  Optical forces acting on nano-sized particles are typically too small to be
useful for particle manipulation. We theoretically and numerically demonstrate
a mechanism that can significantly enhance the optical force acting on a small
particle through a special type of resonant particle-substrate coupling. The
resonance arises from the singular behavior of the particle's effective
polarizablity in the presence of a metal-dielectric-metal multilayer substrate.
We show that this phenomenon is closely related to the existence of a flat-band
plasmon mode supported by the multilayer substrate.
