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

等离子体镜是由高功率激光束照射和电离固体表面形成的微小的瞬时光反射器，是一种强大的光子装置，用于操纵传统光学无法处理的强光。等离子体反射镜不仅可以重定向和聚焦高功率光，还可以作为超快非线性滤波器，抑制脉冲激光束中的有害噪声，并作为从太赫兹到x射线的频率发生器，用于当前光源无法达到的应用。使用微纳米结构以及超轻材料作为瞬态等离子体反射镜的基础，可以使它们更有效，增强其光修饰特性，并增加新的功能，以推进x射线生物医学成像和超快计量。该项目研究了新型材料结构如何影响强烈的激光等离子体相互作用和等离子体反射镜的频率上转换效率，以产生更高的辐射能量。超密相对论性激光产生的等离子体的高次谐波发射效率与激光场强与等离子体密度的比值有关，这是产生发射相对论性电子的力平衡的结果。被超高对比度激光束完全电离的凝聚相材料往往过于密集，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。降低等离子体有效密度的一种方法是，通过使用独立的纳米级箔（如多层石墨烯）作为基材，使等离子体更薄；另一种方法是使用超轻的纳米多孔材料。该项目的目的是研究在多太瓦峰值功率下，飞秒光脉冲在与密集相对论性激光产生的等离子体相互作用中聚焦到几微米的行为。这些等离子体将在具有微纳米级结构的材料中产生，如v型槽、独立的纳米箔和气凝胶。实验和数值模拟将发展为各种材料结构。这项研究将对等离子体反射镜的频率上转换以及结构和超轻材料的潜力有基本的了解，以最大限度地提高其效率。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.