• acceleration enhancement
• high power laser-matter interaction
• SPR frequency shift
• surface plasmon resonance

A system which presents a periodic optical structure is usually referred to as a photonic
structure, for its light manipulation capabilities. The most simple one dimensional photonic structure that can be made of metallic material is a diffraction grating, which is known
for its "light splitting" properties: an incident wave will be split and diffracted at different
angles. Because of this, gratings are commonly used as monochromators and spectrometers. However, for p-polarised incident light and particular ratios of light wavelength over grating pitch, a diffraction grating can behave like an optical cavity, i.e. all the incident
light can be confined in the proximity of the grating surface and converted into a surface
wave, a kind of wave that propagates parallel to the metallic surface and is evanescent away
from it.
In the present work, since SPR provides a local enhancements of the e.m. field, we investigate
an application of this phenomenon to the field of high power laser matter interaction.
Today’s laser amplification technology is such that, during the interaction, ordinary matter is rapidly ionized and forms an overdense plasma in front of the (typically solid) target. This means that laser light cannot propagate inside the material, therefore making difficult the transfer of the laser energy. The most efficient heating mechanisms are based on the acceleration of electrons that are dragged in the vacuum and re-injected into the plasma with supra thermal velocities. This is usually referred as "vacuum heating", and a crucial role is played by the fields near the plasma-vacuum interface.

Thus, the concept we investigate in this thesis work is the introduction of a periodic modulation to the target surface, which is maintained during the interaction process. The aim is to excite a SPR and confine the laser energy in proximity of the target surface, in order to enhance the absorption process. The interest in this work is in the availability in the near future of high contrast laser pulses, which allows the conservation of the target periodic structure, that until now tends to be erased by the the laser pre-pulse (high pre-pulse = low contrast).
Clearly, the grating depth cannot be as small as in usual gratings, where the depth over pitch ratio is h/d = 1/100 and throughout our work we set the parameter range as h/d = 0,05-0,2. In these conditions we have observed numerically that the SPR is red-shifted at increasing h/d. To account for this fact, we have used a non-perturbative formalism to perform an original analytic calculation which enables us to predict the shifting of the resonant frequencies.

The idea of absorption enhancement via SPR excitation has already been investigated by means of self-consistent particle-in-cell (PIC) simulations, showing that when a modulated target replace a flat one, the absorption increases from 20% to 70%. However, because of the intrinsic noise of PIC methods, it is not evident the actual contribution of the surface
modes to the absorption process.
Therefore, in order to clarify the surface plasmon contribution to electrons acceleration, we simplify the self-consistent numerical problem by splitting it in two steps: at first the surface fields produced by a modulated surface are studied with a FDTD electromagnetic code, assuming fot the plasma a Drude dielectric constant. Then the electrons motion in the surface fields is studied by a test particle approach. With this numerical scheme we are able to show that the electrons dynamics is greatly sensitive to SPR excitation. In particular, the average kinetic energy acquired by the electrons in the resonant fields is much greater than in the not resonant case, and the energy distribution of the accelerated electrons shows an extended "plateau" region, that is absent when the resonance is not excited.

This confirms our hypothesis that the major effect in electrons acceleration grating driven enhancement is due to the possibility to excite collective surface electrons modes, and cannot be explained as a consequence of "hot spot" creation in the field pattern, because of the field lines bending caused by the modulation of the surface. Consequently the energy absorption enhancement observed in PIC simulations has to be considered an effect of the collective surface electron mode excitation.