Eigenmode optical forces arising in symmetrically coupled waveguides have
opposite sign on opposite waveguides and thus can deform the waveguides by
changing their relative separation, but cannot change any other degree of
freedom on their own. It would be extremely desirable to have a way to act on
the center of mass of such a system. In this work we show that it is possible
to do so by injecting a superposition of eigenmodes that are degenerate in
frequency and have opposite parity along the desired direction, resulting in
beating forces that have the same sign on opposite waveguides and therefore act
on the center of mass. We have used both the Maxwell Stress Tensor formalism
and the induced dipole force equation to numerically calculate this transverse
beating force and have found its magnitude to be comparable to the eigenmode
forces. We also show that the longitudinal variation caused by the spatial
beating pattern on the time-averaged quantities used in the calculations must
be taken into account in order to properly employ the divergence theorem and
obtain the correct magnitudes. We then propose a heuristic model that shows
good quantitative agreement with the numerical results and may be used as a
prototyping tool for accurate and fast computation without relying on expensive
numerical computation.Comment: 9 pages, 4 figure
