Extended density matrix model applied to tall barrier quantum cascade lasers

Quantum cascade lasers (QCLs) are promising sources of terahertz (THz) radiation that have applications such as security and medical screening. While optical output power has recently exceeded 1 W, their highest operating temperature is currently limited to ~200 K due to mechanisms such as thermal back filling and non-radiative phonon emission between lasing states. Another possible cause of performance degradation is parasitic leakage currents over barriers into continuum states as subband electron temperatures increase with lattice temperature. Novel designs with new injection schemes remain an intensive research area and new efforts are being made assuming that barrier heights no longer need to be constant. A possible advantage of this is using tall barriers to reduce the leakage current, and in this work we present a theoretical study of recent experimental evidence supporting this. Interface roughness (IFR) scattering scales with the conduction band discontinuity squared and the calculations also assume a typical correlation length Λ and root mean roughness value Δ which are related to growth quality of the individual sample. We take typical values of Λ=60 Å and Δ=3 Å for these parameters. The QCL gain and current output characteristics are calculated using an extended density matrix solver which models transport through the injection barrier coherently. We obtain similar current and gain values at resonance for both structures, indicating that the experimentally observed reduction in current density could be accredited to the reduction of parasitic current leakage. Additionally, this work attempted a similar design with all AlAs barriers which did not lase and it was conjectured that this was due to excessive IFR scattering as well as increased susceptibility to monolayer fluctuations with thinner layers. Our model, which accounts for the lifetime broadening in the gain calculation, confirms that modifying the IFR parameters to Λ=100 Å and Δ=1 Å (i.e. unrealistically sharp interfaces) leads to a significant improvement in performance as shown in Figure 1. We extend this work by proposing designs which aim to balance leakage current reduction and excessive scattering to achieve higher operating temperatures

Influence of barrier height on interface roughness scattering and coherent transport in AlGaAs quantum cascade lasers

Quantum cascade lasers (QCLs) are promising sources of terahertz (THz) radiation that have applications such as security and medical screening. While optical output power has recently exceeded 1 W, their highest operating temperature is currently limited to ~200 K due to mechanisms such as thermal back filling and non-radiative phonon emission between lasing states. Another possible cause of performance degradation is parasitic leakage currents over barriers into continuum states as subband electron temperatures increase with lattice temperature. Novel designs with new injection schemes remain an intensive research area and new efforts are being made assuming that barrier heights no longer need to be constant. A possible advantage of this is using tall barriers to reduce the leakage current, and in this work we present a theoretical study of the effects of increased barrier heights on transport between states in the structure. Similar to previous efforts, we initially restrict the modification of barrier height to the injection barrier; these are typically the thickest in THz QCLs and allow the reduced barrier widths necessary for AlAs barriers to remain above 1 ML

Theory and design of quantum cascade lasers in (111) n-type Si/SiGe

Although most work towards the realization of group IV quantum cascade lasers (QCLs) has focused on valence band transitions, there are many desirable properties associated with the conduction band. We show that the commonly cited shortcomings of n-type Si/SiGe heterostructures can be overcome by moving to the (111) growth direction. Specifically, a large band offset and low effective mass are achievable and subband degeneracy is preserved. We predict net gain up to lattice temperatures of 90 K in a bound-to-continuum QCL with a double-metal waveguide, and show that a Ge interdiffusion length of at least 8 Å across interfaces is tolerable

A scattering rate approach to the understanding of absorption line broadening in near-infrared AlGaN/GaN quantum wells

There has been much interest in the advancement of III-Nitride growth technology to fabricate AlGaN/GaN heterostructures for intersubband transitions (ISBTs). The large conduction band offset in these structures (up to 2 eV) allows transition energies in the near- to the far-infrared region, which have applications from telecommunications, such as in all-optical switches, to infra-red detectors for sensing and imaging. To date, ISBT electroluminescence has been elusive and absorption measurements remain an important method to verify band structure calculations. The growth quality can be inferred from the absorption spectrum, which will have line broadening with contributions that are both inhomogeneous (large-scale interface roughness, and non-parabolicity) and homogeneous (electron scattering related lifetime broadening). In the present work we calculated the contributions of various homogeneous broadening mechanisms (electron interaction with longitudinal-optical (LO) phonons, acoustic phonons, impurities and alloy disorder) to the full linewidth, and also the contribution of band non-parabolicity, which contributes to the inhomogeneous broadening. Calculations are then compared to the measured absorption spectra of several samples

