Dmitry G. Revin

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 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.

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.

High peak power λ∼3.3 and 3.5 μm InGaAs/AlAs(Sb) quantum cascade lasers operating up to 400 K

We demonstrate λ∼3.5 μm and λ∼3.3 μm strain compensated In0.7Ga0.3As/AlAs(Sb)/InP quantum cascade lasers operating in pulse regime at temperatures up to at least 400 K. Peak optical power exceeding 3.5 W at 300 K has been achieved at both wavelengths for 10 μm wide 4 mm long lasers with high reflectivity coated back facets. Threshold current densities of 2.5 kA/cm2 and 3.5 kA/cm2 have been observed at 300 K for the devices emitting at λ∼3.5 μm and λ∼3.3 μm, respectively.

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.

Measurements of optical losses in mid-infrared semiconductor lasers using Fabry–Pérot transmission oscillations

We present a Fabry–Perot resonator technique for room temperature optical loss measurements on mid-infrared (λ∼2–4 μm) lasers. The quality of optical waveguides for λ≈2.3 μm InGaAsSb/AlGaAsSb/GaSb interband lasers and a λ≈3.7 μm strain-compensated InGaAs/InAlAs/InP quantum cascade laser have been estimated using this method. The optical losses for these lasers lie in the range 15–25 cm−1 for interband lasers and 4–5 cm−1 (transverse electric polarization) and 21–23 cm−1 [transverse magnetic (TM) polarization] for the quantum cascade laser. The considerably higher losses for TM polarization in the case of quantum cascade laser are explained by intersubband absorption in the active layers. The method may be applied to structures with only a minimum amount of device processing, facilitating rapid progress in development of mid infrared laser designs in new materials systems.

Improved performance of In0.6Ga0.4As∕AlAs0.67Sb0.33∕InP quantum cascade lasers by introduction of AlAs barriers in the active regions

The authors demonstrate that the performance of strain compensated InP-based InGaAs∕AlAsSb quantum cascade lasers (QCLs) can be improved if AlAsSb barriers in the laser active regions are replaced by AlAs layers. The introduction of AlAs is intended to help suppress compositional fluctuations due to interdiffusion at the quantum well/barrier interfaces. An In0.6Ga0.4As∕AlAs0.67Sb0.33 QCL with AlAs barriers displays pulsed laser operation at wavelength of 4.1μm, for temperatures up to at least 320K, with lower threshold current density and higher output optical power than the reference laser with identical design but with AlAs0.67Sb0.33 barriers throughout the entire core region.

InP-Based Midinfrared Quantum Cascade Lasers for Wavelengths Below 4 μm

We review the recent development of high-performance short-wavelength (λ ~ 3.05-3.8 μm) strain-compensated InGaAs/AlAs(Sb)/InP quantum cascade lasers (QCLs). The lasers are demonstrated in which wavelengths as low as 3.05 μm are obtained at temperatures up to 295 K. We also verify that strain-compensated In0.7Ga0.3As/AlAs(Sb) QCLs with AlAs barriers in the active region operate with much better performance compared with the lasers having identical design but with AlAsSb barriers throughout the whole core region. λ ~ 3.3-3.7 μm laser emission is observed at temperatures up to at least 400 K and up to 20 W of output optical power at 285 K for the QCLs with various core region designs. Room temperature distributed feedback InGaAs/AlAs(Sb) QCLs with buried third-order gratings have been also developed, displaying single-mode operation in the wavelength range of 3.358-3.380 μm for temperatures between 270 and 360 K.

High performance InP-based quantum cascade distributed feedback lasers with deeply etched lateral gratings

We report device results and techniques for fabricating gratings for InP-based quantum cascade (DFB) lasers, grown by MOVPE. Deeply etched lateral gratings are achieved by the development of a novel two-stage ICP etch process