Fig. 1. Experimental setup for diffractive steering of optical tweezers. A high resolution (1920 × 1200 pixels) reflective spatial light modulator (SLM) is illuminated by an expanded collimated laser beam. At the SLM, a number of image windows displaying computer-designed off-axis holograms is presented. Only laser light diffracted from these holograms into the desired first order is guided by a lens to the rear input aperture of a microscope objective. There it is used to trap particles in different kinds of advanced optical traps.

Fig. 2. Examples for holograms displayed at the SLM for producing a “cogwheel” beam, a “doughnut” beam, and four single optical traps in different focal planes. The holograms are directly displayed as pictures on the computer screen, and can there be moved by mouse-dragging. The positions of the corresponding optical traps in the object plane of the microscope follow these mouse movements instantaneously.

Fig. 3. Left: CCD image of the light intensity distribution in the object plane of the microscope, generated by the six holograms displayed in Fig. 2. The lower left “cogwheel” shaped intensity distribution results from a superposition of two counter-propagating doughnut modes with a helicity of 5, the ring-shaped intensity distribution (lower, middle) corresponds to an optical doughnut beam generated by the upper middle hologram in Fig. 2, whereas the other holograms each reconstruct a single optical focus at a different position. Right: Six micro beads (diameters indicated in the figure) trapped simultaneously in the 6 light traps generated by the 6 holograms. The upper middle bead is trapped in another focal plane than the other beads to demonstrate the feasibility of 3-dimensional steering of the holographic tweezers. A mpeg-movie which demonstrates movement of two beads in different focal planes by mouse-dragging the corresponding hologram windows at the computer monitor in attached in “beads.mpg” (2.2 MBytes).