Holographic 3D intensity shaping of evanescent waves

This paper investigates the use of holographic algorithms to create more complex evanescent-wave fields. The authors have written a direct-search/simulated-annealing algorithm for shaping the 3D intensity pattern of evanescent wave fields. This direct-search algorithm includes a forward model that calculates the 3D intensity pattern due to a given number of evanescent plane-wave components. By making random changes to the phase and amplitude of each of these components the algorithm aims to create an approximation to a desired 3D intensity pattern. This algorithm has been demonstrated theoretically and was used to investigate limits to optical manipulation using evanescent waves. The algorithm is capable of creating configurations interesting for optical trapping. In particular, it is possible to create an evanescent light field that does not simply fall of exponentially in the direction of propagation, but that passes through a maximum. For optical trapping this means that dielectric objects can be trapped away from the evanescent-wave-creating surface which may help reduce particle sticking and allow easier manipulation.