The controlled coupling of a laser to plasma has the potential to address grand scientific challenges^1,2,3,4,5,6, but many applications have limited flexibility and poor control over the laser focal volume. Here, we present an advanced focusing scheme called a ‘flying focus’, where a chromatic focusing system combined with chirped laser pulses enables a small-diameter laser focus to propagate nearly 100 times its Rayleigh length. Furthermore, the speed at which the focus moves (and hence the peak intensity) is decoupled from the group velocity of the laser. It can co- or counter-propagate along the laser axis at any velocity. Experiments validating the concept measured subluminal (−0.09c) to superluminal (39c) focal-spot velocities, generating a nearly constant peak intensity over 4.5 mm. Among possible applications, the flying focus could be applied to a photon accelerator⁷ to mitigate dephasing, facilitating the production of tunable XUV sources.

激光与等离子体的受控耦合具有解决^1,2,3,4,5,6巨大的科学挑战的潜力，但是许多应用的灵活性有限且对激光焦距的控制较差。在这里，我们提出了一种先进的聚焦方案，称为“飞行聚焦”，其中，彩色聚焦系统与chi激光脉冲相结合，可使小直径激光聚焦传播近其瑞利长度的100倍。此外，焦点移动的速度（因此峰值强度）与激光器的群速度解耦。它可以以任何速度沿激光轴共向或反向传播。验证概念的实验测量的是腔内（-0.09 c）到腔内（39 c）焦点速度，在4.5毫米范围内产生几乎恒定的峰值强度。在可能的应用中，可以将飞焦应用于光子加速器⁷以减轻相移，从而促进可调谐XUV源的生产。