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Optical Vortices and Optical Wrenches

The optical tweezers described to this point share a severe limitation: they can only trap particles with low absorption coefficients whose index of refraction is moderately greater than that of the surrounding medium. Reflective particles, for example, are driven away from the focal spot by radiation pressure. Sasaki and coworkers [48, 49] ingeniously circumvented this difficulty by scanning a single tweezer with mirrors rapidly enough to form an `optical cage'.

More recently, several groups have used Laguerre-Gaussian laser modes, also known as `optical vortices', to make single-beam `dark' optical tweezers [50, 51, 52] capable of trapping otherwise untrappable particles. An optical vortex has a phase singularity on its axis along which destructive interference causes the beam's intensity to vanish. Intensity maxima spiral around the dark central core at a radius determined by the topological charge tex2html_wrap_inline744 of the phase singularity. When focused, the optical vortex forms a funnel of light down which particles are pushed by radiation pressure and at whose focus they become trapped. So far, optical vortices have been used to trap absorbing and reflecting particles [53], and low-index particles in a high-index fluid [54].

In addition to trapping particles not otherwise amenable to optical manipulation, optical vortex tweezers also can apply controlled torques to trapped particles. Each photon in an optical vortex carries an angular momentum tex2html_wrap_inline746 associated with the beam's topological charge. Photons in circularly polarized beams carry an additional tex2html_wrap_inline748 depending on the direction of polarization. This angular momentum is transferred to an absorbing particle and carried away by viscous coupling to the surrounding fluid. The trapped particle experiences a constant torque in steady state. The net torque applied by conventional tweezers is vanishingly small since strongly absorbing particles cannot be trapped. Particles trapped in optical vortex tweezers are observed to spin [55] at rates determined both by the polarization state and topological charge [56]. This so-called optical `wrench' has great promise for studying rotational dynamics in colloidal suspensions, particularly when combined with the methods of Section 3.


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Next: Optical `Atomizers' Up: Optical Tweezers in Colloid Previous: Optical Ratchets

David G. Grier
Mon Feb 17 21:35:47 CST 1997