Subwavelength focusing of light by a tapered microtube

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 671 nm and a near-field scanning optical microscope probe which was mounted on a three-axis piezonanopositioning stage, a sequence of two-dimensional intensity profiles were obtained. Our experimental results reveal the smallest focal spot with a near diffraction-limited full width at half-maximum of ∼435 nm (0.65λ) at a distance of ∼1.47 μm (2.2λ) from the output end face of microtube. The experimental results are in agreement with our numerical simulation.