Enhanced relativistic harmonics by electron nanobunching

TL;DR

This paper demonstrates that electron nanobunching in relativistic laser-plasma interactions enhances harmonic generation, producing flatter synchrotron spectra and potentially single attosecond pulses with high energy concentration.

Contribution

It introduces the concept of electron nanobunching as a mechanism for enhanced relativistic harmonic generation and provides analytical and numerical analysis of the resulting synchrotron spectra.

Findings

Flatter synchrotron spectra with power-law exponents of 4/3 or 6/5.

Spectral cut-off frequency determined by electron gamma-factor or nanobunch thickness.

Generation of single attosecond pulses containing nearly full optical cycle energy.

Abstract

It is shown that when an few-cycle, relativistically intense, p-polarized laser pulse is obliquely incident on overdense plasma, the surface electrons may form ultra-thin, highly compressed layers, with a width of a few nanometers. These electron "nanobunches" emit synchrotron radiation coherently. We calculate the one-dimensional synchrotron spectrum analytically and obtain a slowly decaying power-law with an exponent of 4/3 or 6/5. This is much flatter than the 8/3 power of the BGP (Baeva-Gordienko-Pukhov) spectrum, produced by a relativistically oscillating bulk skin layer. The synchrotron spectrum cut-off frequency is defined either by the electron relativistic $\gamma$-factor, or by the thickness of the emitting layer. In the numerically demonstrated, locally optimal case, the radiation is emitted in the form of a single attosecond pulse, which contains almost the entire energy of the full optical cycle.