A tiny shutter that opens and closes two thousand times a second has been developed by a group of engineers in Japan. The ultra-fast blinking device might usher in new display technologies such as large-area liquid-crystal display screens.

Most liquid-crystal displays have a shutter that opens or closes over each pixel, letting light through or blocking it off. The development of televisual display screens depends on having shutters that work very fast - these switching speeds have been one of the limiting factors in making flat-screen liquid-crystal TV sets.

The shutters aren't mechanical devices, like those in a camera. Instead, each pixel is a cell containing a liquid crystal between two plates. The cell is switched from transparent to opaque by applying an electric field across the plates. Because of the peculiar nature of liquid crystals, this can totally alter the transparency of the filling.

Most liquid crystals consist of rod-like molecules that line up with one another, a little like logs carried along a river. They act like polarizing filters on a camera lens, letting light pass through only if the plane of polarization is aligned with the direction of the molecules. It is rather like trying to post a letter - you have to get the envelope aligned with the slot.

Because the direction in which liquid crystal molecules point can be influenced by an electric field, the transparency of a film of the stuff to polarized light can be switched electrically. Apply a voltage, so that the molecules all swing round perpendicular to the plane of the polarized light, and you have effectively rotated the polarizing filter, making the cell go dark.

But this reorientation can be slow - typically several milliseconds. In the 14 June issue of Applied Physics Letters, Saulius Juodkazis of the University of Tokushima, Japan and colleagues report a new way to make fast liquid-crystal switches. And it is all done with light, not with electronics. This means that one light beam could be used to control the transmission of another, and so it opens up another application for liquid-crystal shutters. All-optical switching is one of the requirements of a light-based communications technology that uses the output of fibre-optic transmission systems 'raw' rather than converting it back into electronic pulses for signal processing. All-optical communication could be very fast - provided it has very fast switches.

The new all-optical switch does away with clunky plates and electrodes, and uses instead a spinning droplet of liquid crystal. The droplet is smaller than those in a fine mist - just two or three thousandths of a millimetre across. The trick is to suspend the droplet in free space, so that it can swivel quickly in response to a light signal.

To do this, the researchers used 'optical tweezers': laser beams that trap the object in an intense light field. This instrument is widely used in biology for manipulating cells, and works because the light beam exerts a kind of radiation pressure. Juodkazis and colleagues realized that, by using polarized light as the trapping beam, they could not only hold onto their droplet but also control its orientation. The liquid crystals are sensitive to the direction of polarization in the trapping beam - if this is altered, the droplet swivels in free space in response.

This means that the polarized laser beam can be used to control the transparency of the droplet to a second, weaker polarized light beam. If the droplet's molecules are parallel to this beam, it passes through; if the whole droplet is turned, the light is blocked. It is rather like rotating the entire cell in a conventional liquid-crystal display - except that in this case the 'cell' is just a tiny droplet hanging in space, so that it takes less than a millisecond to rotate. And then, within the mist, light beams start talking.