Photonic Crystal Enables Surface-Emitting Quantum-Cascade Laser
Daniel S. Burgess
By incorporating a photonic crystal resonator into the design of a quantum-cascade laser, researchers in the US have developed compact, surface-emitting devices with potential applications in chemical sensing, spectroscopy and imaging. Produced by a collaboration of Bell Labs in Murray Hill, N.J., and California Institute of Technology in Pasadena, the mid-IR lasers also may serve as model systems for investigations into photonic crystals and microcavity effects.
A scanning electron microscope image of a wire-bonded device illustrates the structure of a quantum-cascade laser etched with a photonic crystal. The device displays surface emissions in a vertical structure.
Quantum-cascade lasers have demonstrated great promise since their development nearly a decade ago. Their uniqueness stems from the use of an intersubband optical transition: Electrons undergo a quantum jump between quantized conduction band states, called subbands, of a suitably designed multi-quantum-well structure composed of standard III-V semiconductor compounds. The wavelength may be tuned at the growth stage by changing the thickness of the layers in a device.
The transverse magnetic polarization of these intersubband transitions intrinsically favors edge-emitting configurations for quantum-cascade lasers, said Raffaele Colombelli of Université Paris-Sud in Orsay, France, formerly a postdoctoral researcher at Bell Labs. Surface emitters, however, can be interesting for various applications because they may be integrated into arrays.
To produce the new device, the researchers used a chlorine-based dry etch to inscribe a hexagonal pattern of airholes through the three-well, InGaAs/AlInAs, which was grown lattice-matched on InP, and into the lower cladding and substrate. This photonic crystal acted to provide feedback for laser action as a microcavity and to diffract the approximately 8-µm radiation vertically from the surface of the semiconductor, Colombelli explained. It also reduced diffraction into the substrate. They repeated the process with different hole radii with lattice spacings of 2.69 to 3 µm to yield an array of 50-µm-diameter devices that could be individually addressed.
The researchers measured the emission from individual lasers in operation using a nitrogen-cooled HgCdTe detector and a microbolometer equipped with a polarizer. They found that the devices predominantly displayed pulsed, single-mode operation and could tune the output wavelength from the array by addressing lasers with different hole radii and lattice spacings.
Much work lies ahead before the surface emitters achieve performances comparable to today's quantum-cascade lasers, Colombelli said. Further avenues of inquiry may include filling the holes of the photonic crystal with various nonlinear materials and may concentrate also on more fundamental issues.
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