Spatio-Temporal Tailoring of Light Fields for Sub-Cycle Resolved Measurements of Strong-Field Effects

持续时间短到足以探测电子时间标度的激光脉冲可以在XUV和IR-VIS体系中产生。高次谐波产生可用于在XUV中产生孤立的阿秒脉冲和脉冲串，并且由于最近光脉冲的相干合成被用于在IR-VIS区产生亚周期光波形。在光子能量为5-15 eV的情况下，如此极短持续时间的脉冲尚未被证明。气体电池或灯丝中的频率转换产生相对较短的脉冲，但非线性光-物质相互作用的复杂相互作用和显著的线性色散阻止了接近飞秒势垒。对于光谱应用，如瞬态吸收光谱，紫外/VUV中的宽带和亚飞秒脉冲将非常有用，因为它们允许直接探测许多材料中的带隙。本项目的核心任务是研究在非共线几何结构中，飞秒近红外脉冲在固体目标中产生的UV/VUV场。使用厚度为15-100µm的薄块状固体作为产生介质，低次谐波产生和自衍射的级联过程在UV/VUV中产生了多层面的发射图案。一个特殊设计的光谱仪记录了产生脉冲的平分线及其以外的发射模式，并使用UV/VUV场中的干涉来仔细检查非线性光-物质相互作用。实验方法的目标是跟踪在微扰和强场光学之间的过渡状态下带结构和/或种群动力学的变化。在非共线几何条件下，可以优化短紫外/VUV脉冲的产生，在选定的发射角下可以产生< 2 fs的脉冲。利用圆极化反旋转产生脉冲，可以实现变椭圆率的紫外/紫外光场。一个普遍的特点是紫外/紫外脉冲通过非共线几何结构与基本场分离，这在薄金属滤波器不能用于与基本脉冲分离的情况下特别有益。产生的UV/VUV脉冲不能用于传统的泵浦探测实验，因为它们的强度非常低，并且它们仅作为裁剪场存在，在产生介质后面具有时空耦合，防止光束传输或重新聚焦。然而，当探针样品直接位于生成样品之后时，它们可以用于瞬态吸收实验。

Laser pulses with durations short enough to probe the electronic timescale can be generated in the XUV and in the IR-VIS regime. High-order harmonic generation can be used to generate isolated attosecond pulses and pulse trains in the XUV, and since very recently coherent synthesis of optical pulses is used to generate subcycle optical waveforms in the IR-VIS regime. At photon energies in the regime 5-15 eV, pulses with such extremely short durations have not yet been demonstrated. Frequency conversion in a gas cell or filamentation yields comparatively short pulses, but the complex interplay of nonlinear light-matter interaction and significant linear dispersion has prevented to approach the femtosecond barrier. For spectroscopic applications, such as transient absorption spectroscopy, broadband and sub-femtosecond pulses in the UV/VUV would be very useful, because they allow the direct probing of the bandgap in many materials. The central quest of this project is to investigate the generation of UV/VUV fields in solid targets by femtosecond NIR pulses in a non-collinear geometry. Using thin bulk solids (thickness 15-100 µm) as generation media, cascaded processes of low-order harmonic generation and self-diffraction yield a multifaceted emission pattern in the UV/VUV. A specially designed spectrometer records the emission pattern in the bisector of the generating pulses and beyond, and interferences in the UV/VUV field are used to scrutinize the nonlinear light-matter interaction. The goal of the experimental methods is to track changes in the band structure and/or population dynamics in the transition regime between perturbative and strong-field optics. The generation of short UV/VUV pulses can be optimized in the noncollinear geometry, and < 2 fs-pulses can be generated at selected emission angles. Using circularly polarized counter-rotating generation pulses, fields in the UV/VUV with varying ellipticity can be achieved. A generic feature is that the UV/VUV pulses are separated from the fundamental field through the noncollinear geometry, which is especially beneficial in a regime where thin metal filters cannot be used for the separation from the fundamental pulses. The generated UV/VUV pulses cannot be used in traditional pump-probe experiments, because they have very low intensity and they exist only as tailored fields with spatiotemporal couplings behind the generation medium, preventing beam transport or refocusing. However, they can be utilized in transient absorption experiments when a probe sample is located directly after the generation sample.