Space-time wave packets

TL;DR

Space-time wave packets are a class of optical fields that maintain their shape during propagation, exhibit unique controllable velocities, and have recently seen rapid experimental advances due to Fourier synthesis innovations.

Contribution

This paper reviews the fundamental properties, unique behaviors, and recent experimental developments of space-time wave packets, highlighting their potential in structured and ultrafast optics.

Findings

Propagation-invariant in linear media

Controllable group velocities in free space

Recent experimental synthesis techniques

Abstract

"Space-time" (ST) wave packets constitute a broad class of pulsed optical fields that are rigidly transported in linear media without diffraction or dispersion, and are therefore propagation-invariant in absence of optical nonlinearities or waveguiding structures. Such wave packets exhibit unique characteristics, such as controllable group velocities in free space and exotic refractive phenomena. At the root of these behaviors is a fundamental feature underpinning ST wave packets: their spectra are not separable with respect to the spatial and temporal degrees of freedom. Indeed, the spatio-temporal structure is endowed with non-differentiable angular dispersion, in which each spatial frequency is associated with a single prescribed wavelength. Furthermore, deviation from this particular spatio-temporal structure yields novel behaviors that depart from propagation invariance in a precise manner, such as acceleration with an arbitrary axial distribution of the group velocity, tunable dispersion profiles, and Talbot effects in space-time. Although the basic concept of ST wave packets has been known since the 1980's, only very recently has rapid experimental development emerged. These advances are made possible by innovations in spatio-temporal Fourier synthesis, thereby opening a new frontier for structured light at the intersection of beam optics and ultrafast optics.