Adaptive spatial control of soft x-rays for coherent microscopy applications

对分子尺度系统的结构和运动进行成像在科学中具有重要意义。成像如此微小的空间特征需要使用波长为几纳米量级的光，而要在这些系统中解析快速动力学需要持续时间为阿秒(1as=十亿分之一纳秒)的光脉冲。生产满足这些要求的明亮光源是一个巨大的挑战，但潜在的回报是巨大的，应用范围从生物医学科学到先进工程和纳米技术。短波辐射的一个来源是高次谐波(HHG)。这是一个非线性过程，当强烈的激光脉冲聚焦到气体中时，会导致原子发出软X射线脉冲。由于产生的软X射线束是相干的，或类似激光的，并且可以使用紧凑的激光系统产生，因此HHG对于许多应用来说是一个非常有吸引力的来源。HHG脉冲也非常短--大约在阿秒量级--与电子运动的自然时标相对应。最近的实验表明，HHG可以用于驱动相干衍射成像(CDI)实验，这在以前只能在大型设施中实现。在CDI中，物体是由一束相干的X射线照射的，记录的是衍射光。然后使用算法来根据衍射图案确定对象的形状。在大规模设施中进行的CDI已被证明是一种非常强大的技术，能够确定细胞和纳米晶体的结构。因此，HHG驱动的实验室规模的CDI是一条有吸引力的紧凑、超快、纳米级成像的途径，将是一种革命性的科学工具。然而，实现HHG全部潜力的关键一步是控制这些光束的空间结构。CDI对照明光束的相干性和形状非常敏感，而高次谐波过程的非线性会导致光束呈现复杂的结构。控制这些光束不是一件容易的事，因为传统的光学设备，如透镜，在这些波长上有很强的吸收。在这个项目中，我们将通过实施两项创新来实现对HHG光束的精确控制。第一种是使用一种新的方法来操纵强激光脉冲的形状，这是由我们在牛津大学的项目合作伙伴开发的。这项技术结合了两个可编程的光学元件，一个反射，一个折射，以便允许对超快激光光束进行任意整形。高次谐波过程对驱动激光的敏感性意味着，通过精确控制激光光束，我们将能够改变所产生的谐波的性质。第二个创新是，我们将第一次能够直接测量随着驱动光束的变化而发生的高次谐波的结构变化。为了做到这一点，我们将实施我们最近开发的一项技术，该技术使用HHG光束中的一对针孔来创建干涉图案。通过分析针孔移动时的干涉图样，我们可以高精度地确定高倍频光束的强度、曲率和相干性。这一控制系统将使我们能够显著提高HHG驱动的CDI的能力。例如，我们将能够大大增加HHG源的亮度，改善其相干性，并控制其波前曲率。这将带来更清晰的图像、更高的分辨率和更快的图像获取速度。它还将使我们能够成像复杂的目标，如病毒和纳米颗粒。这一研究计划将汇集脉冲整形和X射线计量学方面的强大新技术，以显着扩大紧凑型X射线成像的范围。这种系统的开发对高对比度生物成像、先进光刻、材料工程和医学等多个领域至关重要。

Imaging the structure and motion of molecular-scale systems is of fundamental importance in science. Imaging such small spatial features requires the use of light with wavelengths of the order of a few nanometers, while resolving fast dynamics in these systems requires pulses of light with durations of attoseconds (1as = 1 thousandth of a billionth of a nanosecond). Producing bright light sources which meet these requirements represents a significant challenge but the potential rewards are huge, with applications spanning biomedical science to advanced engineering and nanotechnology. One source of short wavelength radiation is high-harmonic generation (HHG). This is a nonlinear process which can occur when an intense laser pulse is focussed into a gas, causing the atoms to emit bursts of soft x-rays. HHG is a very attractive source for many applications since the resulting soft x-ray beams are coherent, or laser-like, and can be generated using compact laser systems. The HHG pulses are also extremely short - on the order of attoseconds - and correspond to the natural timescales of electronic motion. Recent experiments have shown that HHG can be used to drive coherent diffractive imaging (CDI) experiments, previously only possible at large-scale facilities. In CDI an object is illuminated by a coherent beam of x-rays and the diffracted light recorded. Algorithms are then used to determine the shape of the object from the diffracted pattern. CDI performed at large-scale facilities has proven to be an extraordinarily powerful technique, enabling the structures of cells and nanocrystals to be determined. Laboratory-scale CDI driven by HHG is therefore an attractive route towards compact, ultrafast, nanoscale imaging, and would be a revolutionary scientific tool.However, a key step in realizing the full potential of HHG is to control the spatial structure of these beams. CDI is very sensitive to the coherence and shape of the illuminating beam while the nonlinearity of the HHG process can result in beams which exhibit complex structures. Controlling these beams is non-trivial since conventional optics, such as lenses, absorb strongly at these wavelengths. In this programme we will achieve precise control of HHG beams by implementing two innovations. The first uses a new approach for manipulating the shape of intense laser pulses which was developed by our project partners at Oxford University. This technique combines two programmable optical elements, one reflective and one refractive, in order to allow arbitrary shaping of an ultrafast laser beam. The sensitively of the HHG process to the driving laser means that by precisely controlling the laser beam we will be able to modify the properties of the generated harmonics. The second innovation is that we will be able to measure directly, for the first time, the changes in the structure of the HHG field as the driving beam is varied. To do this we will implement a technique which we developed recently which uses a pair of pinholes in the HHG beam to create an interference pattern. By analysing the interference pattern as the pinholes are moved we can determine the intensity, curvature, and coherence of the HHG beam with high accuracy. This control system will allow us to dramatically improve the power of HHG-driven CDI. For example, we will be able to greatly increase the brightness of the HHG source, improve its coherence, and control its wavefront curvature. This will result in much sharper images, higher resolutions, and faster image acquisitions. It will also enable us to image complex targets, such as viruses and nanoparticles. This research programme will bring together powerful new techniques in pulse-shaping and x-ray metrology to dramatically extend the scope of compact x-ray imaging. The development of such a system is crucial for a variety of areas such as high-contrast biological imaging, advanced lithography, materials engineering, and medicine.