Extreme high field plasmonics: electron acceleration and XUV harmonic generation from ultrashort surface plasmons*

Propagating surface plasmons (SP) are collective electromagnetic (EM) modes localized across a sharp interface between vacuum and a metal or plasma. Plasmonics exploits SP for EM field confinement and enhancement with several applications. In a series of experiments and simulations [1] it has been shown that "relativistic" SP of high amplitude can be excited in the interaction of intense sub-picosecond laser pulses with solid targets.

By using a femtosecond laser system with extremely high contrast, SP have been excited on "grating" (periodically engraved) targets. "Surfing" acceleration of high charge (up to 650 picoCoulomb) electron bunches along the SP has been demonstrated [2,3]. The extreme ultraviolet (XUV) harmonics diffracted by the grating are strongly enhanced in the grazing direction [4], quasi-collinear to the electron bunches. Simulations show that the XUV harmonics are boosted by nanobunching in the SP field of the electrons which scatter the laser field, similarly to free electron laser operation. Ongoing work include the effect of shortening the SP down to the single-cycle limit by using a laser driver with rotating wavefront [5].

[1] A. Macchi, "Surface plasmons in superintense laser-solid interactions", Phys. Plasmas 25 (2018) 031906
[2] L. Fedeli et al, "Electron acceleration by relativistic surface plasmons in laser-grating interaction", Phys. Rev. Lett. 116 (2016) 015001
[3] G. Cantono et al, "Extensive study of electron acceleration by relativistic surface plasmons", Phys. Plasmas 25 (2018) 031907
[4] G. Cantono et al, "Extreme ultraviolet beam enhancement by relativistic surface plasmons", Phys. Rev. Lett. 121 (2018) 264803
[5] F. Pisani, L. Fedeli, A. Macchi, "Few-cycle Surface Plasmon Polariton Generation by Rotating Wavefront Pulses", ACS Photonics 5 (2018) 1068