Adaptive control, generation, and characterization of bright soft x-rays by quasi-phase-matching

该方案利用牛津小组在产生和控制放大超短激光脉冲串方面取得的最新进展，以及在波导中产生高能光子的新方法，并设法将其应用于产生非常明亮的相干、超短、软X射线辐射源-能量接近X射线光子能量的类似激光的辐射脉冲，相干软X射线辐射可以通过称为高次谐波产生的过程产生。在这个过程中，可以通过电离原子并将产生的电子碰撞到其母离子中来产生多个频率的强激光脉冲，产生可以短至100阿秒（1阿秒=十亿分之一微秒）的辐射脉冲。光子的能量接近X射线的能量可以通过这种方式产生。然而，该过程的效率非常低，并且在更高的能量下变得更低，严重限制了许多应用。这种低效率的主要原因是失相效应。简单地说，这意味着激光脉冲和产生的软X射线以不同的速度传播，从而阻止了软X射线的持续增长。克服这种失相的一种方法是采用称为准相位匹配的技术，由此谐波生成过程以等于相干长度的周期打开和关闭，相干长度即驱动激光脉冲和所生成的辐射变得异相的距离。如果这在N个相干长度上实现，则软X射线信号将增加因子N平方。因此，为了使这种技术的功率最大化，有必要在尽可能多的相干长度上进行准相位匹配。打开和关闭谐波过程可以通过多种方式实现。一种方法涉及使用一系列（或一列）在与产生激光脉冲的方向相反的方向上行进的紧密间隔的非常短的激光脉冲。在EPSRC的授权EP/C 005449下，牛津小组最近开发了一种方法，用于产生多达100个脉冲的激光脉冲序列，可以精确控制。这有可能匹配多达100个相干长度，这将使软X射线产量增加10，000倍！这些脉冲序列的自适应性质也将允许采用遗传算法来充分优化软X射线源。作为该计划的一部分，该小组将寻求利用这些新的自适应脉冲序列的力量，从谐波产生中产生软x射线源，其亮度远远超过以前记录的任何东西。在EP/C 005449的资助下，牛津小组与贝尔法斯特女王大学（QUB）的一个小组合作，也发现了一种控制谐波过程的新方法。这依赖于操纵激光脉冲通过狭窄的中空玻璃纤维传播的方式。通过精确控制激光脉冲及其如何耦合到光纤中，可以激发光纤中的模式，使得它们在光纤内产生干涉图案，从而打开和关闭谐波产生。牛津大学和QUB小组成功地利用这种模式从谐波产生中创造了有史以来最亮的水窗X射线源，作为该计划的一部分，他们将寻求进一步发展这种方法。这种明亮的软x射线源的发展对于各种科学应用至关重要，例如化学反应的超快测量，高对比度生物成像，先进光刻，原子和分子光谱学以及超快x射线衍射。

This programme uses recent advances made by the Oxford group in producing and controlling trains of amplified, ultrashort laser pulses, as well as a new method for producing high-energy photons in waveguides, and seeks to apply this to generating very bright sources of coherent, ultrashort, soft x-ray radiation - laser-like pulses of radiation at energies approaching x-ray photon energies, with extremely short time durations.Coherent soft x-ray radiation can be produced through a process called high harmonic generation . In this process multiple frequencies of an intense laser pulse can be generated by ionizing an atom and colliding the resulting electron into its parent ion, producing a pulse of radiation which can be as short as 100 attoseconds (1 attosecond = 1 billionth of a microsecond). Photon energies approaching x-ray energies can be generated this way. However, the efficiency of this process is very low, and gets much lower at higher energies, severely limiting many applications. The main cause for this low efficiency is the effect of dephasing . Simply put, this means that the laser pulse and the generated soft x-rays travel at different speeds, preventing the continuous growth of the soft x-ray. One way to overcome this dephasing is to employ a technique known as quasi phase-matching, whereby the harmonic generation process is switched on and off with a period equal to the coherence length , i.e. the distance over which the driving laser pulse and the generated radiation become out of phase. If this is achieved over N coherence lengths the soft x-ray signal will increase by a factor N-squared. Therefore, to maximize the power of this technique it is necessary to quasi phase-match over as many coherence lengths as possible. Switching the harmonic process on and off can be achieved in a number of ways. One method involves using a series ( or train ) of closely spaced, very short laser pulses travelling in the opposite direction to the generating laser pulse. Under EPSRC grant EP/C005449 the Oxford group has recently developed a method for producing trains of laser pulses with up to 100 pulses which can be accurately controlled. This has the potential to match up to 100 coherence lengths, which would increase the soft x-ray yield by a factor of 10,000! The adaptive nature of these pulse trains will also allow genetic algorithms to be employed to fully optimize the soft x-ray source. As part of this programme the group will seek to exploit the power of these new adaptive pulse trains to produce a soft x-ray source from harmonic generation with a brightness far exceeding anything previously recorded. Under grant EP/C005449 the Oxford group, in collaboration with a group at Queen's University Belfast (QUB), also discovered a new method for controlling the harmonic process. This relies on manipulating the way a laser pulse propagates through a narrow, hollow glass fibre. Through precise control of the laser pulse and how it couples into the fibre it is possible to excite modes in the fibre such that they create an interference pattern within the fibre which switches the harmonic generation on and off. The Oxford and QUB groups successfully used such a pattern to create the brightest source of water window x-rays from harmonic generation ever reported and, as part of this program will seek to develop this method even further. The development of such bright soft x-ray sources is crucial for a variety of scientific applications such as ultrafast measurements of chemical reactions, high-contrast biological imaging, advanced lithography, atomic and molecular spectroscopy, and ultrafast x-ray diffraction.