CAMBRIDGE, Mass., Feb. 25 -- Engineers and applied physicists have laid the foundations for a new type of "plug and play" laser, the Raman injection laser. The device represents several key innovations in laser technology, combining the advantages of nonlinear optical devices and semiconductor injection lasers with a compact design, and may one day lead to wide-ranging applications in imaging and detection. The proof-of-concept model was developed by Mariano Troccoli, Ertugrul Cubukcu and Federico Capasso, all in the Harvard University Division of Engineering and Applied Sciences; Alexey Belyanin of Texas A&M University; and Deborah L. Sivco and Alfred Y. Cho, both of Bell Laboratories, Lucent Technologies.
Their work was published in the Feb. 24 issue of Nature.
Conventional Raman lasers depend on a fundamental phenomenon in physics called the Raman effect, the change in the frequency of monochromatic light (such as a laser) when it passes through a substance. When light from an intense exciting laser beam, known as the "pump," deflects off the molecules of certain materials, some of the incident photons lose part of their energy. As a result, a secondary laser beam with a frequency shifted from that of the exciting laser emerges from the material.
"Raman lasers have been used for a long time," said Troccoli. "In general, they require a large and powerful pump to compensate for the beam's attenuation, or weakening, as it propagates through the material. There is a downward shift because of the nonlinear coherent oscillations in the molecules of the material being used. In our work, we were able to put the pump and the material itself into a single device."
The team's ability to combine the power source and the Raman material together, literally creating a laser-within-a-laser, has resulted in several key innovations. The injection laser is the first current-driven Raman laser; in essence, it can be plugged in. The current generates an internal laser beam (the pump) within the material, which in turn generates the Raman laser radiation. Because the pump laser is now self-generated, the device is incredibly efficient (30 percent of the pump power is converted into the Raman laser beam), suppressing the standard decline that happens when an external power source is used.
"Everything is contained in one single cavity only a few microns wide and a couple of millimeters long," said Capasso, who codeveloped the groundbreaking quantum cascade laser more than a decade ago. "This means there's a very strong interaction and that you do not need a very high-powered source to get the desired effect."
As a result, the apparatus needed to generate the laser can be made much smaller, with a footprint of less than about a millimeter. This is comparable to the size of commercial diode lasers that are found in everyday applications such as DVD players and bar-code readers or used by physicians for procedures such as laser eye correction. The new device was created using molecular beam epitaxy (MBE), the thin-film growth technique pioneered by Alfred Y. Cho in the late 1960s that is now widely used in commercial products such as light-emitting diode (LED) displays. In MBE, atoms are "spray painted" on a substrate to produce semiconductor films only a few atomic layers thick.
"Perhaps what's most important is that the new laser is not dependent on the conventional Raman shift," Troccoli said. "Instead, we use an internal electronic oscillation mode that can be designed or tailored over a broad range by changing the laser thickness of the constituent materials. This makes the device far more flexible and allows us to use it in room-temperature conditions. We can design it to emit in the midinfrared range, where most molecules have their telltale absorption "fingerprints," and eventually broaden it to work in the terahertz range (a wavelength between 3 and .003 millimeters), where many materials appear transparent."
Although the current version of the injection laser is not as powerful as standard Raman lasers, it overcomes fundamental barriers for practical use. Ultimately, by better understanding the physics involved and through improving the device's design, the researchers hope their work will lead to a new generation of "tunable" compact lasers that can operate at almost any wavelength of the invisible light spectrum. Because these lasers can easily penetrate through packages and clothing, they may also have broad future potential applications related to areas such as homeland security.
Capasso said, "While our paper merely demonstrated proof of concept, one day it may lead to the sort of security experts dream of having: a portable device that you could use to detect things like weapons or explosives simply shining an invisible light to see what someone might be hiding,. The work also represents an important advance in quantum design since we are now able to engineer, from the bottom-up, a new Raman material and laser and tailor its property for a given application"
The finding was partially supported by Texas A&M University's Telecommunications and Informatics Task Force (TAMU TITF) initiative.
For more information, visit: www.deas.harvard.edu