Subwavelength focusing of light by a tapered microtube

TL;DR

This paper demonstrates a method for achieving subwavelength light focusing using a tapered glass microtube, with experimental results closely matching numerical simulations, advancing optical focusing techniques.

Contribution

It introduces a novel tapered microtube design for subwavelength focusing at optical frequencies, validated by experiments and simulations.

Findings

Achieved a focal spot of ~435nm at 671nm wavelength.

Focal spot was near diffraction-limited in size.

Focusing occurred at ~1.47μm from microtube endface.

Abstract

We propose a mechanism for subwavelength focusing at optical frequencies based on the use of a tapered microtube fabricated from a glass capillary tube. Using coherent illumination at 671nm and a near-field scanning optical microscope probe which was mounted on a 3-axis piezo nanopositioning stage, a sequence of 2-D intensity profiles were obtained. Our experimental results reveal the smallest focal spot with a near diffraction-limited full width at half-maximum of ~435nm(0.65{\lambda})at a distance of ~1.47{\mu}m (2.2{\lambda}) from the output endface of microtube. The experimental results are in excellent agreement with our numerical simulation.