Optimising laser driven electron nanobunches from ultrathin foil interactions: Coherent synchrotron emission and relativistic electron mirrors

面向普通读者的研究概述最令人兴奋的科学前沿之一是对发生在阿秒时间尺度上的现象的研究(如，S)。想象一下如此短的时间，假设光在一秒内从这里到达月球，但在一飞秒内仅传播0.0003毫米(S，10^-15)。把它放在上下文中，这大约是10^-15年S(或1000as)头发宽度的1/300。阿秒是原子过程/跃迁发生的时间尺度--例如，一个电子在~24As(所谓的‘原子时间单位’)中绕着氢原子转。为了研究和在未来的控制中，这种超快过程的动力学需要具有前所未有的质量和精度的测量工具--持续时间为阿秒的光脉冲。这比任何可见光(紫色~1.3f，红色~2.5f)的单个周期要短得多，相反，需要对极紫外线(XUV)/X射线辐射进行临床精确控制，以实现超短持续时间。然而，这一努力的回报是巨大的；研究人员可以在比以前更短的时间尺度上，以一定程度的空间清晰度来研究微观宇宙，从而使他们能够看到使用传统的XUV/X射线源(如同步加速器)而通常被“模糊”的事件。必须使用比光谱中可见区域的波长更短的波长来合成阿秒脉冲，这其中存在一个重大问题--比可见光谱更短的波长，即紫外线和X射线，在大多数材料中被强烈吸收。因此，用传统的激光建造技术不可能建造阿秒激光器。相反，需要下一代方法。目前世界各地的激光实验室正在研究两种主要的介质--强烈的激光-气体相互作用和使用固体密度靶形成的相对论激光等离子体--用于产生阿秒脉冲。在拟议的研究中，我们重点研究第二种介质-相对论激光等离子体。所研究的产生强阿秒脉冲的基本机制是在高功率激光与超薄碳膜相互作用过程中产生相对论电子纳米束。这一新概念是基于我们最近的工作，表明在相对强的驱动激光场的作用下，可以在超薄碳膜的正面形成和快速加速10 nm尺度(nm=纳米=10^-9m)的致密电子束，然后从超薄碳膜的背面出现。本研究将详细研究产生的两种机制--相干同步辐射(CSE)和相对论电子镜(REM)。直到最近才证明，CSE和REM为了解相对论激光等离子体相互作用提供了一个新的窗口，我们的工作不仅将揭示这些机制的微观动力学，还将显示产生明亮阿秒脉冲的直接途径。

Summary of research for a general audience One of the most exciting frontiers of science is the study of phenomena that take place on the timescale of attoseconds (as, 10^-18 s). To imagine such an incredibly short period of time, consider that light travels from here to the moon in one second, but only travels 0.0003mm in one femtosecond (fs, 10^-15 s). To put it in context, that is about 1/300th the width of a human hair in 10^-15 s (or 1000 as). Attoseconds are the timescales on which atomic processes/transitions occur - for example, an electron circles the hydrogen atom in ~24 as (the so called 'atomic unit of time'). To investigate, and in future control, the dynamics of such ultrafast processes measurement tools of unprecedented quality and precision are required - pulses of light with attosecond duration. This is much shorter than a single cycle of any visible light wave (violet~1.3fs, red~2.5fs), requiring instead extreme-ultraviolet (XUV)/ X-ray radiation to be controlled with clinical accuracy to achieve ultrashort durations. However, the pay-off for this effort is substantial; researchers can investigate the microcosm with a degree of spatial clarity and on shorter time scales than previously possible, thus allowing them to see events that are ordinarily 'blurred' using conventional XUV/X-ray sources such as synchrotrons.Attosecond pulses must be synthesized using wavelengths shorter than those in the visible region of the spectrum and therein lies a significant problem - wavelengths shorter than the visible spectrum i.e. ultraviolet and X-rays, are strongly absorbed in most materials. It is therefore impossible to build an attosecond laser using conventional laser building techniques. Instead next generation methods are required. Two principle media are currently being studied at laser laboratories around the world - intense laser-gas interactions and relativistic laser plasmas formed using solid density targets - for the production of attosecond pulses.In the proposed research we focus on the second medium - relativistic laser plasma. The underlying mechanism under investigation for the generation of intense attosecond pulses is the production of relativistic electron nanobunches during high power optical laser interactions with ultrathin carbon foils. This novel concept is based on our recent work showing that dense bunches of electrons with sub 10nm scale (nm = nanometer = 10^-9m) can be formed and rapidly accelerated on the front surface by the relativistically intense driving laser field and subsequently emerge from the rear surface of ultrathin carbon foils. Two resulting mechanisms will be studied in detail in this research - Coherent Synchrotron Emission (CSE) and Relativistic Electron Mirrors (REM). Only recently demonstrated, CSE and REM offer a novel window onto the relativistic laser plasma interaction and our work will not only reveal the microscopic dynamics of these mechanisms but also show a direct path to the generation of bright attosecond pulses.