Theory of Relativistic Surface Plasmon Excitation on Smooth Surface by High-Intensity Laser

TL;DR

This paper develops a classical theory for relativistic surface plasmon excitation at smooth plasma-vacuum interfaces driven by intense lasers, analyzing the effects of geometry, drive spectrum, and relativistic modifications.

Contribution

It introduces a comprehensive analytical model for RSP excitation considering surface geometry, drive spectrum, and relativistic effects, with implications for particle acceleration and surface wave control.

Findings

Surface geometry influences RSP excitation and mode selection.

Relativistic effects modify dielectric response and excitation efficiency.

Cylindrical surfaces enable nonlinear wakefield generation for particle acceleration.

Abstract

We present a classical theory of relativistic surface plasmon (RSP) excitation at a smooth plasma-vacuum interface driven by either a ponderomotive force or an electric field of an intense laser pulse. Starting from Maxwell equations coupled to a cold-fluid plasma response, we derive a general driven wave equation for the RSP and solve it analytically. We show that an infinite planar surface enforces conservation of the in-plane wavevector. A finite longitudinal interaction length or axial modulation supplies a finite kz spectrum, while cylindrical curvature replaces one continuous transverse in-plane wavenumber by a discrete azimuthal mode index m. This partially relaxes the planar in-plane constraint, while axial phase matching remains controlled by the longitudinal spectrum of the drive. The excitation strength is controlled by the overlap between the drive and the surface eigenfield, which is determined by the surface geometry. This provides a general principle for controlling RSP excitation. We also show that relativistic effects can substantially modify the dielectric response and can be preliminarily verified by particle-in-cell simulations. Within the local relativistic dielectric model, the overlap-normalised planar source saturates at large a0, and cylindrical curvature partially alleviates this reduction before strong surface softening develops. The role of surface geometry is analysed. A cylindrical surface can sustain an on-axis accelerating field, enabling highly nonlinear wakefield generation for particle acceleration. In addition, the cylindrical geometry imposes a precise mode-selection rule that provides intrinsic control over RSP excitation. Axisymmetric ponderomotive drive selects fundamental mode m=0. A linearly polarised laser field selects a superposition of m=+1 and m=-1 modes, and a circularly polarised laser field selects a single helical mode.