Spatiotemporal control of laser intensity

The controlled coupling of a laser to plasma has the potential to address grand scientific challenges(1-6), 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 accelerator(7) to mitigate dephasing, facilitating the production of tunable XUV sources.