Search

A significant enhancement in the output power of a GaAs-based terahertz quantum cascade laser (THz QCL) was achieved by implementing a relatively high impurity doping concentration. The QC structure was precisely designed using the nonequilibrium green function method by considering the band bending effect caused by a higher doping concentration. This enabled us to avoid the subband misalignment induced by the strong band bending effect, to provide efficient carrier transport even at high doping concentrations, and to improve the output power. The maximum output power of a GaAs/Al0.16Ga0.84As 3.5 THz-band QCL at 10 K was enhanced from 390 to 600 mW by increasing the doping concentration at the depopulation layers from 6 × 1016 to 1.2 × 1017 cm−3. The average power of 21 mW was achieved by implementing the appropriate pulse drive conditions. [ABSTRACT FROM AUTHOR]

We present a systematic comparison of the results from our non-equilibrium Green's function formalism with a large number of AlGaAs-GaAs terahertz quantum cascade lasers previously published in the literature. Employing identical material and simulation parameters for all samples, we observe that the discrepancies between measured and calculated peak currents are similar for samples from a given group. This suggests that the differences between experiment and theory are partly due to a lacking reproducibility for devices fabricated at different laboratories. Varying the interface roughness height for different devices, we find that the peak current under lasing operation hardly changes, so that differences in interface quality appear not to be the sole reason for the lacking reproducibility. [ABSTRACT FROM AUTHOR]

A novel surface emitting distributed feedback quantum cascade laser emitting around λ 4.6 µm is demonstrated by employing an equivalent phase shift (EPS) of quarter-wave (λ/4). The EPS is fabricated through extending one sampling period by 50% in the center of a sampled Bragg grating. Single-lobed far-field radiation pattern with a low divergence angle of about 0.6º x 16.8º is obtained. Selective single-mode lasing with a mean side mode suppression ratio above 20 dB and wavelength coverage range of 72 nm is achieved simultaneously on a single wafer only by changing the sampling period. [ABSTRACT FROM AUTHOR]

In this work, the electrons effective masses in double barrier resonant tunneling diodes fabricated on the InGaAs/GaAsSb material system are investigated by magnetotunneling experiments. It is found that due to the nonparabolic band structure in this material system, the electron effective masses increase significantly on samples with smaller well width and higher resonance energies. Surprisingly, a decreasing effective mass is observed with increasing Landau levels index. This mass decrease can be explained in terms of the quantum confined Stark effect influencing the resonant level positions inside the resonant tunneling diodes. [ABSTRACT FROM AUTHOR]

Terahertz time-domain spectroscopy is shown to provide a convenient and rapid means to measure the conductivity of individual layers in semiconductor heterostructures such as terahertz quantum cascade lasers. By modeling the complex transmission at terahertz frequencies, the electron density and the in-plane momentum scattering time of the active regions and doped contact layers were determined for both GaAs/AlGaAs and InGaAs/InAlAs epilayers. The measured temperature dependence of the electron scattering rate revealed the significance of impurity and LO phonon scattering. The implications for laser operation at room temperature are discussed by considering the changes in absorption and resonant tunneling current with temperature. [ABSTRACT FROM AUTHOR]

We present the design and fabrication of photonic crystal (PhC) based resonators for terahertz quantum-cascade laser with a gain maximum at 2.7 THz. The PhC provides the confinement in lateral direction, for the vertical confinement a double-metal waveguide is used. We show theoretical and experimental analyses of devices based on a central gain region, which is surrounded by a PhC mirror. The devices are lasing in the bandgaps or at high-symmetry points of the PhC, depending on the period. These concepts allow us to tune the emission of the lasers in the range of a few hundred gigahertz. [ABSTRACT FROM AUTHOR]

A midinfrared quantum cascade laser with Mach–Zehnder cavity and split contacts is investigated with respect to interference effects. By increasing the temperature in one of the two coupled active waveguides, the value of the effective refractive index is varied and the modal phase is shifted. As a result, destructive interference is observed within the resonator, which manifests itself in a minimum of the modulated output power. The dissipated heat is controlled by locally adding a continuous current to the drive current pulses. In the first step, thermal properties, threshold values, and far fields are analyzed and compared to a Fabry–Pérot resonator to gain insight into the physical principles of the monolithic interferometer. Based on these findings, the temperature distribution is calculated in a two-dimensional heat transfer simulation, which leads to a match between the thermal change of the effective refractive index and the condition for destructive interference; a phase shift of π between the two interfering beams is confirmed. By modulating the effective refractive index using evanescent fields instead of temperature variations, a monolithic midinfrared interferometric sensing device becomes feasible. [ABSTRACT FROM AUTHOR]

The effect of doping concentration on the performance of short-wavelength quantum-cascade lasers based on the strain-compensated InGaAs/InAlAs/AlAs heterostructure on InP, emitting at 3.8 μm, is investigated for average doping concentrations between 0.3 and 3.9×1017 cm-3 (sheet densities between 1.6 and 20.9×1011 cm-2). Although the threshold current density is rather independent of doping concentration, the maximum current density increases with doping and exhibits a saturation for the highest doping level. Other important performance characteristics such as differential quantum efficiency, peak optical emission power, slope efficiency, and maximum operating temperature are observed to be maximized for structures with an average doping of 2-3×1017 cm-3, corresponding to a sheet density of about 1.5×1012 cm-2. [ABSTRACT FROM AUTHOR]

