arXiv subjects

Peeter Saari

Publications and source records attributed to Peeter Saari.

15 recordsLinked to original sources

arXiv2026

Orbital and spin current density backflow in unidirectional monochromatic electromagnetic fields in vacuum

In this study, energy backflow in the Poynting vector, as well as its orbital and spin current density components, has been examined for a 2-dimensional causal unidirectional vector-valued monochromatic electromagnetic wave. Linear transverse electric (TE), transverse magnetic (TM), and circular polarization cases are considered and studied in detail, including both electric and magnetic contributions to the current density components. Spin current backflow has been found to be unexpectedly strong. A study of the energy backflow is also presented in the scalar version of the 2-dimensional monochromatic wave. A detailed study has been carried out of the correlation of the positions of energy backflows with local wavenumbers and their signs, the zeros of appropriate intensities and the presence of vortices.

physics.optics↗

arXiv2025

Wave pulses with unusual asymptotical behavior at infinity

The behavior of wave signals in the far zone is not only of theoretical interest but also of paramount practical importance in communications and other fields of applications of optical, electromagnetic or acoustic waves. Long time ago T. T. Wu introduced models of 'electromagnetic missiles' whose decay could be made arbitrarily slower than the usual inverse distance by an appropriate choice of the high frequency portion of the source spectrum. Very recent work by Plachenov and Kiselev introduced a finite-energy scalar wave solution, different from Wu's, decaying slower than inversely proportional with the distance. A physical explanation for the unusual asymptotic behavior of the latter will be given in this article. Furthermore, two additional examples of scalar wave pulses characterized by abnormal slow decay in the far zone will be given and their asymptotic behavior will be discussed. A proof of feasibility of acoustic and electromagnetic fields with the abnormal asymptotics will be described.

math-ph↗

arXiv2024

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

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).

physics.optics↗

arXiv2024

Matters Arising: On physical nature of the optical deDroglie-Mackinnon wave packets. ARISING FROM Layton A. Hall and Ayman F. Abouraddy Nature Physics https://doi.org/10.1038/s41567-022-01876-6 (2023)

Hall and Abouraddy [1] have reported first experimental observation of optical de Broglie-Mackinnon wave packets, which is a seminal achievement in the study of so-called non-diffracting optical pulses. These wave packets propagate in free space without spreading with subluminal relativistic velocities, i.e., with speeds slower but close to the velocity of light in vacuum. The experiments in [1] became possible thanks to the application of quite a witty method. Unfortunately, the explanation of the physical nature of the wave packets and their graphical and mathematical descriptions in the theoretical part of [1] suffer from some ambiguities that need to be clarified.

physics.optics↗

arXiv2022

Energy backflow in unidirectional spatiotemporally localized wavepackets

Backflow, or retro-propagation, is a counterintuitive phenomenon where for a forward-propagating wave the energy or probability density locally propagates backward. In this study the energy backflow has been examined in connection with relatively simple causal unidirectional finite-energy solutions of the wave equation which are derived from a factorization of the so-called basic splash mode. Specific results are given for the energy backflow arising in known azimuthally symmetric unidirectional wavepackets, as well as in novel azimuthally asymmetric extensions. Using the Bateman-Whittaker technique, a novel finite-energy unidirectional null localized wave has been constructed that is devoid of energy backflow and has some of the topological properties of the basic Hopfion.

physics.optics↗

arXiv2021

Backward energy flow in simple 4-wave electromagnetic fields

Electromagnetic energy backflow is a phenomenon occurring in regions where the direction of the Poynting vector is opposite to that of the propagation of the wave field. It is particularly remarkable in the nonparaxial regime and has been exhibited in the focal region of sharply focused beams, for vector Bessel beams, and vector-valued spatiotemporally localized waves. A detailed study is undertaken of this phenomenon and the conditions for its appearance are examined in detail in the case of a superposition of four plane waves in free space, the simplest electromagnetic arrangement for the observation of negative energy flow, as well as its comprehensive and transparent physical interpretation. It is shown that the state of polarization of the constituent components of the electromagnetic plane wave quartet determines whether energy backflow takes place or not and what values the energy flow velocity assumes. Depending on the polarization angles, the latter can assume any value from c (the speed of light in vacuum) to -c in certain spatiotemporal regions of the field.

physics.optics↗

arXiv2020

Localized wave solutions of the scalar homogeneous wave equation and their optical implementation

