Quantum Wire Exhibits Lasing
Gallium-arsenide quantum wires produced with a novel crystal growth technique have demonstrated single-mode lasing. At low energies, one-dimensional active gain regions are expected to have a higher density of states than their two- or three-dimensional counterparts, offering a low lasing threshold and stable, single-mode lasing.
A quantum wire created at the intersection of two perpendicular sets of semiconductor planes lases under optical stimulation. The percentages reflect the concentration of Al in the AlGaAs, and the contours illustrate the constant probability of photons and electrons in the structure. The devices may provide insights into the physics of one-dimensional systems. Courtesy Yuhei Hayamizu.
Researchers at the University of Tokyo's Institute for Solid-State Physics and at Lucent Technologies' Bell Labs in Murray Hill, N.J., fabricated the devices using a technique called cleaved-edge overgrowth. They deposited layers of AlGaAs and GaAs using traditional molecular beam epitaxy, but then cleaved the wafer in the middle to expose the sandwiched layers. They again employed molecular beam epitaxy to deposit additional layers on top of the exposed edge, covering the structure with perpendicular planes of GaAs and AlGaAs.
In this second step, they deposited a 6-nm-thick "arm well" of GaAs across the cleaved edge, which intersected the plane of a 14-nm-thick GaAs "stem well" to create a "T" in cross section. The intersection of the planes defined a 14 x 6-nm quantum wire, with the wave function of the one-dimensional ground state at the center of the intersection, explained researcher Yuhei Hayamizu. The scientists then cleaved the devices into 500-µm-long sections and coated one face with gold.
They employed 753-nm radiation from a Ti:sapphire laser to pump the devices in the plane of the stem well, and they measured the output at the gold-coated face. The optical pump threshold was 5 mW, and several modes around 785.8 nm were excited at 8.3 mW of pump power. As the pump power was increased to 17 mW, the emission changed to single-mode with a center at approximately 786 nm. With the pump power increased to 260 mW, the quantum wire exhibited redshifted single-mode emission at 786.5 nm.
The low threshold, the single-mode stability and the small redshift were all in contrast to the behavior of 2-D lasers fabricated with the extended regions of the T-profile. Those devices exhibited thresholds three times (stem well) and 50 times (arm well) higher, as well as a propensity for multimode operation, and the redshift of the stem-well devices was nearly four times as large.
Although most of the proof-of-principle device characteristics matched expectations, some details of the 1-D emission were unexpected. For example, the energy separation between the modes at high pump energy was twice as large as antic-ipated. Also, the emitted intensity did not smoothly increase as pump power was increased.
Hayamizu noted that, although the output of a single quantum-wire laser is small because of the small lasing region, it should be possible to increase the output power by creating multiple quantum wires in the active region. The team is working on the geometry of the devices to improve the confinement, he added.
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