Light carries momentum that can push on an object, but it can also move an object through thermal forces. A new experiment combines both of these effects to manipulate a tiny gold plate that can slide along a tapered optical fiber. The momentum-transfer and thermal forces act in opposite directions, allowing the plate to be driven first one way and then the other. The device might one day be used to generate mechanical energy from light or as a micro-transport system for a miniature chemical processing device called a lab-on-a-chip.
When a light beam reflects or scatters off an object, the object will recoil. This so-called optical force is used, for example, to trap glass beads in optical tweezers. Light can also exert force through the photophoretic effect, where preferential absorption of light on one side of an object leads to a temperature difference that causes the object to move. The air molecules on the hot side heat up, and their collisions with the object deliver more momentum than those on the colder side, producing motion, as in the rotation of a radiometer (light mill).
In most situations, both the optical and photophoretic forces will act on an object exposed to light, but usually one will dominate, says Min Qiu of Zhejiang University in Hangzhou, China. For example, in water, the optical force dominates because the liquid prevents any significant heat buildup in one location. Researchers have devised experiments in which an object is affected by both forces, but typically the force directions are the same or the motion is limited (see 9 July 2012 Viewpoint). Qiu and his colleagues’ new setup has the two forces driving an object back and forth over relatively long distances.
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