Reexamination of group velocities of structured light pulses

TL;DR

This paper clarifies the conflicting results on the group velocities of structured light pulses by analyzing different measurement schemes and their relation to measurable travel times, using Bessel-Gauss pulses as a model.

Contribution

It provides a detailed analysis of how various definitions of group velocity relate to measurable pulse travel times, resolving contradictions in previous studies.

Findings

Different group velocity definitions are appropriate depending on beam structure and measurement method.

The analysis shows subluminal and superluminal regions in Bessel-Gauss pulse propagation.

Measurement scheme influences the observed group velocity, clarifying previous controversies.

Abstract

Recently, a series of theoretical and experimental papers on free-space propagation of pulsed Laguerre-Gaussian and Bessel beams was published, which reached contradictory and controversial results about group velocities of such pulses. Depending on themeasurement scheme, the group velocity can be defined differently.We analyze how different versions of group velocity are related to the measurable travel time (time of flight) of the pulse between input (source) and output (detecting) planes. The analysis is tested on a theoretical model---the Bessel-Gauss pulse whose propagation path exhibits both subluminal and superluminal regions. Our main conclusion from resolving the contradictions in the literature is that different versions of group velocity are appropriate, depending on whether or not the beam is hollow and how the pulse is recorded in the output plane---integrally or with spatial resolution.