Controlling the velocity of ultrashort light pulses in vacuum through spatio-temporal couplings

Due to their broad spectral width, ultrashort lasers provide new possibilities to shape light beams and control their properties, in particular through the use of spatio-temporal couplings. In this context, we present a theoretical investigation of the linear propagation of ultrashort laser beams that combine temporal chirp and a standard aberration known as longitudinal chromatism. When such beams are focused in a vacuum, or in a linear medium, the interplay of these two effects can be exploited to set the velocity of the resulting intensity peak to arbitrary values within the Rayleigh length, i.e. precisely where laser pulses are generally used. Such beams could find groundbreaking applications in the control of laser-matter interactions, in particular for laser-driven particle acceleration.