The light of a conventional laser
excites tiny cavities within the film
to produce red "random laser"
light; © Polson/University of Utah
"Nobody knew how it worked," says Valy Vardeny, senior author of the new study. "We succeeded in imaging the cavities. This is a big step in our understanding of this phenomenon."
Lasers "are carefully designed and constructed to produce laser emission," says study co-author Randy Polson. "Surprisingly, there is a class of lasers where laser emission happens from materials that are literally shaken together.” These are called random lasers since the emission happens from an uncontrolled configuration – a disordered rather than crystal structure within the "lasing medium" – the material used to generate the laser.
In the study, the researchers created images to reveal the natural cavities within a "pi-conjugated polymer film," which is a thin film of an organic polymer that conducts electricity even though it is a plastic-like material. The microscopic cavities – natural irregularities within the plastic – act together like the mirrors in regular resonators that help amplify the light in a conventional laser.
To make pictures of laser-generating cavities within the polymer, the scientists focused a green, conventional laser pulse on a thin film of the polymer, stimulating red random laser emission from the film. By focusing a lens in a certain way, they made 10,000 images of the red laser light spectrum emitted by microscopic squares within the film, then assembled them into an image of the entire piece of polymer film.
They revealed the mirror-like cavities by showing adjacent pixels with the same red light spectrum and the same above-average index of refraction, which is a measure of how fast light travels within a material. When they mapped areas of above-average refraction index within the polymer, those areas were connected to form loops, which act as mirror-like cavities to generate laser light, Vardeny says.
In 2004, the researchers infused human colon tissue – normal and tumour – with a red fluorescent dye that generates random laser light when stimulated. They could distinguish tumour from normal colon tissue because the tumour emitted more individual lines of random laser light, possibly because its structure was more disordered than normal tissue.
COMPAMED.de; Source: University of Utah