Subluminal and superluminal light pulses have attracted considerable attention in recent decades^1,2,3,4, opening perspectives in telecommunications, optical storage and fundamental physics⁵. Usually achieved in matter, superluminal propagation has also been demonstrated in vacuum with quasi-Bessel beams^6,7 or spatio-temporal couplings^8,9. Although, in the first case, the propagation was diffraction free, but with hardly controllable pulse velocities and limited to moderate intensities, in the second, high tunability was achieved, but with substantially lengthened pulse durations. Here we report a new concept that extends these approaches to relativistic intensities and ultrashort pulses by mixing spatio-temporal couplings and quasi-Bessel beams to independently control the light velocity and intensity. When used to drive a laser-plasma accelerator¹⁰, this concept leads to a new regime that is dephasing free, where the electron beam energy gain increases by more than one order of magnitude.

近几十年来，亚腔内和超腔内的光脉冲引起了人们极大的关注^1,2,3,4，这在电信，光学存储和基础物理学^5方面开辟了前景。通常在物质上实现，超光速传播也已在真空中用准贝塞尔光束^6,7或时空耦合^8,9进行了证明。。尽管在第一种情况下，传播是无衍射的，但脉冲速度难以控制且强度仅限于中等强度，但在第二种情况下，实现了高可调性，但脉冲持续时间却大大延长。在这里，我们报告了一个新概念，通过混合时空耦合和准贝塞尔光束来独立控制光速和光强，从而将这些方法扩展到相对论强度和超短脉冲。当用于驱动激光等离子体加速器^10时，该概念导致了一种新的自由移相方案，其中电子束能量增益增加了一个以上数量级。