Coherent Thomson backscattering from laser-driven relativistic ultra-thin electron layers

TL;DR

This paper investigates the generation of coherent Thomson backscattering from laser-driven relativistic electron layers in ultra-thin foils, using analytic theory and simulations to explore Doppler shifts and methods to optimize pulse generation.

Contribution

It introduces new methods to control electron momentum and recover full Doppler shifts, enhancing the design of intense attosecond pulses.

Findings

Doppler shift proportional to b3_x^2, with b3_x related to electron velocity.

Transverse electron momentum reduces Doppler shift from its maximum value.

Proposed methods effectively recover full Doppler shift, enabling better pulse control.

Abstract

The generation of laser-driven dense relativistic electron layers from ultra-thin foils and their use for coherent Thomson backscattering is discussed, applying analytic theory and one-dimensional particle-in-cell simulation. The blow-out regime is explored in which all foil electrons are separated from ions by direct laser action. The electrons follow the light wave close to its leading front. Single electron solutions are applied to initial acceleration, phase switching, and second-stage boosting. Coherently reflected light shows Doppler-shifted spectra, chirped over several octaves. The Doppler shift is found to be proportional to \gamma_x^2=1/(1-\beta_x^2), where \beta_x is the electron velocity component in normal direction of the electron layer which is also the direction of the driving laser pulse. Due to transverse electron momentum p_y, the Doppler shift by 4*\gamma_x^2=4*\gamma^2/(1+(p_y/mc)^2) ~= 2*\gamma is significantly smaller than full shift of 4*\gamma^2. Methods to turn p_y -> 0 and to recover the full Doppler shift are proposed and verified by 1D-PIC simulation. These methods open new ways to design intense single attosecond pulses.