CHAMPAIGN, Ill. -- A novel concept proposed two years ago for making a tunable, mid-infrared semiconductor laser has moved a significant step closer to reality with the recent demonstration of intersubband stimulated emission from an actual device structure, says a team of researchers from the University of Illinois and from the University of Paris in Orsay, France.
"By demonstrating intersubband stimulated emission -- that is, that emitted photons will be confined in the device long enough to induce further emission by other excited electrons -- we have shown that a new type of layered structure can be optimized for operation as an optically pumped, unipolar solid-state laser," said Jean-Pierre Leburton, a U. of I. professor of electrical and computer engineering and a researcher at the university's Beckman Institute for Advanced Science and Technology.
The proposed laser would emit in the mid- to far-infrared spectrum, a range of wavelengths above 10 microns that has been accessible only to gas lasers and to one other recently developed solid-state laser, Leburton said. The new semiconductor design is much simpler than the other design and also would be "tunable" to particular wavelengths by changing the thickness and composition of the layers.
"The core of the new device structure consists of special cells built of alternating layers of gallium-arsenide and aluminum-gallium-arsenide," Leburton said. "The cells are designed to exhibit three energy levels within the main conduction band, which can be excited by optical pumping. Photons are emitted when excited electrons jump from higher to lower energy levels."
In the special cell structure, population inversion -- a necessary condition for lasing that requires that more electrons exist at a higher energy level than at a lower energy level -- is achieved by drastically reducing the lifetime of electrons in the lower energy level.
"That was one of the trickier feats to accomplish," Leburton said. "First we had to calculate the physical separation required between the lower energy level and the ground energy level in order to produce this extremely short electron lifetime. Then we had to very carefully grow the device layers to exacting dimensions and electron concentrations. The potential laser operation of the device is really a function of the unique nature of these carefully designed and constructed cell structures."
A tunable, mid-infrared solid-state laser would be useful for a variety of applications, including optical communications, environmental testing and atmospheric studies, Leburton said.
The recent experiment was the result of a collaborative effort between two teams of scientists. The U. of I. team, headed by Leburton, performed modeling and simulation studies to design the desired cell structure. The French team, directed by Francois Julien, built the device using molecular beam epitaxy and conducted the initial lasing experiments.
The scientists reported their findings at the workshop on compound semiconductors
at the French National Center for Scientific Research (CNRS) in Chantilly,
France, in January.