Twisting light in 4D: Helical wave packets with unique properties

We demonstrate the coherent manipulation of the spatial and temporal properties of ultrashort pulsed laser beams to generate wave packets that trace helical trajectories in space-time. This is achieved by introducing a non-separable correlation between the beam's frequency (related to its temporal structure) and its orbital angular momentum (that defines its spatial topology). The demonstration relies on a diffractive axicon grating to separate the frequencies of a pulsed beam into colinear radial rings which are then independently modulated with an azimuthal phase using a digital spatial light modulator. The all-digital setup allows to tailor the topological-spectral correlations, affording control over a range of beam properties, while more complex correlations realize exotic space-time wave packets with extended beam dynamics. These unique beams open new horizons in ultrafast light-matter interactions, microscopy and multiplexing.