Effect In Quantum Cascade Lasers Research Articles

Frequency combs induced by optical feedback and harmonic order tunability in quantum cascade lasers

This study investigates the interaction between frequency combs and optical feedback effects in Quantum Cascade Lasers (QCLs). The theoretical analysis reveals new phenomena arising from the interplay between comb generation and feedback. By considering the bias current corresponding to free-running single mode emission, the introduction of optical feedback can trigger the generation of frequency combs, including both fundamental and harmonic combs. This presents opportunities to extend the comb region and generate harmonic frequency combs with different orders through optimization of external cavity parameters, such as losses and length. Furthermore, this study demonstrates that optical feedback can selectively tune the harmonic order of a pre-existing free-running comb by adjusting the external cavity length, particularly for feedback ratios around 1%, which are readily achievable in experimental setups. Under strong feedback conditions (Acket parameter C > 4.6), mixed states emerge, displaying the features of both laser and external cavity dynamics. While this study is predominantly centered on terahertz QCLs, we have also confirmed that the described phenomena occur when utilizing mid-infrared QCL parameters. This work establishes a connection between comb technology and the utilization of optical feedback, providing new avenues for exploration and advancement in the field. In fact, the novel reported phenomena open a pathway toward new methodologies across various domains, such as the design of tunable comb sources, hyperspectral imaging, multi-mode coherent sensing, and multi-channel communication.

Sensitive dependence of the linewidth enhancement factor on electronic quantum effects in quantum cascade lasers

The linewidth enhancement factor (LEF) describes the coupling between amplitude and phase fluctuations in a semiconductor laser and has recently been shown to be a crucial component for frequency comb formation in addition to linewidth broadening. It necessarily arises from causality, as famously formulated by the Kramers–Kronig relation, in media with nontrivial dependence of the susceptibility on intensity variations. While thermal contributions are typically slow, and thus can often be excluded by suitably designing the dynamics of an experiment, the many quantum contributions are harder to separate. In order to understand and, ultimately, design the LEF to suitable values for frequency comb formation, soliton generation, or narrow laser linewidth, it is, therefore, important to systematically model all these effects. In this comprehensive work, we introduce a general scheme for computing the LEF, which we employ with a nonequilibrium Green's function model. This direct method, based on simulating the system response under varying optical intensity and extracting the dependence of the susceptibility to intensity fluctuations, can include all relevant electronic effects and predicts the LEF of an operating quantum cascade laser to be in the range of 0.1–1, depending on laser bias and frequency. We also confirm that many-body effects, off-resonant transitions, dispersive (Bloch) gain, counter-rotating terms, intensity-dependent transition energy, and precise subband distributions all significantly contribute and are important for accurate simulations of the LEF.

Simulation of nonequilibrium effects in quantum cascade lasers

AbstractQuantum cascade lasers exploit optical transitions between quantized electronic energy levels in multi‐quantum‐well structures for light generation and detection. This design principle enables the realization of compact, semiconductor‐based lasers in the mid‐infrared and terahertz spectral regions. In this contribution, the modeling of such devices is discussed based on an ensemble Monte Carlo approach, enhanced by modifications obtained from the density matrix formalism. A special emphasis is put on the inclusion of nonequilibrium electron and phonon effects. The simulations are validated against available experimental data for a terahertz quantum cascade laser.

Two-band finite difference method for the bandstructure calculation with nonparabolicity effects in quantum cascade lasers

We present a two-band finite difference method for the bandstructure calculation of quantum cascade lasers (QCLs) based on the equivalent two-band model of the nonparabolic Schrödinger equation. Particular backward and forward difference forms are employed in the discretization procedure instead of the common central difference form. In comparison with the linearization approach of the nonparabolic Schrödinger equation, the method is as accurate and reliable as the linearization approach, while the velocity of the method is faster and the matrix elements are more concise, therefore making the method more practical for QCLs simulations.

