Optical Forces between Coupled PT-symmetric Waveguides

Evanescent wave coupling between two dielectric waveguides results in an optical force between the waveguides. This force is attractive (repulsive) when the coupled waveguide system is excited with the symmetric (anti-symmetric) mode and is transverse to the axis of wave propagation. Through energy conservation considerations, it is shown that the optical bonding force is proportional to the gradient of the propagation index of the symmetric and anti-symmetric supermodes with respect to the distance between the two guides. However, when gain or loss is involved in the waveguides, this derivative becomes undefined at an exceptional point singularity. In order to investigate forces at the exceptional point singularities, here we consider a PT-symmetric model of two coupled waveguides and calculate the force through Maxwell’s stress tensor. Our results show that the optical bonding force is finite at the exceptional point. In addition, we find an extra force component which is directed along the axis of propagation.