Self assembly of microparticles in stable ring structures in an optical trap

TL;DR

This study demonstrates the spontaneous formation of stable microsphere rings in a conventional optical tweezers setup, revealing the influence of experimental parameters and stratified media on ring stability and structure.

Contribution

It introduces the observation of ring formation in standard Gaussian beam optical tweezers with standing wave geometry, supported by theoretical simulations.

Findings

Ring structures depend on stratified media thickness.

Standing wave geometry influences ring stability.

Geometrical aberrations cause ring formation.

Abstract

Micro-particle self assembly under the influence of optical forces produced by higher order optical beams or by projection of a hologram into the trapping volume is well known. In this paper, we report the spontaneous formation of a ring of identical microspheres (each with diameter 1.1 $\mu$m) in conventional single beam optical tweezers with a usual TEM$_{00}$ Gaussian beam coupled into a sample chamber having standing wave geometry with a cover slip and glass slide. The effects of different experimental parameters on the ring formation are studied extensively. The experimental observations are backed by theoretical simulations based on a plane wave decomposition of the forward and backward propagating Gaussian beams. The ring patterns are shown to be caused due to geomterical aberrations produced by focusing the Gaussian beam using a high numerical aperture microscope objective into stratified media. It is found that the thickness of the stratified media and the standing wave geometry itself play a critical role in formation of stable ring structures. These structures could be used in the study of optical binding, as well as biological interactions between cells in an optical trap.