PORTLAND, Ore. Semiconductor lasers emit at wavelengths from 375 nanometers to 1.8 microns (1,800 nanometers), but then skip to wavelengths of about 4 microns. In that gap, current semiconductors appear to be incapable of lasing.
Now researchers at Binghamton University in New York claim to have devised a strategy to bridge the lasing gap with new materials and architectures that could enable applications not now possible.
"We are interested in creating lasers at wavelengths that are not currently available today, as well as to employ new light emitting processes to make new types of devices," said professor Oana Malis, a physicists at the University of Binghamton. "There is currently a gap in the near infrared between about two microns and four microns wavelength, which has applications in both sensing and covert communications."
To plug the lasing gap, Malis plans to develop quantum cascade lasers using gallium nitride. In a separate project, quantum cascade lasers would operate in the valence band, thereby enabling surface emission devices not possible today. For instance, 2- to 4-micron lasers could be used as spectral scanners to reveal chemical composition and for tight-beam communications channels that are nearly impossible to intercept.
Conventional lasers energize electrons that emit a single photon by jumping from the semiconductor's conduction band to its valence band. By contrast, a quantum cascade laser uses a stair-step arrangement of layers consisting of from 25 to 75 quantum wells--each at a progressively lower energy level. In that way, electrons can cascade down the energy staircase, emitting a photon at each step. Such quantum cascades have already revolutionized a variety of applications, from pollution monitoring to chemical sensing to medical diagnostics and homeland security.
Today, quantum cascade lasers fabricated from indium phosphide lose their ability to lase at about 2 microns, but Malis believes that by going to gallium nitride--from which blue and ultraviolet laser diodes are made today--that she can coax a quantum cascade laser into the lasing gap.
"Current semiconductors cannot go past about 2-micron wavelengths, but the nitride family can reach this range," said Malis. "I would like to make a quantum cascade lasers our of gallium nitride instead of indium phosphide, which we believe will make new wavelengths accessible in the that gap."
Malis' second project is to make quantum cascade lasers that work within the valence band, which she believes will enable 2- to 4-micron vertical cavity surface emitting lasers in place of the current method: squeezing out from the edge of chips like quantum cascade lasers.
"Today, quantum cascade lasers made from aluminum gallium arsenide can operate at 5 to 10 micron wavelengths in the conduction band," said Malis. "But I want to make lasers in the valence band, which has the potential of enabling new types of devices, such as true surface-emitting devices, which has not yet been achieved elsewhere."
The quantum cascade laser research was funded by the National Science Foundation and Research Corp. (Tucson, Ariz.).