Table-top femtosecond X-ray dynamical imaging

我们如何才能拍摄一部原子--甚至是电子--在分子内部运动的电影？这是许多物理、化学和生物领域的一个基本问题。为此，我们需要持续时间比原子或电子运动的特征时间短得多的光脉冲。对于原子来说，这通常是几飞秒(1fs是十亿分之一纳秒)；电子的移动速度更快，在阿秒尺度上(1阿秒是千分之一飞秒！)。我们还需要非常短的波长，例如X射线的波长，以便在纳米尺度上达到必要的分辨率。满足这些要求是一个巨大的挑战，但在从医学到材料工程的应用方面，回报是巨大的。这种类型的切割边缘成像实验已经通过在大型设施中使用X射线源来实现。然而，它们的庞大规模和操作成本阻碍了它们成为广泛使用的工具。有一种更方便、更紧凑的方法来产生非常短的X射线脉冲。如果我们将短脉冲的可见光照射到气体射流上，如Ar，气体中的原子就会通过一种被称为高次谐波产生(HHG)的过程发射出极强的紫外线和软X射线辐射来响应这种光的存在。这些脉冲用于探测原子和分子中的电子动力学的适用性已经在一系列开创性的实验中得到了测试。然而，HHG光源的亮度远远无法与大型设施的亮度相提并论。我们将研究使HHG成为一项完全可行的技术的前景，该技术具有足够小的系统，足以用于普通研发实验室的“分子电影”拍摄。我们已经找到了克服当前限制的解决方案：特别是，我们将致力于选择可能产生HHG辐射的最佳可见光，以及使用“相位匹配”技术，即控制光如何通过喷流传播，以提高产生效率。HHG光束类似于X射线激光，它们具有相干特性。这意味着，如果我们收集远离目标的光的幅度和相位的完整信息，我们可以使用复杂的计算机代码来重建这个物体的形状。这避免了使用难以制造的X光镜头。此外，通过调谐X射线束的波长，可以仅选择对象中的指定原子元素并对其成像。我们将在铝合金的原理验证实验中演示我们计划开发的明亮HHG光束的实用性。这些合金对航空航天、汽车和电子工业至关重要，它们的强度来自于热处理过程中不均匀的形成。然而，它们的微观结构和力学性能之间的关系还不是很清楚，我们的演示实验可能为探索这些重要问题开辟了一条新的途径。从基本的观点来看，电磁场包含了光学实验中可以获得的关于物体的最大可能信息。因此，我们还将研究能够全面描述物体散射的X射线场的方法，从而跟踪物体的空间和结构动力学。总而言之，我们计划通过开发明亮、紧凑的脉冲软X射线源和测量方法，在原子空间和时间尺度上朝着实验室规模的成像迈出重要一步，这些光源和测量方法返回入射到被研究物体上和从被研究物体散射的辐射场的全部细节。因此，这一研究计划有可能在纳米尺度的时空成像中实现一步一步的改变

How can we make a movie of atoms - or even electrons - moving inside molecules? This is a fundamental problem in many fields of physics, chemistry and biology.For this, we need pulses of light with a duration which is much shorter than the characteristic times of the movements of the atoms or electrons. For the case of atoms this is typically a few femtoseconds (1fs is one billionth of a nanosecond); electrons move even faster, on the attosecond scale, where (1 attosecond is one thousandth of a femtosecond!). We also need very short wavelengths, such as those of X-rays, so to achieve the necessary resolution at the nanometre scale. Meeting these requirements is a formidable challenge, but the pay-off in terms of applications, ranging to medical science to material engineering, is enormous.Cutting-edge imaging experiments of this type have already been achieved by using X-ray sources in huge facilities. However, their large scale and operating cost prevents them from becoming a widespread tool.There is a more convenient and compact way of producing very short X-ray pulses. If we shine short pulses of visible light on a jet of gas, such as argon, the atoms of the gas respond to the presence of this light by emitting bursts of extreme ultraviolet and soft X-ray radiation by a process called "high harmonic generation" (HHG). The applicability of these pulses for probing electronic dynamics in atoms and molecules has been tested in a series of pioneering experiments. However, the brightness of HHG sources is far from being comparable with that of large-scale facilities. We will investigate the prospects for making HHG a fully viable technique for taking "molecular movies" with a system small enough for an ordinary R&D laboratory. We have identified solutions for overcoming current limitations: in particular, we will work on choosing the best possible visible light for producing HHG radiation, as well as on employing techniques of "phase-matching", i.e. controlling how the light propagates through the jet, to increase the efficiency of generation.HHG beams are akin to an X-ray laser, with which they share properties of coherence. This implies that, if we collect the full information on the amplitude and the phase of the light far from our target, we can use sophisticated computer codes to reconstruct the shape of this object. This avoids using lenses for X-rays, which are difficult to manufacture. Further, by tuning the wavelength of the X-ray beam it is possible to select and image only a specified atomic element in the object. We will demonstrate the utility of the bright HHG beams we plan to develop in proof-of-principle experiments on aluminium alloys. These alloys - which are of crucial importance to the aerospace, automotive, and electronic industries - derive their strength from the formation of inhomogeneities during heat treatment. However, the relation between their microscopic structure and mechanical properties is not well understood; our demonstration experiments may open a new route for exploring these important issues.From a fundamental viewpoint, the electromagnetic field contains the maximum possible information about an object that can be obtained in an optical experiment. Hence we will also investigate methods able fully to characterize the X-ray field scattered from an object, allowing the spatial and structural dynamics of the object to be tracked.In summary, we plan to take major steps towards laboratory-scale imaging at atomic spatial and temporal scales by developing bright, compact pulsed soft-X-ray sources and measurement methods that return the full details of the radiation field incident on, and scattered from, the object under study. This research programme therefore has the potential to deliver a step change in what is possible in spatio-temporal imaging at the nanoscale