A spatial filter placed in the inner focal plane of either telescope removes spurious rays created by imperfections in the low-cost diffractive optic. Conventional white-light illumination is used to form an image of the trapped particles. The suspension was sandwiched between a microscope slide and cover glass separated by 40 m. Optical tweezer array created from a single laser beam using a holographic array generator. Trajectories of the particles in the field of view after being released.
Dark traces indicate particles initially trapped in the array.
Shorter tracks indicate particles which diffused out of the nm depth of focus. Since the focal plane is several sphere diameters from the nearest wall, each sphere is trapped stably in three dimensions. Comparable trapping efficiency would be expected for a single conventional optical tweezer operating at equivalent light intensity. Two additional spheres are trapped less strongly at spurious peaks in the diffraction pattern. Studying the dynamic relaxation of artificially structured colloidal crystals is just one application we foresee for manipulating soft matter with holographically patterned light.