For release: May 17, 2001
Contacts:
Costas Soukoulis, Condensed Matter Physics, (515) 294-2816
soukoulis@ameslab.gov, http://cmpweb.ameslab.gov/personnel/soukoulis/
Saren Johnston, Public Affairs, (515) 294-3474
LASER BEAMS FROM SCATTERED LIGHT - THE BIG SURPRISE
Ames Laboratory Researchers Shed Light on Random Lasing Phenomenon
AMES, Iowa - A computer model developed by theoretical physicists at the U.S. Department of Energy's Ames Laboratory is lending credibility to observations of unexpected and amazing laser activity in materials that trap and scatter light. Their findings have helped open a new area in the study of multiple light scattering and could advance the development of a novel class of lasers for medical, industrial and military applications.
The prospect of obtaining an intense, narrow beam of laser light from any medium that does not possess an orderly atomic structure would have been considered absurd 10 years ago. But Nature never gives up on its mission to astonish us. So, what if the precise, focused wavelengths of laser light that come from a light-amplifying (or gain) medium with an orderly arrangement of atoms could also come from a gain medium that scatters photons of light quite efficiently in all directions? Can such a disordered medium produce laser light?
Yes, it can, says a new theoretical model developed by Ames Lab senior physicist Costas Soukoulis and Iowa State University graduate student Xunya Jiang. A new kind of laser can be created using a disordered medium, such as a finely ground crystal powder. The unusual laser can also be made using a gain medium that contains random scatterers, for example, titanium oxide. The disordered system does not impede lasing as traditionally thought - it accounts for it.
When light is shined into a substance that scatters it well, the Photons get bounced in random directions. If this happens often enough, it's likely that the trajectory of the photons inside the gain medium will be extremely long and that the photons will travel many times through the same crystal grains, ricocheting from side to side as they go. Under these conditions, light can be amplified tremendously. The process is similar to the way light from an ordinary laser travels back and forth between the two mirrors positioned in the laser's cavity. If the electrons in a disordered gain medium get pumped to a higher energy level while traveling their long, random routes, the result could be amplification to laser light. The gain medium, whether a crystal powder or a material containing random scatterers, would, in effect, become a laser.
Soukoulis' and Jiang's theoretical work represents some of the very first efforts to understand and describe the mechanics of random lasing. They have interpreted the laboratory observations of experimentalists Hui Cao of Northwestern University, Ad Lagendijk of the University of Amsterdam and Val Vardeny of the University of Utah, breathing logic and reason into the random lasing phenomenon.
"We wanted to understand all the sharp, narrow lasing peaks the experimentalists were seeing in the output spectrums of disordered systems, such as the zinc oxide nanocrystals Cao was studying," said Soukoulis, who is also an ISU physics professor. "So we simulated the real experiment in the computer, using a complex numerical technique known as finite difference time domain."
Soukoulis' and Jiang's FDTD computer simulation is a one-dimensional version of a three-dimensional experiment. The plus is that it lets them see the dynamic process of the random system and how the electric field is building up inside. The simulation actually makes it possible to track the evolution of the electric field.
The ability to track the population of the electrons in a random system as they are pumped to a higher energy level by an outside energy source, fall to a lower energy level, and then return to the ground state has allowed Soukoulis and Jiang to arrive at several important results. One is the ability to determine the threshold value of lasing in the amplifying medium. Knowing the threshold value is critical because it defines the amount of energy needed to pump the electrons in the amplifying substance to the intensity where lasing takes place.
"Another thing we discovered is that the threshold value of lasing decreases as disorder increases," said Soukoulis. "So, the higher the concentration of scatterers, the lower the threshold of lasing."
The researchers also discovered that the sharp peaks Cao's and Vardeny's teams had observed in the output spectrums were coming from specific lasing modes of the disordered systems. Soukoulis explained that for disordered systems the lasing has a greater probability of occurring in some particular regions of the system than in others. "As the electrons in the amplifying random substance are pumped to a higher energy level, the light waves retain their shape, but they become bigger and bigger," he said. "Eventually, they will start lasing."
In addition, Soukoulis' and Jiang's model revealed that increasing the pumping intensity beyond a maximum value does not affect the number of lasing modes in the random system. "They do not increase anymore; rather, they saturate to a constant value that is determined by the degree of randomness in the system and its length," said Soukoulis.
He adds, "With our FDTD model we can predict exactly where the lasing modes will be in a random system; the mode that will lase best; and the wavelength, which determines the color of the emission light."
The ability to predict the lasing modes in a given random system is critical when considering potential uses for random lasers. They hold promising properties for brightening the pixels in flat-panel displays. By reducing the size of the phosphor grains that emit light in these displays, it might be possible for the electron emitter in each pixel to excite the phosphor's electrons and initiate lasing, which would brighten the pixels.
Used with chemical sensors, random lasers could provide a sophisticated, noninvasive tool for diagnosing problems within the human body. And paint-on random lasers may one day light the way to more efficient and economical search-and-rescue missions to identify downed ships and airplanes, and even individual passengers.
Ames Laboratory is operated for the DOE by ISU. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.
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