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Holographic trapping patterns:

Placing a helical phase plate in a Fourier plane with respect to a phase objects gives rise to isotropic edge enhancements (see Spiral Phase Contrast Link). We have used this technique to construct closed line traps of arbitrary shape which trap high-field seeking micro-particles.

Additionally these planar trapping patterns exert transverse scattering forces related to the orbital angular momentum of the light, which can be exploited to induce motion along the line with a velocity which depends on the local curvature of the line.

Fig. 1 Trapping pattern formed with a spiral phase plate.

Movie (475KB)

The above non-holographic (Fourier filtering) method has some severe limitations: for example only closed lines are possible, and the phase gradients that can be reached are modest (basically they are limited by a total phase of 2π around the loop, which is a consequence of using a filter with helical charge ± 1).

Using holographic methods we have overcome these drawbacks: Two cascaded phase-diffractive elements can shape both, amplitude and phase of the field. A possible implementation is to use the two halves of a spatial light modulator (SLM) panel and a folded setup.

Fig. 2 Three-dimensional trapping pattern for silica beads formed by shaping both, amplitude and phase with a SLM. Particles are trapped along the high intensity lines and can be driven in or out by tailoring the phase gradients.

Movie (438KB)

Since this approach reconstructs an actual wavefront, transverse intensity gradients and axial scatering forces can be chosen independently, as seen in the movie above.

Publications:

A. Jesacher, C. Maurer, S. Fürhapter, A. Schwaighofer, S. Bernet, and M. Ritsch-Marte: Optical tweezers of programmable shape with transverse scattering forces, Opt. Commun. 281, 2207-2212 (2008).

A. Jesacher, C. Maurer, S. Fürhapter, A. Schwaighofer, S. Bernet, and M. Ritsch-Marte: Full phase and amplitude control of holographic optical tweezers with high efficiency, Opt. Express 16, 4479-4486 (2008).