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Laser writing of parabolic micromirrors with a high numerical aperture for optical trapping and rotation

In this paper, we present a fast and customisable technique for creating trapping arrays with high numerical aperture. Using a continuous wave CO2 laser, we write micromirror structures through ablation. First, we expose the glass substrate for ∼100 ms to a focused laser, resulting in a nearly parabolic mirror profile. The glass is then coated with a thin gold layer to achieve a smooth reflective surface. We show that through tuning the ablation parameters, the diameter and depth of the micromirrors can be controlled accurately. To demonstrate the platform’s viability, we use a high NA micromirror to trap 5 μm vaterite and 2 μm silica particles suspended in D2O. We characterise the optical performance of the mirror, then proceed to use it to reverse the direction of rotation of a vaterite particle. Through this, we demonstrate the potential of micromirrors as a simple-to-fabricate and versatile lab-on-chip optical platform.