Bell Labs builds novel semiconductor laser using photonic crystals

3 November 2003 Murray Hill, NJ Lightwave -- A team led by scientists from Bell Labs, the research and development arm of Lucent Technologies, has built a surface-emitting quantum cascade laser that may have applications ranging from optical communications to chemical detection. The small size of the new laser may lead to various laser-on-a-chip applications, the researchers say.

3 November 2003 Murray Hill, NJ Lightwave -- A team led by scientists from Bell Labs, the research and development arm of Lucent Technologies, has built a surface-emitting quantum cascade laser that may have applications ranging from optical communications to chemical detection. The small size of the new laser, a proof-of-concept device described in an article published last Friday on the Web by the journal Science, may lead to various laser-on-a-chip applications, the researchers say.

The new device exploits a photonic crystal, a highly engineered material with superior optical properties, and was made in collaboration with scientists from the New Jersey Nanotechnology Consortium, California Institute of Technology, and Harvard University.

Quantum cascade (QC) lasers were invented at Bell Labs in 1994. QC lasers are made by stacking many ultra thin atomic layers of standard semiconductor materials (such as those used in photonics) on top of one another. By varying the thickness of the layers, it is possible to select the particular wavelength at which a QC laser will emit light, allowing scientists to custom design a laser. When an electric current flows through a QC laser, electrons cascade down an energy "staircase," and every time an electron hits a step, a photon of infrared light is emitted. The emitted photons are reflected back and forth inside the semiconductor resonator that contains the electronic cascade, stimulating the emission of other photons. This amplification process enables high output power from a small device.

In the decade since their invention, QC lasers have proved to be very convenient light sources, and are commercially available, having been licensed by Lucent to laser manufacturers. They are compact, rugged, and often portable, in addition to being powerful. However, they are inherently devices that emit light from the edges. In particular, they cannot emit laser light through the surface of the device.

The Bell Labs team overcame this challenge by using the precise light-controlling qualities of a photonic crystal to create a QC laser that emits photons perpendicular to the semiconductor layers, resulting in a laser that emits light through its surface. Photonic crystals are materials with repeating patterns spaced very close to one another, with separations between the patterns comparable to the wavelengths of light. When light falls on such a patterned material, the photons of light interact with it; with proper design of the patterns, it is possible to control and manipulate the propagation of light within the material.

Using an electron beam lithography facility at the New Jersey Nanotechnology Consortium, located at Bell Labs headquarters in Murray Hill, NJ, the researchers were able to superimpose a hexagonal photonic crystal pattern on the semiconductor layers that made the QC laser. The final laser was only 50 microns across -- about half the diameter of a human hair.

"The most exciting part of this work is that we combined photonic and electronic engineering to create a new surface-emitting QC laser," said Al Cho, adjunct vice president of semiconductor research at Bell Labs. "The photonic crystal approach has real potential for new applications. The production of surface-emitting compact lasers only 50 micrometers across enables large arrays of devices to be produced on a single chip, each with its own designed emission properties."

Such lasers-on-chips, if fabricated in the future, may lead to new possibilities for optical communications, as well as other optoelectronics and sensing applications. QC lasers have already been used to make extremely sensitive sensors, including sensors that have been used by the National Aeronautics and Space Administration for atmospheric monitoring.

In addition to Cho, the interdisciplinary team that designed and fabricated the new laser at Bell Labs included researchers Deborah Sivco, Michael Sergent, Raffaele Colombelli and Claire Gmachl; Don Tennant from the New Jersey Nanotechnology Consortium; Kartik Srinivisan and Oskar Painter from the California Institute of Technology; and Federico Capasso and Mariano Troccoli from Harvard University. Colombelli, now at the University of Paris-South, was a post-doctoral researcher at Bell Labs when the research was done; Gmachl is a professor at Princeton University and a Bell Labs consultant.

"The next step is to see if we can use this sort of technique to get sensing done within the laser," said Capasso, who is also a Bell Labs consultant and one of the inventors of the QC laser. "If we can fill the holes of the photonic crystal in this laser with nanoliters of fluid or other special material, we may get some interesting physics as well as a whole new world of applications."

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