In recent years the topic of localized wave solutions of the homogeneous scalar wave equation, i.e., the wave fields that propagate without any appreciable spread or drop in intensity, has been discussed in many aspects in numerous publications. In this review the main results of this rather disperse theoretical material are presented in a single mathematical representation - the Fourier decomposition by means of angular spectrum of plane waves. This unified description is shown to lead to a transparent physical understanding of the phenomenon as such and yield the means of optical generation of such wave fields.

physics.optics↗

arXiv2020

Relativistic aberration and null Doppler shift within the framework of superluminal and subluminal nondiffracting waves

The relativistic aberration of a wavevector and the corresponding Doppler shift are examined in connection with superluminal and subluminal spatiotemporally localized pulsed optical waves. The requirement of a null Doppler shift is shown to give rise to a speed associated with the relativistic velocity composition law of a double (two-step) Lorentz transformation. The effects of such a transformation are examined both in terms of four-coordinates and in the spectral domain. It is established that a subluminal pulse reverses its direction. In addition to a change in direction, the propagation term of a superluminal pulse becomes negative. The aberration due to a double Lorentz transformation is examined in detail for propagation invariant superluminal waves (X wave, Bessel X wave), as well as intensity-invariant superluminal and subluminal waves. Detailed symmetry considerations are provided for the superluminal focus X wave and the subluminal MacKinnon wavepacket.

physics.optics↗

arXiv2019

Reactive energy in non-diffracting localized waves

It is well known that although the group velocity of structured light pulses propagating in vacuum can be subluminal or superluminal, the upper limit of the energy flow velocity is c, the speed of light in vacuum. This inequality can be explained in terms of the reactive energy left behind by the fields. Energy and reactive energy densities have been calculated for vector-valued two-dimensional (light sheet) superluminal electromagnetic nondiffractive pulses, as well as scalar-valued and TM three-dimensional superluminal and subluminal spatiotemporally localized electromagnetic waves. Emphasis is placed on the physical formation of the reactive energy due to interference of the plane-wave constituents of the structured light waves.

physics.optics↗

arXiv2019

Reexamination of energy flow velocities of non-diffracting localized waves

A universal relation has been established between the local energy transport velocity along the direction of propagation and the group velocity of scalar and vector-valued propagation-invariant spatiotemporally localized superluminal and subluminal electromagnetic waves in free space. Under specific restrictions, this relationship is very closely valid for physically realizable almost propagation-invariant spatiotemporally confined subluminal and superluminal electromagnetic fields. In both cases, although the group velocity may be either superluminal or subluminal, the universal relation is in accord with the well-established result that the upper limit of the energy transport velocity is c, the speed of light in vacuum.

physics.optics↗

arXiv2018

Reexamination of group velocities of structured light pulses

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.

physics.optics↗

arXiv2018

White-light hyperbolic Airy beams

Ultra-broadband hyperbolic Airy beams are experimentally realized and spatio-temporally characterized. Transmissive (refractive) and reflective cubic phase elements were used to impose a cubic phase on the input beam. Nondispersing beams are produced in reflective geometry, while the main lobe of the hyperbolic Airy beam created with a transmissive refractive phase element suffered from lateral dispersion.

physics.optics↗

arXiv2016

Observation of subluminal twisted light in vacuum: comment

Our analysis based on analytical and numerical calculations leads to conclusion that the promising results obtained in [F. Bouchard, J. Harris, H. Mand, R. W. Boyd, and E. Karimi, Optica 3, 351, 2016] are questionable in several respects.

physics.optics↗

arXiv2016

Comments on "Slowing of Bessel light beam group velocity"

In a recent article [R. R. Alfano and D. A. Nolan, Opt. Commun. 361 (2016) 25] the group velocity reduction below the speed of light in the case of certain Bessel beam pulses has been considered and an idea of its application for a natural optical buffer presented. However, the authors treat the problem as if only one type of Bessel pulse existed, no matter how it is generated. The deficiencies of the article stem from not being familiar with an extensive literature on Bessel pulses, in particular, with a couple of papers published much earlier in the J. Opt. Soc. Am. A, which have studied exactly the same problem more thoroughly.

physics.optics↗

arXiv2001

Superluminal Localized Waves of Electromagnetic Field in Vacuo

Presented is an overview of electromagnetic versions of the so-called X-type waves intensively studied since their invention in early 1990.-ies in ultrasonics. These waves may be extremely localized both laterally and longitudinally and - what has been considered as most startling - propagate superluminally without apparent spread. Spotlighted are the issues of the relativistic causality, variety of mathematical description and possibilities of practical applications of the waves.

physics.optics↗