Development of a dual-focus metalens using a hybrid phase profile for high-resolution photonic force microscopy

Abstract

Optically trapped nanorod with high aspect ratios has received growing attention as potential probe tips in photonic force microscopy (PFM). A high resolution using a small footprint of a nanorod is expected, but accurate control of the motion is required. Even though a holographic optical tweezer can control a single nanorod by capturing both ends, it requires complex alignments and bulky equipment, including diffractive optical elements such as a spatial light modulator and its driver. These might hinder the practical use and more applications. In this dissertation, we developed a dual-focus metalens with a hybrid phase profile for controlling the trapped nanorod. A hyperbolic and a quadratic phase profile, which are widely used in metalens researches, are difficult to be used for optical tweezers that require an appropriate scanning range and high numerical aperture (NA) at the same time. A hyperbolic phase profile can achieve high NA but has a very narrow scanning range. On the other side, a quadratic phase profile can achieve a wide scanning range, but it is hard to have a high numerical aperture. To overcome this problem, we designed a hybrid phase profile consisting of a hyperbolic phase profile inside and a quadratic phase profile outside. The hyperbolic phase inside the boundary and the large effective aperture, which results from the quadratic form outside the boundary, lead to high NA and tolerance at oblique incidence. We investigated the properties of a hyperbolic, a quadratic, and a hybrid phase profile using the ray traces and the simulated focal spots. Furthermore, we fabricated a single-focus metalens and a dual-focus metalens using a hybrid phase profile, and then experimentally demonstrated that a metalens with a hybrid phase profile achieves high NA while minimizing other aberrations. These results imply that the successful control of a nanorod would be possible, which opens unprecedented opportunities in the various research fields, especially in a high-resolution PFM.