Synergistic Longitudinal Acceleration and Transverse Oscillation in High-order Harmonic Generation

TL;DR

This paper introduces a novel mechanism for high-order harmonic generation involving surface wave excitation and electron nanobunch acceleration, resulting in a flatter power-law spectrum and potential for larger attosecond pulses.

Contribution

It demonstrates a new synergistic process combining longitudinal acceleration and transverse oscillation for efficient harmonic generation from overdense plasma.

Findings

Relativistic harmonics follow a power-law decay with exponent -1.

Surface wave excitation leads to dense electron nanobunches.

The mechanism enables generation of larger, flatter attosecond pulses.

Abstract

We propose and demonstrate that relativistic harmonics with a slowly decaying power law are generated from a femtosecond lase pulse incident parallel to a micro-scale overdense plasma. It is shown that due to the excitation of a strong surface wave, dense electron nanobunches are continuously accelerated forward while oscillating in the transverse laser field. Even around the stationary phase point, relativistic gamma factors of the nanobunches increase considerably, leading to a much stronger attosecond burst, compared to the case with constant gamma. Our two-dimensional particle-in-cell simulations and analytical theory show that this synergistic function promises a power-law harmonic spectrum $I_n/I_0 = n^{-1}$. This is much flatter than the other well-known radiation mechanisms and paves the way to unprecedentedly large energy attosecond pulses.