Designing a compact accelerator one would hardly think of a microscale silicon chip and a fiber laser normally used for telecom. Yet, this is exactly what the future might look like. The authors of this Optics Letters article demonstrate for the first time relativistic electrons being accelerated “on a chip”. The laser electric field is transverse to the propagation direction and oscillates quickly. This makes it impossible to accelerate relativistic particles in vacuum: the particle gains energy along one half of the laser wavelength and inevitably loses it in the second half – when the laser field changes its sign. The authors designed a “phase-reset” dielectric grating microstructure that flips the laser phase when the electrons reach the second half of the laser wavelength. In this way, direct laser acceleration becomes feasible. Using ultra-short (sub-picosecond) laser pulses, the authors could use laser fields as high as 3.8 GV/m and still stay below the dielectric damage threshold. The average of the accelerating gradient over a laser period was 690 MV/m. This is a record for dielectric laser accelerators and significantly larger than any conventional metal-based accelerator structure can sustain.
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