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Multidimensional mapping of the restoring force of an optical trap using triangular wave flow

A method for mapping the spatial dependence of the restoring force of an optical trap in multiple dimensions using triangular wave flow is considered. A theoretical description of the transient motion of an optically trapped massless microsphere in triangular wave flow is introduced and verified experimentally. Guided by theory, the method is first demonstrated using one-dimensional (1D) triangular wave flow to measure the transverse and axial dependence of the restoring force of an optical trap for polystyrene microspheres with diameters from 360 to 3000 nm and is compared to more conventional measurements. While the experimental results are self-consistent, the expected size dependence predicted by theory Nieminensps2007 is not observed for the optical traps examined here. Two-dimensional (2D) triangular-wave flow is then used to the measure the spatial dependence of the restoring force in the transverse–axial plane. The resulting map shows that a spatial asymmetry, not predicted by theory, is present in the optical force field.