Instabilities in transverse optical binding of dielectric particles

In counter propagating plane waves, spherical beads occupy the interference fringes and optically bind with separations of around a wavelength. The binding strength depends on particle size and refractive index with the particle arrangement also dependent on the polarisation. Some combinations of refractive index and size do not optically bind: instead, the particles experience a force in the propagation direction that leads to a transition to longitudinal binding. These instabilities are assessed computationally with different particle numbers and polarisation states.