Classification of propagation-invariant space-time wave packets in free space: Theory and experiments

TL;DR

This paper classifies and experimentally synthesizes different types of propagation-invariant space-time wave packets in free space, revealing how spectral correlations can control their propagation properties.

Contribution

It introduces a comprehensive classification of 10 classes of propagation-invariant wave packets based on their spectral properties and demonstrates experimental synthesis of all classes.

Findings

Identified 10 classes of propagation-invariant wave packets.

Developed a method to synthesize arbitrary spatio-temporal spectral correlations.

Validated the classification through experimental synthesis and characterization.

Abstract

Introducing correlations between the spatial and temporal degrees of freedom of a pulsed optical beam (or wave packet) can profoundly alter its propagation in free space. Indeed, appropriate spatio-temporal spectral correlations can render the wave packet propagation-invariant: the spatial and temporal profiles remain unchanged along the propagation axis. The spatio-temporal spectral locus of any such wave packet lies at the intersection of the light-cone with tilted spectral hyperplanes. We investigate (2+1)D 'space-time' propagation-invariant light sheets, and identify 10 classes categorized according to the magnitude and sign of their group velocity and the nature of their spatial spectrum - whether the low spatial frequencies are physically allowed or forbidden according to their compatibility with causal excitation and propagation. We experimentally synthesize and characterize all 10 classes using an experimental strategy capable of synthesizing space-time wave packets that incorporate arbitrary spatio-temporal spectral correlations.