Axial Spectral Encoding of Space-Time Wave Packets

Space-time (ST) wave packets are propagation-invariant pulsed optical beams whose group velocity can be tuned in free space by tailoring their spatiotemporal spectral structure. To date, efforts on synthesizing ST have striven to maintain propagation invariance. Here, we demonstrate that one degree of freedom a packet---its on-axis spectrum---can isolated and purposefully controlled independently the others. Judicious amplitude phase modulation yields with programmable changes along axis, including red-shifting or blue-shifting spectra, more sophisticated axial encoding bidirectional shifts accelerating spectra. In all cases, shift larger than initial bandwidth, while preserving propagation-invariance other degrees freedom, packet profile. These results may useful range finding microscopy remote sensing via stamping.