Thermal effects in quantum cascade lasers at λ∼4.6 μm under pulsed and continuous-wave modes

The thermal effects in InGaAs/InAlAs quantum cascade lasers (QCLs) emitting at λ∼4.6 μm under pulsed and continuous-wave (CW) modes using a three-dimensional (3D) heat dissipation model were investigated. Based on the experimentally measured results, the thermal characteristics were theoretically analyzed for various device and heatsinking structures. Also, the heat accumulation effects and dissipation processes were studied in detail under pulsed operation. High cooling efficiencies were achieved by a relatively fast heat diffusion rate from the active core region for the epilayer-down bonded single ridge waveguide buried heterostructure (BH) with a thick electroplated Au around the laser ridge. A further improvement was made by the use of InP embedding layer. In CW mode, the thermal conductance (G th) value of 445 W/(K cm2) at 298 K was obtained for the epilayer-down bonded double-channel ridge waveguide QCL with AlN submount, which indicates a reasonable consistency with the available experimental data. By optimizing the device and heatsinking structures, the G th was improved to a high value of 673 W/(K cm2) at 298 K for the epilayer-down bonded single ridge waveguide BH QCL with InP embedding layer on diamond submount in CW mode.

Impact of momentum dependent matrix elements on scattering effects in quantum cascade lasers

AbstractResults for our fully frequency‐ and wavevector‐dependend self‐consistent nonequilibrium Green's function simulations of electron transport through quantum cascade lasers are compared to those from a simplified approach where scattering rates are only evaluated at representative ‘typical values’ of the in‐plane momentum due to the complexity of the underlying many‐body problem and the limited computational resources available. We find a noticeable impact on electronic lineshapes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of optical Kerr effect in quantum-cascade lasers with multiple resonance levels

In this paper, we investigated the optical Kerr lensing effect in quantum-cascade lasers with multiple resonance levels. The Kerr refractive index n2 is obtained through the third-order susceptibility at the fundamental frequency chi(3)( omega; omega, omega,-omega). Resonant two-photon processes are found to have almost equal contributions to chi(3)( omega; omega, omega,-omega) as the single-photon processes, which result in the predicted enhancement of the positive nonlinear (Kerr) refractive index, and thus may enhance mode-locking of quantum-cascade lasers. Moreover, we also demonstrate an isospectral optimization strategy for further improving n2 through the band-structure design, in order to boost the multimode performance of quantum-cascade lasers. Simulation results show that the optimized stepwise multiple-quantum-well structure has n2 approximately 10-8 cm2/W, a twofold enhancement over the original flat quantum-well structure. This leads to a refractive-index change (delta)n of about 0.01, which is at the upper bound of those reported for typical Kerr medium. This stronger Kerr refractive index may be important for quantum-cascade lasers ultimately to demonstrate self-mode-locking.

Investigation of Thermal Effects in Quantum-Cascade Lasers

The development of a thermal model for quantum cascade lasers (QCLs) is presented. The model is used in conjunction with a self-consistent scattering rate calculation of the electron dynamics of an InGaAs-AlAsSb QCL to calculate the temperature distribution throughout the device which can be a limiting factor for high temperature operation. The model is used to investigate the effects of various driving conditions and device geometries, such as epilayer down bonding and buried heterostructures, on the active region temperature. It is found that buried heterostructures have a factor of eight decrease in thermal time constants compared to standard ridge waveguide structures in pulsed mode and allow a /spl sim/78% increase in heat sink temperature compared to epilayer down mounted devices in continuous-wave mode. The model presented provides a valuable tool for understanding the thermal dynamics inside a quantum cascade laser and will help to improve their operating temperatures.

Influence of longitudinal electric field on the hot-phonon effect in quantum wells

Using the Huang-Zhu model [K. Huang and B.-F. Zhu, Phys. Rev. B 38, 13377 (1988)] for the optical phonons and associated carrier-phonon interactions in semiconductor superlattices, the effects of longitudinal electric field on the energy-loss rates (ELRs) of hot carriers as well as on the hot-phonon effect (HPE) in GaAs∕AlAs quantum wells (QWs) are studied systematically. Contributions of various bulklike and interface phonons to the hot-carrier relaxation are compared in detail, and comprehensively analyzed in relation to the intrasubband and intersubband scatterings for quantum cascade lasers. Due to the broken parity of the electron (hole) states in the electric field, the bulklike modes with antisymmetric potentials are allowed in the intrasubband relaxation processes, as well as the modes with symmetric potentials. As the interface phonon scattering is strong only in narrow wells, in which the electric field affects the electron (hole) states little, the ELRs of hot carriers through the interface phonon scattering are not sensitive to the electric field. The HPE on the hot-carrier relaxation process in the medium and wide wells is reduced by the electric field. The influence of the electric field on the hot-phonon effect in quantum cascade lasers is negligible. When the HPE is ignored, the ELRs of hot electrons in wide QWs are decreased noticeably by the electric field, but slightly increased by the field when considering the HPE. In contrast with the electrons, the ELRs of hot holes in wide wells are increased by the field, irrespective of the HPE.