paper

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

arXiv:1609.02573 · doi:10.1038/s41467-017-00490-y

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 , and we report measurements of longitudinally polarized electric fields propagating between and . 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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