单周期太赫兹脉冲粒子加速的亚光速传播演示
Demonstration of sub-luminal propagation of single-cycle terahertz pulses for particle acceleration
摘要：
在自由空间中，电磁波的亚光速相速度通常是无法获得的，这与被禁止的快于光的群速度密切相关。在激光驱动粒子加速方案中，对有效亚光速相速度的要求对自由空间中可实现的总加速施加了基本限制，并且需要使用介电结构和波导来扩展场-粒子相互作用。在这里，我们展示了一个新的旅行源和自由空间传播的方法，以克服亚光速传播的限制。该方法利用了相对容易的产生超快光源与慢群速度传播，并通过材料真空边界附近的非线性光学相互作用的组到相位的前端转换。该概念证明了两个太赫兹产生过程，非线性光学整流和电流浪涌整流。的相速度是可调的，以上和以下的真空光速$c$，我们报告的纵向偏振电场之间传播的测量$0.77c$和$1.75c$。证明了扩展到多MV/m场强的能力。我们的方法铺平了道路，实现廉价和紧凑的粒子加速器与前所未有的飞秒尺度的粒子控制。
英文摘要：
The sub-luminal phase velocity of electromagnetic waves in free space is generally unobtainable, being closely linked to forbidden faster than light group velocities. The requirement of effective sub-luminal phase-velocity in laser-driven particle acceleration schemes imposes a fundamental limit on the total acceleration achievable in free-space, and necessitates the use of dielectric structures and waveguides for extending the field-particle interaction. Here we demonstrate a new travelling-source and free space propagation approach to overcoming the sub-luminal propagation limits. The approach exploits the relative ease of generating ultrafast optical sources with slow group velocity propagation, and a group-to-phase front conversion through non-linear optical interaction near a material-vacuum boundary. The concept is demonstrated with two terahertz generation processes, non-linear optical rectification and current-surge rectification. The phase velocity is tunable, both above and below vacuum speed of light $c$, and we report measurements of longitudinally polarized electric fields propagating between $0.77c$ and $1.75c$. The ability to scale to multi-MV/m field strengths is demonstrated. Our approach paves the way towards the realization of cheap and compact particle accelerators with unprecedented femtosecond scale control of particles.
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