  We demonstrate the measurement of laterally induced optical forces using an
Atomic Force Microscope (AFM). The lateral electric field distribution between
a gold coated AFM probe and a nano-aperture in a gold film is mapped by
measuring the lateral optical force between the apex of the AFM probe and the
nano-aperture. Torsional eigenmodes of an AFM cantilever probe were used to
detect the laterally induced optical forces. We engineered the cantilever shape
using a focused ion beam to enhance the torsional eigenmode resonance. The
measured lateral optical force agrees well with simulations. This technique can
be extended to simultaneously detect both lateral and longitudinal optical
forces at the nanoscale by using an AFM cantilever as a multichannel detector.
This will enable simultaneous Photon Induced Force Microscopy (PIFM) detection
of molecular responses with different incident field polarizations. The
technique can be implemented on both cantilever and tuning fork based AFMs.
