  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.
