R.P. Green

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.

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.

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.

Room-temperature operation of InGaAs/AlInAs quantum cascade lasers grown by metalorganic vapor phase epitaxy

We report the room-temperature operation of λ≈8.5 μm InGaAs/AlInAs quantum cascade lasers, grown by low-pressure metalorganic vapor phase epitaxy. The necessary control of interfacial abruptness and layer thicknesses was achieved by the use of individually purged vent/run valves and a growth rate of 0.8 μm/h for the active region. Low-temperature threshold current densities of ∼1.5 kA cm−2 and a maximum operating temperature of 290 K have been measured in pulsed operation. These values are comparable with those reported for structures of a similar design grown using molecular beam epitaxy.

Room-temperature operation of an InAs–GaAs–AlAs quantum-cascade laser

We report the shortest-wavelength (λ∼8.5 μm) room-temperature laser operation so far achieved for GaAs-based quantum-cascade (QC) lasers. By depositing InAs monolayers in the device active regions during growth, we are able to both reduce the emission wavelength and minimize thermally activated carrier leakage into quasicontinuum states. This approach results in laser operation up to 305 K, with a peak optical power of ∼10 mW at room temperature. A reduced temperature sensitivity of the threshold current compared with similar GaAs–Al0.45Ga0.55As QC lasers is also observed.

λ∼4-5.3 μm intersubband emission from InGaAs-AlAsSb quantum cascade structures

The In0.53Ga0.47As–AlAs0.56Sb0.44 materials system, lattice matched to InP, is an attractive candidate for short wavelength quantum cascade lasers due to the very large conduction band discontinuity (∼1.6 eV) and compatibility with well established quantum cascade laser waveguide design and fabrication technology. In this letter we report the operation of In0.53Ga0.47As–AlAs0.56Sb0.44 quantum cascade structures emitting in the wavelength range λ∼4–5.3 μm. Clear intersubband electroluminescence peaks are observed close to the design wavelengths, with full widths at half maximum in the range of ∼30–40 meV.

Polaron relaxation channel in InAs/GaAs self-assembled quantum dots

Polaron relaxation in n-type InAs quantum dots has been studied by picosecond time-resolved pump–probe spectroscopy. Due to inhomogeneous broadening of the absorption features associated with transitions between s-like ground state and p-like first excited state, the energy dependence of the polaron decay time has been measured over a wide spectral region from 40 to 60 meV. The polaron lifetime increases continuously from 20 ps at 40 meV to 65 ps at 54 meV. By analysing the temperature dependence of the polaron lifetime the main polaron decay channel has been identified as the cubic overtone (2LA) decay channel. By fitting the experimental data we extract the LO-phonon lifetime of 10 ± 2 ps (at 33 meV) for InAs quantum dots and electron–phonon coupling strength of 6.2 ± 0.5 meV.

Surface emission from MBE and MOVPE grown quantum cascade lasers

We report high power single mode surface emission from second order distributed feedback quantum cascade lasers. Two different laser materials are used for the lasers: MOVPE grown InGaAs/InAlAs and MBE grown GaAs/AlGaAs