Axial Spectral Encoding of Space-Time Wave Packets

Abstract

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 wave packets have striven to maintain their propagation invariance. Here, we demonstrate that one degree of freedom of a ST wave packet---its on-axis spectrum---can be isolated and purposefully controlled independently of the others. Judicious spatiotemporal spectral amplitude and phase modulation yields ST wave packets with programmable spectral changes along the propagation axis, including red-shifting or blue-shifting spectra, or more sophisticated axial spectral encoding including bidirectional spectral shifts and accelerating spectra. In all cases, the spectral shift can be larger than the initial on-axis bandwidth, while preserving the propagation-invariance of the other degrees of freedom, including the wave packet spatiotemporal profile. These results may be useful in range finding in microscopy or remote sensing via spectral stamping.