Ultrashort laser pulse propagation in materials with strong-coupling carriers

超短、强飞秒激光脉冲在光学材料中的传播是由光-物质相互作用决定的。它导致能量密度、相位和极化在空间和时间上的发展，以及技术上的相关效应，如自相位调制和自聚焦。直到最近，才有研究表明，在具有强耦合载流子的材料中，非线性极化的额外非瞬时贡献出现在依赖载流子诱导的晶格变形的材料中。典型的是，地层动力学是在低于500秒的时间范围内发现的，即典型的现代商业超短激光脉冲系统。因此，与传播的飞秒激光脉冲的相互作用成为可能，导致自包含的非线性光学效应和脉冲耦合现象。从固体物理的角度来看，非瞬时的非线性极化依赖于载流子的自捕获，通过与晶格的强耦合作为自捕获的激子或小的强耦合极化子，以及由此产生的电子偶极矩的变化。为了理解传播的超短脉冲，需要对耦合电荷载流子与光的非线性相互作用有全面的了解，这是目前文献中所缺乏的。已建立的超快测量技术依赖于固定的脉冲持续时间，由于非线性极化的准瞬时和非瞬时贡献，这阻碍了脉冲传播影响的选择性分辨。缺乏依赖于脉冲持续时间的非线性光学效应的多尺度建模；在具有强耦合载流子的材料中完全控制脉冲传播是不可能的。该项目旨在填补这一空白，并将以脉冲持续时间为中心参数进行脉冲传播和耦合的系统实验和数值研究。通过光谱和非线性光学方法的结合，可以选择性地分配非瞬时非线性极化对自困激子和小的强耦合极化子的贡献以及它们对准瞬时极化的分离。由于超短激光脉冲与这些准粒子明显的共振相互作用，有必要在宽光谱范围内精确表征复杂的光谱特征。我们期望以铌酸锂为模型系统的研究结果，将为超快光子学的应用领域，例如光学参量放大器，带来关于脉冲传播的重要信息，同时也可能引发新的应用。

The propagation of ultrashort, intense femtosecond laser pulses in optical materials is determined by light-matter-interactions. It results in a spatial and temporal development of energy density, phase and polarisation and in technologically relevant effects, such as self-phase modulation and self-focussing.Only recently, it has been shown that additional, non-instantaneous contributions to the nonlinear polarisation appear in materials with strong-coupling carriers, that rely on a carrier-induced lattice deformation. Characteristically, the formation dynamics is found in the sub-500-fs time regime, i.e. typical for modern, commercial ultrashort laser pulse systems. Thereby, an interaction with propagating femtosecond laser pulses becomes possible, that result in self-contained nonlinear optical effects and pulse coupling phenomena.From the viewpoint of solid state physics, the non-instantaneous, nonlinear polarisation relies on the self-trapping of carriers by strong coupling to the crystal lattice as self-trapped excitons or small, strong-coupling polarons and the resulting change of the electronic dipole moment. For the understanding of propagating ultrashort pulses, a comprehensive knowledge on the nonlinear interaction of coupled charge carriers with light is necessary, that is missing in literature, so far. Established, ultrafast measurement techniques rely on fixed pulse durations, that hinder the selective resolution of pulse propagation influence due to quasi-instantaneous and non-instantaneous contributions to the nonlinear polarisation. Multiscale modelling of nonlinear optical effects dependent on the pulse duration are missing; a complete control of pulse propagation in materials with strong-coupling carriers is not possible.The project aims to fill this gap and a systematic experimental and numeric study of pulse propagation and coupling using the pulse duration as central parameter will be performed.By combination of spectroscopic and nonlinear optical methods, a selective assignment of contributions of the non-instantaneous nonlinear polarisation to self-trapped excitons and small, strong-coupling polarons and their separation to the quasi-instantaneous polarization becomes possible. Due to pronounced resonant interactions of ultrashort laser pulses with these quasiparticles, it is necessary to precisely characterize the complex spectral signature over a broad spectral range. We expect that our results, with lithium niobate as a model system, will bring important information about pulse propagation in the applicationary field of ultrafast photonics, such as e.g. for optical parametric amplifiers, but also may trigger novel applications.