We present a substantial improvement in the CW performance of GaAs-based quantum cascade lasers with operation up to 150 K. This has been achieved through suitable changes in device processing of a well-characterized laser. The technology optimizes the current injection in the laser by reducing the size of the active stripe whilst maintaining-a strong coupling of the optical mode to preserve low current densities. The reduction of total dissipated power is critical for these lasers to operate CW. At 77 K, the maximum CW optical power is 80 mW, threshold current is 470 mA, slope efficiency is 141 mW/A, and lasing wavelength lambda similar to10.3 mum.

The intersubband light emission of GaAs/GaAlAs quantum cascade lasers is measured under pulsed magnetic fields parallel to the current, up to 60 T. A giant modulation of the laser intensity is observed with complete suppression of the laser emission when the energy spacing between intersubband Landau quantized states matches the GaAs optical phonon energy. When the level separation is not equal to the phonon energy, the laser output increases as a result of quenched phonon emission from the upper subband electrons. In this situation, the laser threshold current was found reduced by a factor of two.

We use micro-probe photoluminescence in continuous wave operating GaAs quantum cascade lasers to measure the lattice and electronic temperatures, which determine the electrical power dependence of the threshold current and the slope efficiency, respectively.

We demonstrate the highest continuous wave operating temperature for GaAs based quantum cascade lasers to date 130 K. This has been achieved through optimised device processing, that reduces the joule heating in the laser.

Simultaneous laser emission at wavelengths 8.6 and 10.4/spl mu/m is reported for a GaAs based quantum cascade laser. These devices are an ideal compact source for frequency mixing applications over the mid/far infrared.

The facet temperature profile and the thermal resistance of operating quantum-cascade lasers (QCLs) have been assessed using a microprobe band-to-band photoluminescence technique. Substrate-side and epilayer-side-mounted QCLs based on GaInAs/AlInAs/InP and GaAs/AlGaAs material systems have been compared. The dependence of the thermal resistance on the CW or pulsed injection conditions and its correlation with the output power have been studied. These results were used as inputs for a two-dimensional heat-diffusion model which gives the heat fluxes and the thermal conductivity of the active regions, in order to design QCLs with improved thermal properties.

We present a comprehensive study of the thermal behavior of GaAs/AlGaAs quantum cascade lasers as a function of the aluminum content in the barrier layers, and results in a new QC-structure designs based on the possibility to change the Al content during the MBE growth. In the first sample, we found a steady improvement of the threshold current dependence with the temperature when the Al content in the barriers is increased. The influence of indirect barrier states seems to play no significant role, which can be explained by the strong quantum confinement of the barrier states impeding carrier transport through the satellite valleys. Moreover, these results show that the GaAs/AlGaAs material system may be exploited over its entire composition range for QC lasers. Also, for a sample with AlAs “spikes” of 1–2 monolayers in the Al0.45Ga0.55As barriers, we found an improvement of 45K in the T0, depending on the structures. We also studied a new sort of injector barrier made of two different compositions of Al. The “step” in energy is chosen such that when the injector ground state aligns with the upper active region state, it provides optimal injection; at the same time it inhibits parasitic injection from the injector state to the lower states of the active region. As a result we observe a noticeable reduction of the threshold current.

Measurements of the light emission under strong magnetic field from quantum cascade lasers emitting at 9 and 11 μm are reported. The laser intensity shows strong oscillations as a function of the magnetic field. This effect is due to changes in the lifetime of the upper state of the laser transition, which is controlled by electron-optical phonon scattering. This process is strongly modified by the extra confinement imposed by a magnetic field applied perpendicular to the plane of the layers, which breaks the electron dispersion into discrete Landau levels. The experimental results are in remarkable agreement with our calculations of the phonon-limited lifetime. We also show that this experiment provides direct indications of the ratio of the scattering rates associated with the two nonradiative transitions in the active region.

The thermal behavior of AlAs/GaAs quantum cascade structures is used to demonstrate the existence of confined and interface optical-phonon modes. The high conduction-band offset of this material system greatly improves the confinement of the electronic states and allows the realization of electroluminescence structures,, where the optical power as a function of the,temperature solely depends on the change of the upper-state lifetime induced by the longitudinal-optical-phonon population., We observed that the latter is proportional to the Bose-Einstein factor, with an appropriate phonon mode energy, which does not simply correspond to that of the GaAs bulk phonon (36-meV energy). In particular we show that the agreement between theory, and experiment can be substantially improved when the confined and interface modes are taken into account in our calculation. Our results are corroborated by the threshold temperature dependence of lasers, with an active region based on similar structures.

Electron scattering with longitudinal-optical phonons is the main energy relaxation process between two-dimensional states in semiconductor heterostructures. This effect can be modified when the electron dispersion is broken into series of discrete states, similar to the energy spectrum of quantum boxes. This has been verified by applying a magnetic field parallel to the current of a quantum cascade laser. Light intensity as a function of the magnetic field shows pronounced oscillations, in excellent agreement with our calculations of the electron-phonon scattering rates between Landau levels.

