Quantum Cascade Lasers Achieve 19µm Operation
John Baliga, Associate Editor -- Semiconductor International, 12/1/2000
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| John Baliga, Associate Editor |
In a recent issue of Applied Physics Letters,1 researchers at Lucent Technologies' Bell Laboratories (Murray Hill, N.J.) presented the results of a quantum cascade (QC) laser operating at a wavelength of about 19 µm. QC lasers are multiple quantum well (MQW) structures in which electrons experience a cascade of radiative transitions, so that each electron generates multiple photons.
From an energy standpoint, QC lasers are well suited for longer-wavelength lasers. Because the radiative transitions involve small amounts of energy, many of them can be done with a reasonably sized bias. Longer-wavelength lasers are used for atmospheric and gas composition analysis.
Going to longer wavelengths, however, requires that absorption mechanisms be eliminated or minimized. Large carrier concentrations are often used to maintain the necessary population inversion, but they also increase free carrier absorption. Also, the amount of material involved should be minimized to reduce absorption. The 19 µm device uses a structure with minimal doping and a metal strip to provide a surface plasmon mode waveguide.
The laser is made of InGaAs and AlInAs layers lattice-matched to an InP substrate (In0.53Ga0.47As/Al0.48In0.52As), so that the conduction band forms the MQW structure (Figure). Molecular beam epitaxy (MBE) is used to make the structure, since the thicknesses of the layers vary from 6 to 60 Å.
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| After producing a photon in one active region, an electron travels to the next active region (right to left), repeating the process through multiple stages. The chirped multiple quantum well structure produces minibands optimal for this operation. (Source: Lucent Technologies) |
MQW structures have two very important properties for laser applications. First, the wavelength of the laser is determined by the thickness of the wells. Second, the transitions occur between subbands of the conduction band. Those subbands are similar, so the transition will have nearly the same energy regardless of an electron's momentum.
The chirped structure of the injector and active regions serves some important purposes. First, as the well portions gradually widen from one end of a region to the other, the ground-state energies of the wells gradually get lower. When the device is biased, those states "line up" at one common energy, allowing the formation of the miniband in the region. Second, the thickness of the barrier layers influences the coupling of the well states, which affects the size of the minibands. As the barriers get wider, the coupling gets weaker and the minibands get smaller.
One key to the QC laser's operation is the injection region, which allows electrons that have just made a radiative transition to go to the next active region. The miniband must be as wide as possible to collect the "spent" electrons as efficiently as possible. Sweeping the spent electrons out helps maintain the needed population inversion.
At the other end of the injector region, the barriers are thicker, narrowing the miniband. This narrowing puts the electron at the very bottom of the second miniband in the next active region. This keeps the electron from traveling through to the next injector region. This confines the electron until it makes a radiative transition, which also helps maintain the population inversion.
Putting a metal strip on the device, instead of a dielectric waveguide strip, forms an interface plasmon mode waveguide in the laser material. This type of waveguide has a higher degree of confinement, reducing the required thickness of the semiconductor from 9 to 4 µm. This not only reduces the amount of material that can absorb photons, it also enhances stimulated emission in the laser. A two-metal grating pattern can be used to produce distributed feedback needed for single-mode operation. •
REFERENCES
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A. Tredicucci, C. Gmachl, M. Wanke, F. Capasso, A. Hutchinson, D. Sivco, S.-N. Chu, A. Cho, "Surface plasmon quantum cascade lasers at l ~ 19 µm," Applied Physics Letters, vol. 77, p. 2286-2288 (2000).
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