A laser that changes colour when it stretches has been developed by a team of scientists from Germany and the US. Heino Finkelmann of Albert-Ludwigs University in Freiburg, Germany, and co-workers based the device on a liquid crystal with a spiral structure that behaves like a spring. It emits green light in its unstressed state, but this shifts to red when the spring is stretched (H Finkelmann et al 2001 Advanced Materials at press). The tunable laser could be invaluable in the communications industry to send light of different wavelengths through fibre optic cables.
All of the molecules in one plane of a so-called cholesteric liquid crystal are aligned, and the molecules in each successive plane are rotated slightly with respect to those in the previous plane. This creates a series of parallel helices within the liquid crystal. Over a certain length of the helix, the molecules gradually turn through a full circle – this distance is known as the pitch.
When light shines into the liquid crystal along the axis of the helixes, the light in a short frequency range – with a central wavelength exactly equal to the pitch length – is strongly reflected. This effect can be exploited to make a laser by adding a fluorescent dye to the liquid crystal. The dye is chosen such that the peak in its emission coincides with the wavelength reflected by the liquid crystal. When a laser stimulates this structure, the emission profile is modified and the dye emits laser light at the wavelengths that correspond to the edges of the ‘reflection band’.
Finkelmann’s team realised that this effect could be controlled by adding the dye to a type of highly uniform elastic liquid crystal known as a cholesteric liquid single crystal elastomer, and exciting the system with a laser. When elongated, they found that the laser light shifted from green to red, with a wavelength spread of just 0.3 nanometres. Most lasers have mirrors that form an optical cavity in which the light is amplified, but in the elastic laser the liquid crystal itself behaves as a ‘distributed’ cavity in which the laser light is continually internally reflected.
