Theoretical Research of Optical Pulse Response of Quantum Confined Excitons

我们建立了超短脉冲的光响应理论，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一致性。非线性响应可以处理。采用该方法对物质系统的麦克斯韦方程和密度矩阵方程进行了数值求解。以旋转波近似和随时间缓慢变化的包络近似为基础，减少激振态的个数，使计算变得可行。将此方法应用于超薄膜约束激子，我们澄清了以下几点。即使在几百飞秒的短脉冲中，响应场也表现出显著的纳米尺度空间结构，而在线性和非线性情况下，长波长下的选择规则被打破。我们明白，在这种情况下，我们的做法是必要的。本文首次阐明了经多次反射的短脉冲内部场的实时特性。非线性信号的时间特征反映了上述内场在时间上的运动，因此，内场的纳米尺度空间结构对非线性响应的时间特征起着重要作用。同时指出，如果我们忽略了内场的这种空间结构，我们根本不能正确地描述非线性信号的时变。

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.