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

Ayman F. Abouraddy,H. Esat Kondakci

doi:10.1038/s41467-019-08735-8

Ayman F. Abouraddy, H. Esat Kondakci

Open Access

https://doi.org/10.1038/s41467-019-08735-8

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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.

Highlights

Controlling the group velocity of an optical pulse typically requires traversing a material or structure whose dispersion is judiciously crafted
Vg is dependent on the size of the field spatial profile, and the maximum group delay observable is limited by the numerical aperture
Instead of manipulating separately the field spatial or temporal degrees of freedom and attempting to minimize unavoidable space-time coupling, tight spatio-temporal correlations are intentionally introduced into the wave packet spectrum, thereby resulting in the realization of arbitrary group velocities: superluminal, luminal, or subluminal, whether in the forward direction propagating away from the source or in the backward direction traveling toward it

Summary

Introduction

Controlling the group velocity of an optical pulse typically requires traversing a material or structure whose dispersion is judiciously crafted. Velocities slightly lower than c (≈0.99999c) have been accessible in the experiments performed to date with maximum observed group delays of ~30fs, corresponding to a shift of ~10 μm over a distance of 1 m (or 1 part in 105) Another potential approach to controlling the group velocity of a pulsed beam in free space relies on sculpting the spatiotemporal profile of propagation-invariant wave packets[17,18]. Associating each wavelength in the pulse spectrum with a particular transverse spatial frequency traces out a conic section on the surface of the light-cone while maintaining a linear relationship between the axial component of the wave vector and frequency[19,20] The slope of this linear relationship dictates the wave packet group velocity, and its linearity eliminates any additional dispersion terms. There have been no experimental reports to date on negative group velocities in free space

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