Study of energy backflow in unidirectional monochromatic and space-time waves

TL;DR

This paper analytically investigates energy backflow phenomena in various unidirectional monochromatic and space-time wave packets, revealing new insights into their energy propagation characteristics and providing explicit expressions for the Poynting vector.

Contribution

It introduces novel analytic expressions for energy backflow in (2+1)D unidirectional waves and space-time wave packets, advancing understanding of their energy dynamics.

Findings

Analytic expressions for backflow in (2+1)D waves are derived.

Backflow occurs in superluminal and luminal unidirectional wave packets.

Insights into energy propagation in non-diffracting space-time waves are provided.

Abstract

Backflow, or retropropagation, is a counterintuitive phenomenon whereby for a forward-propagating wave the energy locally propagates backward. In the context of backflow, physically most interesting are the so-called unidirectional waves, which contain only forward propagating plane wave constituents. Yet, very few such waves possessing closed-form analytic expressions for evaluation of the Poynting vector are known. In this study, we examine energy backflow in a novel (2+1)-dimensional unidirectional monochromatic wave and in a (2+1)D spatio-temporal wave packet, analytic expressions which we succeeded to find. We also present a detailed study of the backflow in the "needle" pulse. This is an interesting model object because well-known superluminal non-diffracting space-time wave packets can be derived from its factored wave function. Finally we study the backflow in an unidirectional version of the so-called focus wave mode--a pulse propagating luminally and without spread, which is the first and most studied representative of the (3+1)D non-diffracting space-time wave packets (also referred to as spatiotemporally localized waves).