Optical space-time wave packets having arbitrary group velocities in free space

TL;DR

This paper demonstrates a method to precisely control the group velocity of optical wave packets in free space by sculpting their spatio-temporal spectrum, enabling arbitrary velocities above, below, or even backwards relative to the speed of light.

Contribution

It introduces a novel technique for controlling optical wave packet velocities in free space through joint spatio-temporal spectrum modulation, surpassing previous limitations.

Findings

Arbitrary group velocities achieved in free space.

Control over wave packets traveling faster, slower, or backwards.

Propagation-invariant optical wave packets demonstrated.

Abstract

Controlling the group velocity of an optical pulse typically requires traversing a material or structure whose dispersion is judiciously crafted. Alternatively, the group velocity can be modified in free space by spatially structuring the beam profile, but the realizable deviation from the speed of light in vacuum is small. Here we demonstrate precise and versatile control over the group velocity of a propagation-invariant optical wave packet in free space through sculpting its spatio-temporal spectrum. By jointly modulating the spatial and temporal degrees of freedom, arbitrary group velocities are unambiguously observed in free space above or below the speed of light in vacuum, whether in the forward direction propagating away from the source or even traveling backwards towards it.