Modified Plasma Mirrors to Maximize Efficiency of High Harmonic Generation in Solids

等离子反射镜是由高功率激光束照射并电离固体表面形成的微观微小的瞬时光反射器，是一种强大的光子设备，用于处理传统光学无法处理的太强的光。等离子反射镜不仅可以重定向和聚焦高功率光，还可以用作超快非线性滤光器，抑制脉冲激光中不需要的噪声，并为当前光源无法企及的应用提供从太赫兹到X射线的频率发生器。使用微纳米结构以及超轻材料作为瞬变等离子体反射镜的基础，可能会使它们更高效，增强其光修饰性能，并为推进X射线生物医学成像和超快计量增加新的功能。这个项目研究了新的材料结构如何影响强激光-等离子体相互作用以及等离子体反射镜上转换频率以产生更高能量的辐射的效率。由于产生发射相对论电子的力的平衡，来自过密集的相对论激光产生的等离子体的高次谐波发射的效率随激光场强与等离子体密度的比率而变化。被超高对比度激光光束完全电离的凝聚相材料往往密度太高，除了最强大的激光系统外，其他所有系统都无法达到高效率的条件。一种降低等离子体有效密度的方法，同时保持陡峭的密度梯度，是通过使用独立的纳米级箔(如多层石墨烯)作为基础材料来使等离子体变薄；另一种方法是使用超轻纳米多孔材料。这个项目的目的是研究几太瓦峰值功率的飞秒光脉冲在与稠密的相对论激光产生的等离子体相互作用中的行为。这些等离子体将在具有微米和纳米结构的材料中产生，如V型槽、独立的纳米箔和气凝胶。将针对各种材料结构开展实验和数值模拟。这项研究将从根本上了解等离子反射镜中的频率上转换以及结构和超轻材料最大限度地提高其效率的潜力。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Plasma mirrors -- microscopically small, momentary light reflectors formed by high-power laser beams irradiating and ionizing solid surfaces -- are powerful photonic devices for manipulating light that is too intense to be handled by conventional optics. Not only can plasma mirrors redirect and focus high-power light, they can also serve as ultrafast nonlinear filters suppressing unwanted noise in pulsed laser beams, and as generators of frequencies from terahertz to x-rays for applications that are beyond the reach of current light sources. Using micro- and nano-structures as well as ultralight materials as a base for transient plasma mirrors can potentially make them more efficient, enhance their light modification properties, and add new functionalities to advance x-ray biomedical imaging and ultrafast metrology. This project investigates how novel material structures can affect intense laser-plasma interactions and the efficiency of frequency up-conversion by plasma mirrors to produce higher energies of the radiation.The efficiency of high-order-harmonic emission from overdense relativistic laser-produced plasmas scales with the ratio of laser field strength to plasma density as a result of the balance of forces that produces emitting relativistic electrons. Condensed-phase materials that are fully ionized by ultrahigh-contrast laser beams tend to be too dense for all but the most powerful laser systems to reach the condition of high efficiency. One way of reducing the effective density of the plasma, while keeping a steep density gradient, is to make plasma thinner by using a free-standing nanometer-scale foil, such as multi-layer graphene, as a base material; another way is to use an ultralight nanoporous material. The aim of this project is to study the behavior of femtosecond light pulses at multi-terawatt peak powers focused down to a few micrometers in the interaction with dense relativistic laser-produced plasmas. These plasmas will be created in materials with micro- and nanoscale structure, such as v-grooves, free-standing nanometer foils, and aerogels. Experiments and numerical simulations will be developed for a variety of material structures. This research will achieve fundamental understanding of frequency up-conversion in plasma mirrors and the potential of structured and ultralight materials to maximize its efficiency.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.