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Imaging microcavities sheds light on random laser operation

Photonics Spectra
Mar 2010
Jörg Schwartzjoerg.schwartz@photonics.com

SALT LAKE CITY – Researchers from the University of Utah’s Dixon Laser Institute have managed to visualize the microscopic cavities responsible for laser action in random lasers, helping to explain how this relatively new type of laser really works.

Many see random lasers as one of the most exciting areas in current laser research. Discovered only about a decade ago, they come both with questions about what exactly makes them work and with exciting potential applications. As with other lasers, random lasers comprise a medium capable of light amplification via stimulated emission, with a structure that holds the light in the gain medium long enough to compensate for any energy loss. In normal lasers, the latter is typically achieved by carefully aligning reflectors forming a cavity; in random lasers, however, it is the natural structure of the material doing this job – hence the name.


This photo shows a microscopic image of an electrically conducting polymer film that was pumped by a pulse of green light from a conventional laser, exciting tiny cavities within the film to produce red-colored “random laser” light. Photos by Randy Polson, Department of Physics and Astronomy, University of Utah.

Consequently, random lasers require special materials that not only amplify light but also reflect it many times, retaining it within the material for as long as possible. Offering these characteristics are many “distorted” gain media, such as semiconductors, liquid or photonic crystals, or so-called nanocrystals such as zinc oxide clusters. Another option is adding a scatterer such as titanium dioxide into a medium – a dye, for example – that is already doing the job of amplification.

Of course, the gain medium of a random laser must be pumped to facilitate laser operation, but its output, unlike that of conventional lasers, will not be strongly directional after all the scattering. Nevertheless, the output exhibits narrow coherent spectral lines, or modes. These modes, another characteristic of laser emission, and their generation have been among the big unanswered questions about how random lasers work. In a “normal” laser, modes are established because only certain waveforms are allowed in a cavity, but with random lasers, where no well-defined mirrors exist, their existence is harder to explain. One model suggests that the modes – and the cavities generating them – are strongly localized but that their output is combined, whereas the other paradigm presumes nonlocalized distributed light paths inside the disordered medium, with the lines generated by specific photonic states in the medium.


A film of the polymer DOO-PPV, as seen through a microscope, shows clumps of undissolved polymer within the darker, dissolved material. Such irregularities are believed to create the tiny cavities that act as built-in mirrorlike resonators for random lasers.

The two models are the subject of a hot debate, “but our work shows that the emission from random lasers is generated by individual emitters,” said Randal Polson, one of the Utah researchers in professor Z. Valy Vardeny’s group, which has been pioneering random lasers since their discovery. In their most recent work, published in the Jan. 24, 2010, issue of Nature Physics, they pumped pi-conjugated polymer films with green conventional laser light and studied the dependency of the lasing threshold on the excitation area with and without titanium dioxide nanoparticles added to the polymer. Taking optical pictures, they found that, near the threshold, only one microscopic resonator technique linked the red output spectrum with the location where it was generated.

“With better understanding, almost certainly the commercial use of random lasers will gain pace,” Vardeny said. One exciting observation Polson and he published in the Aug. 16, 2004, issue of Applied Physics Letters was that human tissue can support random lasing if it is infiltrated with a dye – and that malignant tissue exhibits more laser lines than healthy samples. This discovery could enable these inexpensive and easy-to-make lasers to automate cancer detection.


GLOSSARY
cavity
In a laser, the optical resonator formed by two coaxial mirrors, one totally and one partially reflective, positioned so that laser oscillations occur.
lasing threshold
The lowest excitation power level at which a laser's output is mainly the result of stimulated emission rather than spontaneous emission.
stimulated emission
Radiation similar in origin to spontaneous emission but determined by the presence of other radiation having the same frequency. Because the phase and amplitude of the stimulated wave depend on the stimulating wave, this radiation is coherent with the stimulating wave. The rate of stimulated emission is proportional to the intensity of the stimulating radiation.
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