When S Krishna Prasad and co-workers at the Center for Light Crystal Research in Bangalore shone a light on a class of liquid crystals, it induced the crystal molecules to change shape on a quick time-scale. This finding suggests that the system is a potential candidate for storage of images or other optical data.
Phase transitions of shape, induced by temperature, are common in nature; for example ice is transformed to water when it is heated. In recent years, however, it has been found that light can mimic the role played by temperature in bringing about a phase transition. This occurs in certain materials with properties that are in between that of a solid and liquid, namely liquid crystals.
Prasad and his colleagues discovered that the key to such phase transitions in these materials (like azobenzene) is the change in shape driven by light. In its ground state, the azobenzene molecule, for instance, exists in an arrangement known as trans conformation. But when it is irradiated with light of suitable wavelength (365 nanometre), it undergoes a conformational change to the cis state.
Along with this conformational transformation, the molecule changes shape — in the trans state the molecule has a rod-like shape, but it the cis configuration it is bent like a boomerang. The reverse transformation can be brought about by illuminating the molecule with visible light, although it can also occur spontaneously in the dark by a process known as thermal back relaxation.
What makes this shape change useful is that the rod-like shape supports liquid crystallinity, whereas the bent shape destabilises the liquid-crystal phase. “The change from the trans to the cis form can induce an isothermal transition from the liquid-crystal phase to a liquid phase,” says Prasad, adding: “The concomitant change in the related optical properties can be exploited to create a high-resolution, light-driven image storing system.”
Furthermore, the changes can be tailor-made to have extremely high long-term stability; holograms (photographic patterns having three dimensions) stored in this way hardly degrade.
Prasad and colleagues summarize various aspects of their investigations by stating that photo-induced phase transition can take place on an extremely quick time-scale of less than a microsecond — a millionth of a second. “Our observations are very significant from the point of view of storing optical images. Moreover, the time-scale of transition can be further improved by embedding the material in a polymer matrix,” concludes Prasad.