On the importance of frequency-dependent beam parameters for vacuum acceleration with few-cycle radially polarized laser beams

Tightly focused, ultrashort radially polarized laser beams have a large longitudinal field, which provides a strong motivation for direct particle acceleration and manipulation in a vacuum. The broadband nature of these beams means that chromatic properties of propagation and focusing are important to consider. We show via single-particle simulations that using the correct frequency-dependent beam parameters is imperative, especially as the pulse duration decreases to the few-cycle regime. The results with different spatio-spectral amplitude profiles show either a drastic increase or decrease of the final accelerated electron energy depending on the shape, motivating both proper characterization and potentially a route to optimization.