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

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  • D.A. Walsh
  • D.S. Lake
  • E.W. Snedden
  • M.J. Cliffe
  • D.M. Graham
  • S.P. Jamison
Article number421
<mark>Journal publication date</mark>4/09/2017
<mark>Journal</mark>Nature Communications
Issue number1
Number of pages9
Publication StatusPublished
<mark>Original language</mark>English


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 sub-luminal phase-velocity in laser-driven particle acceleration schemes imposes a limit on the total acceleration achievable in free space, and necessitates the use of dispersive structures or waveguides for extending the field-particle interaction. We demonstrate a travelling source approach that overcomes the sub-luminal propagation limits. The approach exploits ultrafast optical sources with slow group velocity propagation, and a group-to-phase front conversion through nonlinear optical interaction. The concept is demonstrated with two terahertz generation processes, nonlinear optical rectification and current-surge rectification. We report measurements of longitudinally polarised single-cycle electric fields with phase and group velocity between 0.77c and 1.75c. The ability to scale to multi-megavolt-per-metre field strengths is demonstrated. Our approach paves the way towards the realisation of cheap and compact particle accelerators with femtosecond scale control of particles.