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Researchers Build Semiconductor Laser Using Photonic Crystal
Nov 2003
MURRAY HILL, N. J., Nov. 4 -- In what they call "an experimental tour de force," a team led by scientists from Bell Labs, the research and development arm of Lucent Technologies, has built a novel semiconductor laser that may have numerous applications, from advanced optical communications to sensitive chemical detectors. The small size of the new laser, a proof-of-concept device described in an article published online last week 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.

"This new laser was made possible by taking advances from many different areas in physics and incorporating them into one device," said Cherry Murray, senior vice president of physical sciences research at Bell Labs. "This work will open up new directions not only for optoelectronics and sensing, but could also provide a new tool to investigate very basic physical phenomena."

Quantum cascade class
The new laser belongs to a class of high-performance semiconductor lasers -- known as quantum cascade (QC) lasers --- that were invented at Bell Labs in 1994. QC lasers are made by stacking many ultrathin atomic layers of standard semiconductor materials (such as those used in photonics) on top of one another, much like a club sandwich. 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.

Light control
The Bell Labs team said they 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, and with proper design of the patterns, it is possible to control and manipulate the propagation of light within the material.

Using a state-of-the-art electron beam lithography facility at the New Jersey Nanotechnology Consortium, located at Bell Labs headquarters in Murray Hill, N.J., 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 micrometers across, or 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 and one of the inventors of the QC laser. "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 technologies. 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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optical communications
The transmission and reception of information by optical devices and sensors.
photonic crystal
semiconductor laser
A semiconductor material which is designed and grown for the efficient production of short wavelength stimulated emission through high gain as well as low internal losses. Materials with band gap energies which emit radiation efficiently within the desired wavelength region are used. Diodes may emit vertically in relation to the laser material junction in a VCSEL configuration or horizontally in an edge emitting configuration. Synonymous with laser diode.  Sometimes used...
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