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RUBBER-LIKE lasers may soon be producing light shows worthy of the best night
clubs. Simply stretching the laser changes the colour of the light it produces
from red to green or any colour in between.

“This is the first example of an easy, cheap, adaptable device which can be
used to tune a laser quickly,” says Robert Eason from the Optoelectronics
Research Centre at the University of Southampton.

The stretchy laser could dramatically increase the capacity of fibre-optic
links, because the more wavelengths or colours you can send down a fibre, the
more information it can carry. It is possible to slightly change the wavelength
of some lasers, but usually each colour requires a different laser. A single
laser which could quickly switch to various wavelengths would be much simpler,
quicker and cheaper.

A laser has two key features. First, it needs a reflective cavity that
bounces light back and forth between opposite walls. One of the walls has to be
slightly “leaky”, to let some light out. Secondly, you need a way to boost the
atoms in the cavity into an excited energy state. Then photons already in the
cavity can trigger the atoms to release a shower of photons of the same
wavelength, which emerge from the leaky wall as a laser beam.

In the stretchy laser, developed by a team from the University of Freiburg in
Germany and Kent State University in Ohio, the ends of the cavity are formed by
layers of molecules in a liquid crystal material. In each layer, the molecules
are at a slightly different angle to those in neighbouring layers. “The
molecules are aligned like stairs in a spiral staircase,” says Peter
Palffy-Muhoray of Kent State.

The staircases act as reflective cavities, in which light bounces between
“stairs” with the same orientation directly above or below on the spiral. By
shining a light into the material, the researchers boosted atoms in these
cavities to an excited state, which caused them to emit green laser light with a
wavelength of 544 nanometres.

As well as being a liquid crystal, the material the researchers used is an
elastomer: it can be stretched or squeezed and then returned to its original
shape. When they stretched the material, they found that the colour of the laser
beam changed dramatically, from green to red.

The researchers realised that when they stretched the liquid crystal they
were increasing the “pitch” of the staircases—how far you go to make a
full rotation. In effect, this made the reflective cavities longer, which
increased the wavelength of the light generated by the laser to 630
nanometres.

The researchers will describe their work in a forthcoming edition of the
journal Advanced Materials, and are now working to make the material
easier to use and easier to manufacture into devices for the communications
industry—for sending light of different wavelengths through
fibre-optic cables, for example. “Tunable laser sources are always in demand for
research, for medical applications, for remote sensing,” says Palffy-Muhoray.
“There are many possibilities.”