Holographic Optical Tweezers

An optical tweezer uses the forces exerted by a strongly focused beam of light to trap, manipulate, and transform a small volume of matter. Originally introduced by Bell Labs researchers in 1986, optical tweezers have become indispensible tools for research in physics, chemistry and biology.

Holographic optical trapping, developed in Prof. David Grier's lab, extends this technique by using computer-generated holograms to dynamically structure the light field before

it is brought to focus. In this way, a single laser beam can power hundreds of optical traps that move independently in three dimensions. Holographic control thus lends itself to lends itself to analyzing and sorting microscopic fluid-borne objects and for assembling them into complex three-dimensional structures such as the icosahedral quasicrystal shown in the figure.

Each holographically projected trap, furthermore, can have an independently specified optical mode structure. Wavefront engineering is useful for extending point-like optical tweezers into holographic line traps, ring traps and optical solenoid beams. The force profiles along these extended traps can be precisely specified through control of their intensity and phase profiles. Extended holographic traps therefore constitute model low-dimensional systems for studying colloidal interactions and nonequilibrium statistical physics. They also have immediate practical applications in medical diagnostics.


Figure 1: Holographic Optical Trapping. Light from a laser is imprinted with computer-generated holograms using a computer-addressed spatial light modulator. The engineered light is relayed to the objective lens of a microscope, which focuses the hologram into a fluid-borne sample. The same objective lens is used to create images of trapped objects.


Figure 2: Holographic trapping of micrometer-diameter colloidal spheres in water. (a) through (e) twelve spheres arranged into a rotating icosahedron. (f) 173 spheres organized into a three-dimensional icosahedral quasicrystal, colored by depth. (g) The quasicrystal's optical diffraction pattern at 632.8 nm, displaying ten-fold symmetry.

Representative References:
1. E. R. Dufresne and D. G. Grier, "Optical tweezer arrays and optical substrates created with diffractive optical elements," Review of Scientific Instruments 69, 1974-1977 (1998). 
2. D. G. Grier, "A revolution in optical manipulation," Nature 424, 810-816 (2003). 
3. Y. Roichman, B. Sun, Y. Roichman, J. Amato-Grill and D. G. Grier, "Optical forces arising from phase gradients," Physical Review Letters 100, 013602 (2008).