J. W. Cockburn

Fingerprints of spatial charge transfer in quantum cascade lasers

We show that mid-infrared transmission spectroscopy of a quantum cascade laser provides clear-cut information on changes in charge location at different bias. Theoretical simulations of the evolution of the gain/absorption spectrum for a λ∼7.4 μm InGaAs/AlInAs/InP quantum cascade laser have been compared with the experimental findings. Transfer of electrons between the ground states in the active region and the states in the injector goes hand in hand with a decrease of discrete intersubband absorption peaks and an increase of broad, high-energy absorption toward the continuum delocalized states above the barriers.

InGaAs∕AlAsSb∕InP quantum cascade lasers operating at wavelengths close to 3μm

The authors report the realization of short wavelength (3.05μm⩽λ⩽3.6μm) InP lattice-matched In0.53Ga0.47As∕AlAs0.56Sb0.44 quantum cascade lasers (QCLs). The highest-performance device displays pulsed laser action at wavelengths between 3.4 and 3.6μm, for temperatures up to 300K, with a low temperature (80K) threshold current density of approximately 2.6kA∕cm2, and a characteristic temperature of T0∼130K. The shortest wavelength QCL (λ≈3.05μm) has a higher threshold current density (∼12kA∕cm2 at T=20K) and operates in pulsed mode at temperatures up to 110K.

Room temperature operation of InAs∕AlSb quantum cascade lasers

The room temperature operation of InAs∕AlSb quantum cascade lasers is reported. The structure, grown by molecular beam epitaxy on an InAs substrate, is based on a vertical transition design and a low loss n+-InAs plasmon enhanced waveguide. The lasers emitting near 4.5μm operate in pulse regime up to 300K. The threshold current density of 3.18-mm-long lasers is 1.5kA∕cm2 at 83K and 9kA∕cm2 at 300K.

InGaAs∕AlAsSb quantum cascade lasers

The In0.53Ga0.47As∕AlAs0.56Sb0.44 heterostructure system is of significant interest for the development of high-performance intersubband devices due to its very large conduction band offset (ΔEc∼1.6eV) and lattice-matched compatibility with well-established InP-based waveguide technology. In this letter, we report the realization of In0.53Ga0.47As∕AlAs0.56Sb0.44 quantum cascade lasers emitting at λ∼4.3μm. The highest-performance devices have low-temperature (20K) threshold currents of ∼6kA∕cm2 and display laser action up to a maximum temperature of 240K, with a characteristic temperature of T0∼150K.

λ∼3.1 μm room temperature InGaAs/AlAsSb/InP quantum cascade lasers

Strain compensated In0.67Ga0.33As/AlAs0.8Sb0.2/InP quantum cascade lasers emitting at wavelengths near 3.1 μm at room temperature have been demonstrated. The lasers operate in pulsed mode with threshold current density of 3.6 kA/cm2 at 80 K and 19.2 kA/cm2 at 295 K. The peak optical power for an as-cleaved 3 mm long and 10 μm wide ridge device exceeds 1 W per facet at 80 K and is around 8 mW at 295 K. The observed laser performance suggests that room temperature operation for these lasers remains possible beyond the predicted threshold for Γ-L intervalley scattering of electrons in the upper laser levels.

Single-mode surface-emitting quantum-cascade lasers

We present high-power surface-emitting second-order distributed feedback quantum-cascade lasers in GaAs and InP material systems. The GaAs device, grown by molecular-beam epitaxy, showed single-mode peak output powers of 3 W at 78 K in pulsed operation. With the InP-based devices, which are grown by metalorganic vapor phase epitaxy, we obtained single-mode peak output powers of 1 W at room temperature. These are the highest output powers for surface emission of quantum-cascade lasers reported so far. The InP-based distributed feedback lasers also have very low threshold current densities and are working well above room temperature.

Quantum cascade lasers grown by metalorganic vapor phase epitaxy

We report the growth of GaAs-based quantum cascade lasers using atmospheric pressure metalorganic vapor phase epitaxy. The necessary control of interface abruptness and layer thickness uniformity throughout the structure has been achieved using a horizontal reactor in combination with individually purged vent/run valves. A low-temperature threshold current density of 10 kA/cm2 and maximum operating temperature of 140 K have been measured. These performance levels are comparable with early GaAs-based devices grown using molecular-beam epitaxy. The measured emission wavelength (λ∼11.8 μm) is approximately 3-μm longer than the calculated transition wavelength, which we explain using a model incorporating compositional grading of the active region barriers.

Energy level structure and electron relaxation times in InAs∕InxGa1−xAs quantum dot-in-a-well structures

Complementary interband and intraband optical spectroscopic techniques are used to investigate the band structure and carrier relaxation times in technologically important InAs∕InGaAs∕GaAs quantum dot-in-a-well (DWELL) structures. We determine the dot ground to first excited state energies to be 42meV in the conduction band and 18meV in the valence band. Using intraband pump-probe experiments, electron relaxation times from the well states to the dot ground state are measured to be ∼5ps at 10K. Our results provide important parameters for the design and simulation of DWELL-based interband lasers and intraband midinfrared photodetectors.

High-performance distributed feedback quantum cascade lasers grown by metalorganic vapor phase epitaxy

We report the operation of distributed feedback quantum cascade lasers, grown by metalorganic vapor phase epitaxy. Single-mode laser emission at λ∼10.3μm and λ∼7.8μm is observed from two different samples, with 300 K threshold current densities of Jth∼3 and ∼2.4kAcm−2, respectively. Structural investigation by x-ray diffraction and transmission electron microscopy, and the close correlation between the predicted and observed emission wavelengths indicate exceptional control of the layer thicknesses, including ultrathin (∼8A) barrier layers in the active region. These results confirm metalorganic vapor phase epitaxy as a viable technology for the growth of high-performance quantum cascade lasers.

Intervalley scattering in GaAs–AlAs quantum cascade lasers

We have investigated the importance of intervalley (Γ–Χ) electron transfer between Γ-point quantum well states and X-point barrier states in GaAs-based quantum cascade lasers with indirect band gap AlAs barriers. A series of samples has been studied in which the energy separation between the coupled injector/upper laser levels and the lowest confined X state in the injection barrier is varied. We demonstrate that for lasing to occur, electron injection into the upper laser level must proceed via Γ states confined below the lowest X state in the injection barrier. The limit this places on the minimum operating wavelength (λ≈8 μm) for the present laser design is overcome by utilizing a double injection barrier to achieve lasing at λ=7.2 μm.