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Optics

A light touch

Being good with your hands' is not quite the badge of approval it once was: the tiny scale of the components used in most high-tech industries has made old-fashioned manual dexterity largely redundant. Yet in many sensitive applications, the unique directness of human touch is highly desirable. Now Graeme Whyte et al. introduce an apparatus that could go some way to putting the 'hand' back in manipulation, even on very small scales (Opt. Express 14, 12497–12502; 2006).

The authors' apparatus marries the skill of human digits with the laser-sharp precision of 'optical tweezers'. Such tweezers use the momentum of a highly focused laser beam to trap and move objects as small as a single atom. The technique is increasingly useful, especially in cell biology, where it can be applied to measuring the mechanical, optical and transport properties of cells and biological molecules without the need for a material probe.

Whyte et al. use 'holographic' optical tweezers, in which a diffractive element, known as a spatial light modulator, is used to steer the laser beam. This allows several optical traps to be created that can be moved independently in three dimensions. The authors hook up the spatial modulator to a camera that images the position of beads attached to the fingertips of two gloved human hands. A specially written hologram-calculation algorithm converts the positions of the beads in the photographic plane into the xy position of the optical traps, and the apparent size of the bead in the image into a z-coordinate.

Credit: OPT. EXPRESS

The result is a device in which each trapping beam acts as a digit of an optically controlled hand. The position of this hand on the micrometre scale is controlled by appropriate larger-scale movements of the remote, real hands. The images show how silica beads trapped at the tips of the optical fingers grasp and move a larger, irregularly shaped chrome bead. The hologram algorithm allows 8 frames per second — enough for manipulation in real time — to be processed using a standard desktop computer.

Because the manipulated object does not itself have to be optically trapped, this method could be very useful for dealing with light-scattering metallic particles or light-sensitive materials, such as some biological tissues. But more generally, such techniques could mean that the future of micromanipulation lies in our hands.

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Webb, R. A light touch. Nature 444, 1017 (2006). https://doi.org/10.1038/4441017a

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