Posterior epistaxis: endonasal exposure and occlusion of the branches of the sphenopalatine artery

Intractable posterior epistaxis (PE) is a frequent emergency for which different treatment modalities are available. While nasal packing causes extreme discomfort and angiography with consecutive selective embolization is not available everywhere, recent studies emphasize the value of sphenopalatine artery (SPA) occlusion by different techniques and indicate success rates of 13-33%. In our institution, previously endoscopic management of PE consisted either of isolated coagulation of an identified bleeding source (group A) or cutting and coagulation of arterial branches running through the sphenopalatine foramen (SPF) (group B). According to our neuroradiological and rhinological experience we developed a modification of SPA transsection and coagulation following identification of the division in conchal and septal branches of the SPA (group C). During a 26-month period the success rates of these three techniques in 95 patients were compared prospectively. The three modalities revealed a re-bleeding rate of 3 out of 21 (21%) in group A, 1 in 6 (16.7%) in group B and 3 in 69 (4.3%) in group C. At the level of the SPF, 36 out of 69 patients had one conchal branch, whereas 30 (43.5%) had two and 3 (4.4%) had three. If SPA transsection and coagulation for intractable PE is adopted the anatomic varieties of the SPA with its division in conchal and septal branches have to be taken into account. According to our experience the septal branch of the SPA plays a major role in PE. Its occlusion significantly improves the success rate of PE treatment

Correction to “Temperature-Dependent High-Speed Dynamics of Terahertz Quantum Cascade Lasers”

Corrections to author affiliation information is presented in the above named paper

Mid-infrared entangled photon generation in optimised asymmetric semiconductor quantum wells

The optimal design of asymmetric quantum well structures for generation of entangled photons in the mid-infrared range by spontaneous parametric down- conversion is considered, and the efficiency of this process is estimated. Calcu- lations show that a reasonably good degree of entanglement can be obtained, and that the optical interaction length required for optimal conversion is very short, in the few μm range

Correction to "Mode Selection and Tuning Mechanisms in Coupled-Cavity Terahertz Quantum Cascade Lasers"

In [1], the affiliation for Andrew Grier was incorrect. The correct affiliation where his contribution was made is as follows: A. T. Grier was with the School of Electronic and Electrical Engineering, University of Leeds, LS2 9JT Leeds, U.K. (e-mail: [email protected])

Design considerations for GaN/AlN based unipolar (opto-)electronic devices, and interface quality aspects

We describe the theoretical and experimental studies of GaN/AlGaN based resonant tunnelling diodes, and in particular analyse the effects and typical values of interface roughness, and then discuss the implications of these, realistic material quality parameters on performance of unipolar optoelectronic devices

Mode selection and tuning mechanisms in coupled-cavity terahertz quantum cascade lasers

We present a model for longitudinal mode competition in coupled-cavity (CC) terahertz (THz) quantum cascade lasers (QCLs) by using a scattering matrix method and multi-mode reduced rate equations (RREs). The dependence of the mode selection and tuning characteristics on various device parameters are systematically investigated, including the net waveguide loss, the optical length of the passive cavity, and the heat sink temperature for different relationship between the active and passive cavity lengths. The changes in eigenmode frequencies due to variations of device parameter are calculated before solving the RREs. The mode selection and tuning results obtained from solving the nonlinear RREs could be well explained by linear scattering matrix analysis. The mode tuning process simulated by the proposed model is compared with experimentally measured data, yielding good agreement. Comprehensive study of the influence of the key device parameters on the performance of CC THz QCLs provides potential design rules for single-mode operation with either wide frequency tunability or high stability