How 'random' lasers work

01-27-2010: When University of Utah scientists discovered a new kind of laser that was generated by an electrically conducting plastic or polymer, no one could explain how it worked and some doubted it was real. Now, a decade later, the Utah researchers have found these "random lasers" occur because of natural, mirror-like cavities in the polymers, and they say such lasers may prove useful for diagnosing cancer.

"Nobody knew how it worked until now," says Z. Valy Vardeny, a distinguished professor of physics and senior author of the new study, published in Nature Physics . "We succeeded in imaging the cavities. This is a big step in our understanding of this bizarre phenomenon, which not many people believed."

Materials or "gain media" that generate conventional lasers are put in an ordered structure known as "lossless resonators" – often mirrors – that generate light without losing much of it. Random laser materials, in contrast, are disordered and lose some light. In the new study, Vardeny and colleagues created images to reveal the natural cavities within a "pi-conjugated polymer film," which is a thin film of an organic polymer – named DOO-PPV – 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.

Lasers "are carefully designed and constructed to produce laser emission," says study coauthor Randy Polson, a senior optical engineer at the University of Utah's Dixon Laser Institute. "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.

"There has been some disagreement how random lasers operate. For an analogy, imagine you are outside a sports stadium and hear the roar of the crowd. Is the noise initiated throughout the stadium at the same time (led by cheerleaders shouting 'Go! go! , or are people yelling on their own ('Great catch,' 'Yea! ?"

"Our work has shown that the emission from random lasers happens with one emitter at a time, like individuals in the crowd yelling unconnected shouts," Polson says.