In recent years, the performance of GaAs-AlGaAs-based quantum cascade (QC) lasers has improved markedly, and this material system is now an attractive choice for the fabrication of QC lasers. These devices are capable of pulsed room temperature. operation and can deliver respectable average powers (11 mW at lambda similar to 9 mum) operating on A Peltier cooler. This performance has been achieved by the suppression of thermally activated carrier leakage through increases in the heterobarrier band offset. We demonstrate that QC lasers, with wavelengths lambda > 9 pm, can operate using heterostructures encompassing the entire composition range of AlxGa1-xAs, without encountering potential problems of the satellite X-minima for x >45%. Furthermore, we present particular characteristics of these devices, such as a phonon-limited temperature dependence, electrical and optical self-oscillations, and novel design concepts that exploit this closely lattice matched material system. Finally, We discuss improvements in device fabrication to lower the operating current through a reduction of the area of current injection. Using this technology, devices can be designed to selectively pump the fundamental lateral mode. We, therefore, observe single spatial-mode operation over the entire current range of operation.

We report on the determination of thermal resistance, facet temperature profile, and heat flux of GaAs/Al0.33Ga0.67As quantum-cascade lasers operating in pulsed mode, using a microprobe band-to-band photoluminescence technique. The thermal resistance of epilayer-side mounted lasers is ∼30% smaller than that of substrate-side mounted ones. The dependence of the thermal resistance on the injection conditions and its correlation with the output power is also reported.

The measurement of thermal resistance and facet temperature profile of operating GaAs/AlGaAs quantum cascade lasers (QCLs) as a function of injected current, repetition rate and pulse width is reported. The use of microprobe band-to-band photoluminescence (PL) spectroscopy allows to achieve a spatial resolution <1 mum. Substrate-side and epilayer-side mounted devices with identical laser structures were investigated. At T = 80 K, the thermal resistance of epilayer-side mounted devices (7.8 K/W) is similar to 30% lower than that of substrate-side mounted devices, thus explaining the better performance of the former. The outcome of a two-dimensional model of heat propagation in our structures is compared with the experimental data. (C) 2001 Elsevier Science B.V. All rights reserved.

Lateral current spreading in shallow ridge processed unipolar semiconductor lasers is described using a two-dimensional flow model. In these devices, contrary to bipolar diode lasers, the density of carriers can be considered constant also in the active region. Therefore electron diffusion is a negligible effect and the spatial distribution of the current can be obtained by solving a two-dimensional differential equation for the electric potential. Our calculations prove that the major contribution to the current spreading takes place right before electrons enter the active region and is caused by the discontinuity of the conductivity at the cladding–active region interface.

We describe some key growth issues for Mid-Infrared electroluminescent devices based on a quantum-cascade design using InAs/AlSb heterostructures grown on GaSb substrates. Structural and optical properties of antimonide/arsenide interfaces are first investigated on InAs/AlSb multiple quantum well samples with different types of Sb-like interfaces and various InAs thicknesses. We show that X-ray reflectometry is a powerful complementary tool to High Resolution X-ray Diffraction (HRXRD) to extract both individual layer thicknesses and interface roughnesses using only electronic densities as input parameters. The good structural quality of samples is evidenced by the persistence of sharp high order satellite peaks on HRXRD spectra. The associated optical properties are studied by photo-induced intersubband absorption. Strong E-12 p-polarized intersubband absorptions are observed with a full-width-at-half-maximum (FWHM) around 12 meV at 77K showing good material quality. Absorption peak positions are compared to theoretical simulations based on a 2x9-band k.p calculation. These results allow us to properly design and fabricate InAs/AlSb quantum cascade light emitting devices in the 3-5 gm wavelength window taking into account the growth constraints. Well-resolved Mid-Infrared (3.7-5.3 lam) electroluminescence peaks are observed up to 300 K with FWHM to emission energy ratio (DeltaE/E) around 8%.

We demonstrate an average power of 2 mW at lambda approximate to 11.5 mum and 233 K, for a GaAs-based quantum cascade laser mounted on a Peltier cooler Furthermore, a maximum operating temperature of 285 K has also been demonstrated. These results are attributed to the excellent intrinsic thermal characteristics of these lasers and good heat management, achieved through the structural design and epi-side down mounting. Measurements show a thermal resistance of 6.7 K/W at 233 K, in good agreement with our models.

Midinfrared (3.7–5.3 μm) electroluminescent devices based on a quantum-cascade (QC) design have been demonstrated using InAs/AlSb heterostructures, grown on GaSb substrates. The very high conduction band discontinuity (>2 eV) of this material system allows the design of QC devices at very short wavelengths. Well-resolved luminescence peaks were observed up to 300 K, with a full-width-at-half-maximum to peak wavelength ratio (Δλ/λ) of the order of 8%. The emission wavelengths are in good agreement with the results of our model. The emitted optical power is lower than that predicted, due to a nonoptimized electron injection into the active region.

Summary form only given. Quantum cascade (QC) lasers have already demonstrated very encouraging results at wavelengths longer than 4 /spl mu/m. In these devices based on intersubband transitions, the highest attainable photon energy is limited by the conduction band offset /spl Delta/E/sub c/. Until now, QC devices have been demonstrated on two material systems: InGaAs/InAlAs/InP and Al/sub x/Ga/sub 1-x/As/GaAs. We propose to extend the concepts of QC lasers to InAs/AlSb heterostructures, grown strain compensated on GaSb. In this material system, the /spl Gamma/-point band discontinuity is very large (/spl Delta/E/sub c//spl cong/2.1 eV) and it is therefore possible to design active regions which cover the 3-5 /spl mu/m atmospheric window. Our QC structures were designed following a diagonal scheme. In this design, the wavefunctions of the upper and lower states of the laser transition are spatially separated. This enables to increase the selectivity of the injection of the electrons into the excited state of the transition and therefore to limit direct injection into the lower state of the active region.