We present the design and simulation of a GaAs-based quantum cascade laser (QCL) emitting at 6.7 μm, the shortest room-temperature lasing wavelength projected to date for GaAs-based QCLs. This is achieved by introducing compressive strain only in the active quantum wells, where the optical transition occurs. A Monte Carlo simulation including both Γ- and X-valley transport demonstrates that the proposed QCL achieves room-temperature lasing at a threshold-current density of 14 kA/cm2, lower than that of the conventional 9.4 μm QCL (16.7 kA/cm2). Furthermore, the electron temperature at 300 K lattice temperature is similar to that of the 9.4 μm device. [ABSTRACT FROM AUTHOR]

We report the design of an active region of InxGa1-xAs1-yNy/AlAs quantum cascade laser structure emitting in the near infrared wavelength range based on an eight-band k·p model. The InxGa1-xAs1-yNy/AlAs heterostructure system is of significant interest for the development of short wavelength quantum cascade lasers due to its large conduction band discontinuity (∼1.5 eV) and compatibility with the mature GaAs fabrication process. A detailed analysis of the intersubband transition energy within the conduction band as a function of layer thickness, composition, electric field, and temperature has been carried out. Finally, an optimized combination of In0.2Ga0.8As0.97N0.03/AlAs three-coupled-well structure has been obtained. Under an applied field of 100 kV/cm and at room temperature, a shortest wavelength of 3.4 μm has been achieved by making use of this system without introducing an upper lasing level beyond the X minima of the AlAs barrier. [ABSTRACT FROM AUTHOR]

We present a detailed Monte Carlo simulation of electron transport incorporating both Γ- and X-valley states in GaAs-based quantum cascade lasers (QCLs). Γ states are calculated using the K·p method, while X states are obtained within the effective mass framework. All the relevant electron-phonon, electron-electron, and intervalley scattering mechanisms are included. We investigate the X-valley leakage in two equivalent-design GaAs/AlGaAs QCLs with 33% and 45% Al-barrier compositions. We find that the dominant X-valley leakage path in both laser structures is through interstage X→X intervalley scattering, leading to a parallel leakage current JX. The magnitude of JX depends on the temperature and occupation of the X subbands, which are populated primarily by the same-stage scattering from the Γ-continuum (Γc) states. At 77 K, JX is small up to very high fields in both QCLs. However, at room temperature the 33% QCL shows a much higher JX than the 45% QCL even at low fields. The reason is that in the 33% QCL the coupling between the Γ-localized (Γl) states and the next-stage Γc states is strong, which facilitates subsequent filling of the X states through efficient intrastage Γc→X scattering; with high X-valley population and high temperature, efficient interstage X→X scattering yields a large JX. In contrast, good localization of the Γl states in the 45% QCL ultimately leads to low X-valley leakage current up to high fields. Very good agreement with experiment is obtained at both cryogenic and room temperatures. [ABSTRACT FROM AUTHOR]

We present an advanced technique for the design and optimization of GaAs/AlGaAs quantum cascade laser structures. It is based on the implementation of the simulated annealing algorithm with the purpose of determining a set of design parameters that satisfy predefined conditions, leading to an enhancement of the device output characteristics. Two important design aspects have been addressed: improved thermal behavior, achieved by the use of higher conduction band offset materials, and a more efficient extraction mechanism, realized via a ladder of three lower laser states, with subsequent pairs separated by the optical phonon energy. A detailed analysis of performance of the obtained structures is carried out within a full self-consistent rate equations model of the carrier dynamics. The latter uses wave functions calculated by the transfer matrix method, and evaluates all relevant carrier–phonon and carrier–carrier scattering rates from each quantized state to all others within the same and neighboring periods of the cascade. These values are then used to form a set of rate equations for the carrier density in each state, enabling further calculation of the current density and gain as a function of the applied field and temperature. This paper addresses the application of the described procedure to the design of λ∼9 μm GaAs-based mid-infrared quantum cascade lasers and presents the output characteristics of some of the designed optimized structures. [ABSTRACT FROM AUTHOR]

A broadband midinfrared laser operating up to 340 K in pulsed mode is presented. It was realized by incorporating seven different quantum cascade active regions with 0.2 μm increments to emit at different wavelengths. Despite the relatively large wavelength increments, a continuous lasing spectrum is obtained from 7.7 to 8.4 μm, with significant improvements in threshold current and temperature performance compared with previously reported broadband quantum cascade lasers. A full temperature dependent characterization was performed, and the threshold current density at 300 K was 6.5 kA/cm2, with peak output power of over 100 mW. A characteristic temperature of T0=192 K was obtained. [ABSTRACT FROM AUTHOR]