Optical gradient forces between evanescently coupled waveguides

Mohammad-Ali Miri, Michele Cotrufo, and Andrea Alu

Evanescently coupled dielectric waveguides exert optical forces on each other, which may be attractive or repulsive as a function of the excited optical mode. Through energy conservation considerations, it is possible to show that the optical force between two waveguides is proportional to the derivative of the effective propagation index with respect to the separation between waveguides. Here, we prove analytically that the lateral force calculated from the spatial derivative of the propagation index is equivalent to the one obtained from a formal calculation based on the Maxwell’s stress tensor. Interestingly, this latter approach reveals that the sign and magnitude of the force depend only on the field intensity at the channel interfaces. In addition, our derivation provides insights into the design of the waveguide profile in order to increase or decrease the optical forces between coupled channels.

DOI