Broadband space-time wave packets propagating 70 m

The propagation distance of a pulsed beam in free space is ultimately limited by diffraction and space-time coupling. "Space-time" (ST) wave packets are pulsed beams endowed with tight spatio-temporal spectral correlations that render them propagation-invariant. Here we explore the limits of the propagation distance for ST wave packets. Making use of a specially designed phase plate inscribed by gray-scale lithography, we synthesize an ST light sheet of width $\approx700$~$\mu$m and bandwidth $\sim20$~nm and confirm a propagation distance of $\approx70$~m.