Demonstration of sub-luminal propagation of single-cycle terahertz pulses for particle acceleration

TL;DR

This paper introduces a novel method for generating and controlling sub-luminal and super-luminal terahertz pulses in free space, enabling advanced particle acceleration techniques without traditional dielectric structures.

Contribution

It demonstrates a new traveling-source approach that converts slow group velocity optical pulses into tunable phase velocities in free space, surpassing previous limits.

Findings

Phase velocity tunable between 0.77c and 1.75c

Achieved high field strengths exceeding multi-MV/m

Demonstrated free-space propagation of single-cycle terahertz pulses

Abstract

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.