地考虑了感应电流密度和响应场的微观位置依赖性，考虑了感应电流密度和响应场的自洽性，可以处理非线性响应，采用的方法是使麦克斯韦方程组方程组为：和物质系统的密度矩阵方程进行了数值求解。采用旋波近似和随时间缓变的包络近似，并以减少激子态的数目为基础，使计算变得可行。将这种方法应用于超薄薄膜束缚激子，我们阐明了以下几点：1.即使在几百飞秒的短脉冲下，响应场也出现了显著的纳米尺度空间结构，并且在线性和非线性区域都打破了在长波长情况下应该保持的选择规则。2.首次用多次反射的短脉冲阐明了内场的真实的时间特性3.非线性信号的时间特性反映了内场在时间上的上述运动，因此，内场的纳米尺度空间结构在非线性响应的时间特性中起重要作用。如果忽略内场的这种空间结构，就不能准确地描述非线性信号的时变性。

We have developed the theory for the optical response of the ultra short pulse in which the nanoscale spatial structure of the response field in the media is correctly considered. This theory has following features.The nonlocal response is considered.The microscopic position dependences of the induced current density and the response field are correctly taken into account.The self-consistency of the induced current density and the response field are taken into account.Nonlinear response can be treated.We take the method such that the simultaneous equations of Maxwell eq. and the equation for the density matrix of the matter system are numerically solved. Taking the rotating wave approximation and slowly varing envelope approximation in time, and reducing the number of the exctitonic states as bases, we make this calculation feasible. Appling this method to the ultra thin film confining excitons, we have clarified the following points.1.The remarkable nanoscale spatial structure of the response field appears even for the short pulse of several hundreds femto second, and the selection rule that should hold in the case of long wavelength is broken for both the linear and nonlinear regimes. We understand that our approach is essentially necessary in such conditions.2.We elucidate, for the first time, the real time characteristics of the internal field by the short pulse undergoing multiple reflection.3.The time characteristics of the nonlinear signal reflects the above mentioned motion of the internal field in time, thus, the nano-scale spatial structure of the internal field takes a important role in the time characteristics of the nonlinear response. It is also clarified that if we neglect this spatial structure of the internal field, we can not describe the correct time variation of the nonlinear signal at all.