Optical traps use forces exerted by specially structured beams of light to localize microscopic objects in three
dimensions. In the case of single-beam optical traps, such as optical tweezers, trapping is due entirely to gradients
in the light's intensity. Gradients in the light field's phase also control optical forces, however, and their quite
general influence on trapped particles' dynamics has only recently been explored in detail. We demonstrate
both theoretically and experimentally how phase gradients give rise to forces in optical traps and explore the
sometimes surprising influence of phase-gradient forces on trapped objects' motions.
We describe a class of diffractive optical elements that intended for use in holographic optical trapping systems to project ring-like optical traps. The ring traps' hologram is design by shape-phase algorithm, that incodes amplitude information in Fourier space into shape. The flexibility of the shape-phase algorithm leeds to several advantages over commonly used optical vorteces. While, optical vorteces carry orbital angular momentum which determines the vortex diameter, ring trap dimensions do not depend on the angular momentum. In particular, we formed a ring trap without angular momentum, or tangetial-force. Also unlike most optical vortices or ring-like Bessel beams, these ring traps have strong enough axial intensity gradients to trap objects in three dimensions. We investigated the three dimensional vortex-like trapping experimentaly and by volumetric imaging.
Holographic optical trapping uses forces exerted by computer-generated holograms to organize microscopic materials
into three-dimensional structures. Achieving and verifying accurate three-dimensional placement requires
methods for assessing the accuracy of the projected traps' geometry, as well as methods for measuring trapped
objects' three-dimensional positions. Volumetric imaging of the projected trapping pattern solves one problem.
Holographic video microscopy addresses the other. The combination is exceptionally effective for organizing,
inspecting and analyzing soft-matter systems.
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