Constructing microstructures using the optical trapping map of dielectric spheres

Micro-particle self-assembly provides an insight into the dynamics of particles in a well-understood force environment, interactions between particles, and processes where particles themselves modify the force environment. Various ways have been reported on creating microparticle assembly in optical traps. Yet the basis to understanding the nature of the assembly is to first comprehend trapping of a single sphere in a focused Gaussian laser beam. For spherical dielectric particles that are to be manipulated by a focused Gaussian laser beam, the axial trapping efficiency of this is a function of (i) the particle radius r, (ii) the ratio of the refractive index of particle over the medium, and (iii) the numerical aperture of the delivered light beam. From a comprehensive simulation conducted, we uncovered optical trapping regions in the 3D parameter space forming an iso-surface landscape with ridge-like contours. Using specific points in the parameter space, we drew attention to difficulties in using the trapping efficiency and stiffness metrics in defining how well particles are drawn into and held in the trap. An alternative calculation based on the maximum forward and restoration values of the trapping efficiency in the axial sense, called the trapping quality, was proposed. We also discuss the possibility of coupling optical trapping with other physical methods, notably capillary forces, in order to achieve effective microparticle assembly.