Particle accelerators are essential tools in science, hospitals and industry^1,2,3,4,5,6. Yet their costs and large footprint, ranging in length from metres to several kilometres, limit their use. The recently demonstrated nanophotonics-based acceleration of charged particles can reduce the cost and size of these accelerators by orders of magnitude^7,8,9. In this approach, a carefully designed nanostructure transfers energy from laser light to the particles in a phase-synchronous manner, accelerating them. To accelerate particles to the megaelectronvolt range and beyond, with minimal particle loss^10,11, the particle beam needs to be confined over extended distances, but the necessary control of the electron beam’s phase space has been elusive. Here we demonstrate complex electron phase-space control at optical frequencies in the 225-nanometre narrow channel of a silicon-based photonic nanostructure that is 77.7 micrometres long. In particular, we experimentally show alternating phase focusing^10,11,12,13, a particle propagation scheme for minimal-loss transport that could, in principle, be arbitrarily long. We expect this work to enable megaelectronvolt electron-beam generation on a photonic chip, with potential for applications in radiotherapy and compact light sources⁹, and other forms of electron phase-space control resulting in narrow energy or zeptosecond-bunched beams^14,15,16.

^{粒子加速器是科学、医院和工业1,2,3,4,5,6}中必不可少的工具。然而，它们的成本和巨大的占地面积（长度从米到几公里不等）限制了它们的使用。最近展示的基于纳米光子学的带电粒子加速可以将这些加速器的成本和尺寸降低^7,8,9数量级。在这种方法中，精心设计的纳米结构以相位同步的方式将能量从激光传输到粒子，从而加速它们。将粒子加速到兆电子伏范围及以上，同时将粒子损失降至最低^10,11，粒子束需要被限制在很长的距离上，但电子束相空间的必要控制一直难以捉摸。在这里，我们展示了在 77.7 微米长的硅基光子纳米结构的 225 纳米窄通道中的光学频率下的复杂电子相空间控制。特别是，我们通过实验展示了交替相位聚焦^10,11,12,13，这是一种用于最小损耗传输的粒子传播方案，原则上可以任意长。我们期望这项工作能够在光子芯片上产生兆电子伏特电子束，具有在放射治疗和紧凑型光源⁹以及其他形式的电子相空间控制中应用的潜力，从而产生窄能量或 zeptosecond-bundle 束^14,15,16。