Optimum annular focusing by a phase plate

TL;DR

This paper presents a simple optical setup using a phase plate to generate and optimize annular focal fields from various beams, including Gaussian beams, achieving maximum peak intensity and narrow annular focus.

Contribution

It introduces a method to design phase plates for optimal annular focusing of arbitrary beams, including experimental implementation for Gaussian beams.

Findings

Optimal phase transmittance for maximum peak intensity identified.

Experimental demonstration of optimum annular focusing of Gaussian beam.

Enhanced attributes like high peak intensity and narrow annular section achieved.

Abstract

Conventional light focusing, i. e. concentration of an extended optical field within a small area around a point, is a frequently used process in Optics. An important extension to conventional focusing is the generation of the annular focal field of an optical beam. We discuss a simple optical setup that achieves this kind of focusing employing a phase plate as unique optical component. We first establish the class of beams that being transmitted through the phase plate can be focused into an annular field with topological charge of arbitrary integer order q. Then, for each beam in this class we determine the plate transmittance that generates the focal field with the maximum possible peak intensity. In particular, we discuss and implement experimentally the optimum annular focusing of a Gaussian beam. The attributes of optimum annular focal fields, namely the high peak intensity, intensity gradient and narrow annular section, are advantageous for different applications of such structured fields.