• optics
• laser-plasma interaction
• high intensity regime
• plasmonics

Plasmonics is a branch of nanophotonics dedicated to the study and application of the interaction of electromagnetic waves with the collective oscillations of electrons in a medium. The surface electromagnetic excitation propagating along the interface of two different media is referred to as Surface Plasmon Polariton (SPP). The ability of SPP to confine and enhance the electromagnetic field at the subwavelength scale in the immediate vicinity of the interface makes them ideal candidates for a vast variety of applications in a broad range of research fields including chemistry and biology. For this reason, it is important to know how to control the coupling of incident electromagnetic waves to SPPs. The main issue in coupling laser light to SPP is that this cannot happen at a flat surface. A widely used method, which will be discussed and used in this work, is to use a grating, i.e. a surface with a periodic engraving.
Advances in laser technology have paved the way to shorter and more intense laser pulses, permitting the exploration of increasingly higher field regimes. In these regimes nonlinear relativistic effects, which arise when the amplitude of the oscillating electron momentum becomes close to or exceeds m_ec, are predicted to take place. The extension of the study of plasmonics to the high field regime is motivated by the latest laser technology advancement with powerful laser systems, with intensities up to the order of 10^22 W/cm 2 , expected to become available to the scientific community in the near future. A drawback of high field plasmonics is the current lack of theory in this interaction regime. Indeed, despite numerous experimental and numerical work, nonlinear plasmonics remains a largely unexplored territory. In the experimental and numerical studies realized in [1,2], the role of SPP in the acceleration of electron bunches along grating targets at high laser intensities and related generation of coherent, high harmonic XUV pulses has been demonstrated, with potential in application requiring ultrashort, intense electron and XUV pulses. In the above mentioned context we wanted to investigate how the use an extremely ultrashort SPP, at the limit of single-cycle duration, may affect the duration and spectra of SPP-enhanced electron and XUV emission. A scheme to obtain SPP of nearly single-cycle duration without using a few-cycle laser driver was proposed in [3] using laser pulses with wavefront rotation. This mechanism leads to the generation of ultra-short SPP pulses given that only a fraction of the laser beam will match the resonance condition and excite a SPP.
With these experiments in mind and the upcoming multi-petawatt generation of lasers, the aim of this manuscript is to find favorable conditions to improve the excitation of both ultra-short and high-intensity SPP and related acceleration of energetic electron bunches. To this aim we have studied, via numerical simulations employing two different approaches, the effect of changing parameters for both the grating target (such as the shape and profile of the modulation) and the laser pulse (such as the waist size and the wavefront rotation).

References
[1] L. Fedeli et al. “Electron Acceleration by Relativistic Surface Plasmons in Laser-Grating Interaction”. In: Phys. Rev.
Lett. 116 (1 2016), p. 015001. doi: 10.1103/PhysRevLett.116.015001.
[2] G. Cantono et al. “Extensive study of electron acceleration by relativistic surface plasmons”. In: Physics of Plasmas
25.3 (2018), p. 031907. doi: 10.1063/1.5017706.
[3] F. Pisani L. Fedeli and A. Macchi. “Few-Cycle Surface Plasmon Polariton Generation by Rotating Wavefront Pulses”.
In: ACS Photonics 5.1068-1073 (2018). doi: 10.1063/1.5013321.