A biophysicist marvels at the idea of grabbing microscopic particles with light by tweaking its phase.

Light carries energy and momentum. Have you ever gazed at a comet on a hot summer night? The dust tail seen streaming out from a comet is caused by sunlight bombarding dust particles from its surface and pushing them away from the Sun. The same radiation pressure can be used to 'trap', or hold, microscopic particles. And if an item of interest — for example, a biological molecule — is attached to a particle subject to trapping, it can then be manipulated as the trap is moved.

So how does one generate optical traps? Conventionally, a laser beam is directed through the objective lens of a microscope and focused to a small spot very close to the specimen. The trapping force relies on the gradient of the laser's intensity — the tighter the focus, the greater the intensity change within the focused beam, and the greater the trapping force.

For a long time, this has been the only type of trap available. But not any more! David Grier and his colleagues have created a new type of trap that relies on the gradient of the 'phase' of a laser's light as well as its intensity (Y. Roichman et al. Phys. Rev. Lett. 100, 013602; 2008). Light waves, like ocean waves, have crests and troughs. The phase of a light wave specifies what position within the wave, from crest to trough, the light is in at a given moment. By tweaking the phase of the laser in the trap, the researchers are able not only to hold a particle steady, but also to move it in a line or spin it around in a circle. It is now possible to design optical traps that are more flexible and versatile, and that can generate as much trapping force as before, but with less light.

I would not be surprised if these traps soon become one of the must-have tools in single-molecule biophysics, cell biology and colloidal physics.

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