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A WEIRD crystal whose surface changes shape when light shines on it has been
discovered by Japanese researchers. The change can be reversed by shining light
of a different colour on it—leading experts to believe it could be used to
redirect light in optical communications networks.

Masahiro Irie’s team at Kyushu University in Fukuoka found that light can
change the shape of the diarylethene crystals’ surface. Illuminating one face of
a smooth-surfaced, colourless crystal with ultraviolet light at a wavelength of
366 nanometres created a series of 5 to 10 steps. Each step was about 1
nanometre high, the size of a single molecule. The steps remained stable until
they were illuminated with visible light at a wavelength longer than 500
nanometres, which restored the smooth surface.

The crystal was also photochromic: under UV light it turned blue, while the
visible light made it colourless again.

The researchers were able to switch the crystals between the two coloured
states more than 10,000 times. “The reversibility is unique,” says John Scheffer
of the University of British Columbia.

Molecules are photochromic when light switches them between alternative forms
that absorb light differently. Photochromic sunglasses exploit the effect by
using molecules that darken under bright sunlight, then become transparent when
the light dims.

Light can induce chemical reactions in other crystals, but changing the shape
of the molecules’ normally makes the crystal disintegrate. But this doesn’t
happen with Irie’s crystal. “This one works well because the molecular size
changes very little,” says Scheffer.

Optics specialists says that changeable surface shapes might also be used to
switch or redirect optical signals without the need for electronics or for
microelectromechanical systems with moving parts.