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Posted: May 7, 2008

Lasers and milk: The common denominator

(Nanowerk News) Reading about a "random laser" for the first time, you might wonder whether this term refers to the laser in your CD player which plays the song titles in the random shuffle mode. In physics, however, "random lasers" refer to a class of microlasers which use the principle of random light scattering as an integral part of the laser operation.
In conventional lasers light is trapped between two highly reflecting mirrors where it is amplified by pumping from outside. Only when this amplification process is efficient enough, the laser begins to operate. After the initiation of the modern study of random lasers by Nabil M. Lawandy (Brown University), it was demonstrated by Hui Cao (Northwestern/Yale) and coworkers that you don't necessarily require elaborate mirrors to confine light long enough for lasing from micron sized devices. All you need to do is to put light into a highly disordered medium where scattering in random directions takes place. This mechanism, similar to the multiple scattering of light which makes a glass of milk look white, can prevent the light from escaping too quickly. If the random medium is optically active, pumping it with energy from outside will result in the emission of coherent light at sharply defined frequencies and in random directions.
"In pratice, random lasers are small beads of micrometer size, too small to be seen by the human eye", says Hakan E. Türeci, a research associate in the Quantum Photonics Group at ETH Zurich, who coauthored the article with Li Ge, Stefan Rotter and A. Douglas Stone at Yale University. "Due to their robustness and ease of manufacture, these lasers are sometimes referred to as "laser paint" and have found various applications, currently commercially available, such as document security and remote sensing. There are envisioned application areas in diagnostic imaging and super-fast displays as well".
Laser theory extended
Conventional laser theory tries to describe the operation of a laser by looking at the resonances of the laser cavity. In a random laser these resonances are, due to the lack of any defining mirrors, however, not at all well defined. The resonances are so closely spaced that they cannot be looked at independently of each other. Türeci and co-workers at Yale University have now extended the conventional laser theory such that it can be applied to random lasers, one of the most exotic type of lasers in existence, as well. In recent experiments it was observed that a specific random laser always shines at the same frequencies, but at intensities which differ strongly from measurement to measurement. With their publication in Science the authors show that this result can be traced back to unusually strong interactions between the laser modes.
Türeci: "Future research in designing novel micro and nanolasers will benefit from our approach, and we are implementing some of these ideas already with experimental collaborators to improve, e.g. power output, directional emission, for different kinds of microlasers."
Source: ETH Zurich