An off-focus optical tweezers : A new tool for capturing and manipulating a group of particles

Abstract

A typical optical tweezers(1) system is operated by precisely focusing a laser beam through an objective into a laser point on the sample plane of a microscope. This results in the trap of a single particle into the laser point via gradient dipole force(2). In the past decade, such in-focus optical tweezers have been widely used in capturing, moving, and manipulating a single bio-cell or a single bead(3). However, it will be useful sometimes to capture a group of bio-cells at a time. For this purpose, we propose a novel technique in this paper by shifting the focusing laser point slightly above or below the sample plane, which leaves a relatively large laser spot on the sample plane. We term this type of setup an off-focus optical tweezers. In this paper, we will demo the capture of a group of polystyrene beads by our off-focus optical tweezers system. In addition, by using a water-drag-force method, we show that the off-focus optical tweezers exhibit a larger transverse capturing range but a weaker trapping force within than the traditional in-focus optical tweezers do.