Convergent measure of focal extent, and largest peak intensity for non-paraxial beams

TL;DR

This paper introduces a new measure called the focal concentration area for non-paraxial beams, establishes a rigorous upper bound on peak intensity using a delta distribution limit, and discusses implications for electromagnetic beams.

Contribution

The paper proposes the focal concentration area as a robust measure of beam focal extent and derives a fundamental upper bound on peak intensity for non-paraxial beams.

Findings

Focal concentration area provides a finite measure for beam focal extent.

A rigorous upper bound on peak intensity is established using the Dirac delta limit.

The same lower bound applies to electromagnetic beams as to scalar beams.

Abstract

Second moment beam widths are commonly used in paraxial optics to define the focal extent of beams. However, second moments of arbitrary beams are not guaranteed to be finite. I propose the focal concentration area as a measure of beam focal area, defined to be the ratio of total radial intensity to radial intensity regulated by a unit area Gaussian distribution. I use the Dirac delta limit of this distribution to establish a rigorous upper bound on the peak intensity of non-paraxial beams of a given total intensity, and show that this is achieved by the recently proposed `proto- beam' solution. I discuss the generalisation to electromagnetic beams, and find the same lower bound as the scalar case. This bound cannot be achieved for any physical beam, and as such the physical lower bound must be higher.