  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.
