Universal Properties of Electromagnetic Beams
We survey the existing known universal properties of electromagnetic beams,and discuss the possibility of a new one.The established universal properties are the existence of beam invariants,which follow from conservation laws,and various non-existence theorems arising out of beam localization transversely to the direction of propagation.In both cases the theorems follow directly from Maxwells equations.The conservation of energy,momentum and angular momentum lead to seven beam invariants,the simplest of which is the integral across a transverse section of an electromagnetic beam of the time-averaged longitudinal component of the momentum density.This integral is the same for any transverse section: it is an invariant.This particular invariant comes from the conservation of energy; the conservation of momentum leads to three invariant integrals over components of the electromagnetic stress tensor,and likewise three more invariants come from the conservation of angular momentum.The non-existence theorems show that the properties of the textbook electromagnetic plane wave cannot be realized for real(transversely finite)beams.They assert the non-existence of:(i)Pure TEM beam modes,(ii)Beams of fixed linear polarization,(iii)Beams which are everywhere circularly polarized in a fixed plane,and(iv)Beams within which the energy velocity is everywhere in the same direction and magnitude c.In addition to these established facts,we give an elementary topological argument for the universality of rings of zeros of the beam wave function in the focal plane,leading to wave vortices.
John Lekner
MacDiarmid Institute and School of Chemistry and Physics Victoria University,Wellington,New Zealand
国际会议
Progress in Electromagnetics Research Symposium 2013(2013年电磁学研究新进展学术研讨会)
台北
英文
464-469
2013-03-01（万方平台首次上网日期，不代表论文的发表时间）