We study the linear intersubband absorption spectra of a 15 nm InAs quantum well using the intersubband semiconductor Bloch equations with a three-subband model and a constant dephasing rate. We demonstrate the evolution of intersubband absorption spectral line shape as a function of temperature and electron density. Through a detailed examination of various contributions, such as the phase space filling effects, the Coulomb many-body effects and the nonparabolicity effect, we illuminate the underlying physics that shapes the spectra.
Quantum well intersubband transitions display some of the most interesting many-body effects including various collective excitations, such as intersubband plasmon and Fermi-edge singularity (repellon). To describe these various excitations and to study the effects of scatterings on the intersubband lineshape, we have performed a systematic microscopic theoretical investigation of intersubband transitions. The theory leads to a set of intersubband semiconductor Bloch equations (ISBEs) at the first order of Coulomb interaction. The extension to include the second order Coulomb interaction and LO-phonon interaction leads to optical dephasing or linewidth broadening. Using this theory, we have studied systematically the interplay of collective excitations in quantum well intersubband transitions. Our results show that such interplay leads to dramatic changes in spectral features, such as absorption peak positions and lineshape, compared with a free-particle theory. We will also show that the typical usage of the dephasing rate approximation is generally invalid for intersubband transitions. There is a strong cancellation effects between the in- (off-diagonal) and out-(diagonal) scattering terms at the second order and the out-scattering alone overestimates the linewidth significantly. Such a cancellation is much stronger for intersubband transitions than for interband case, because of the much smaller inhomogeneous broadening in the intersubband case. We also show that there is a cancellation of electron-electron and electron-phonon scatterings in their contributions to the linewidth. Finally we will show that our theory agrees very well with recent infrared absorption experiments.
We developed a method to eliminate the spurious solutions of the
k•p Hamiltonian in the envelope function approximation applied to the quantized states of heterostructures by introducing an off-diagonal k2 term. This results in a modification in the fourth and higher order terms in k of the band dispersion, which keeps the dispersion at the Γ point but modifies it at large k so that converts spurious states to the harmless evanescent ones. We show that the modification to the Hamiltonian leads to the monotonic behavior of the conduction band as a function of k and thus removes the spurious solutions in the calculations of confined states for all popular III-V compounds and their alloys.
Intersubband absorption spectra are analyzed using the density matrix theory under the second Born approximation. The intersubband semiconductor Bloch equations are derived from the first principles including electron-electron and electron-longitudinal optical phonon interactions, whereas electron-interface roughness scattering is considered using Ando's theory. A spurious-states-free 8-band
k•p Hamiltonian is used, in conjunction with the envelope function approximation to calculate the electronic band structure self-consistently for type II InAs/AlSb multiple quantum well structures. We demonstrate the interplay of various physical processes in the absorption spectra in the mid-infrared frequency range.
Electron and hole diffusions in the plane of semiconductor quantum wells play an important part in the static and dynamic operations of semiconductor lasers. It is well known that the value of diffusion coefficients affects the threshold pumping current of a semiconductor laser. At the same time, the strength of carrier diffusion process is expected to affect the modulation bandwidth of an AC-modulated laser. It is important not only to investigate the combined DC and AC effects due to carrier diffusion, but also to separate the AC effects from that of the combined effects in order to provide design insights for high speed modulation. In this presentation, we apply a hydrodynamic model developed by the present authors recently from the semiconductor Bloch equations. The model allows microscopic calculation of the lateral carrier diffusion coefficient, which is a nonlinear function of the carrier density and plasma temperature. We first studied combined AC and DC effects of lateral carrier diffusion by studying the bandwidth dependence on diffusion coefficient at a given DC current under small signal modulation. The results show an increase of modulation bandwidth with decrease in the diffusion coefficient. We simultaneously studied the effects of nonlinearity in the diffusion coefficient. To clearly identify how much of the bandwidth increase is a result of decrease in the threshold pumping current for smaller diffusion coefficient, thus an effective increase of DC pumping, we study the bandwidth dependence on diffusion coefficient at a given relative pumping. A detailed comparison of the two cases will be presented.
Electron and hole diffusions in the plane of semiconductor quantum wells play an important part in the static and dynamic operations of semiconductor lasers. In this paper, we apply a hydrodynamic model developed from the semiconductor Bloch equations to numerically study the effects of nonlinearity in the diffusion coefficient on single mode operation and direct modulation of a gain-guided InGaAs/GaAs multiple quantum well laser, operating not too far from threshold. We found that a small diffusion coefficient is advantageous for lowering the threshold current and increasing the modulation bandwidth. Most importantly, the effects of nonlinearity in the coefficient can be approximately reproduced by replacing the coefficient with an effective constant diffusion coefficient, which corresponds roughly to the half height density of the density distribution. This conclusion is the same as in Ref. 8, but we will discuss the disagreements in reaching it. Finally, the beam profile is slightly modified in the nonlinear case.
Carrier diffusion and thermal conduction play a fundamental role in the operation of high-power, broad-area semiconductor lasers. Restricted geometry, high pumping level and dynamic instability lead to inhomogeneous spatial distribution of plasma density, temperature, as well as light field, due to strong light-matter interaction. Thus, modeling and simulation of such optoelectronic devices rely on detailed descriptions of carrier dynamics and energy transport in the system. A self-consistent description of lasing and heating in large-aperture, spatially-inhomogeneous edge- or surface-emitting lasers (VCSELs) require coupled diffusion equations for carrier density and temperature. In this paper, we derive such equations from the Boltzmann transport equation for the carrier distributions. The derived self- and mutual-diffusion coefficients are in general nonlinear functions of carrier density and temperature including many-body interactions. We study the effects of many-body interactions on these coefficients, as well as the nonlinearity of these coefficients for large-area VCSELs. The effects of mutual diffusions on carrier and temperature distributions in gain-guided VCSELs will be also presented.
Many-body Coulomb effects influence the operation of quantum- well (QW) laser diode (LD) strongly. In the present work, we study a two-band electron-hole plasma (EHP) within the Hatree- Fock approximation and the single plasmon pole approximation for static screening. Full inclusion of momentum dependence in the many-body effects is considered. An empirical expression for carrier density dependence of the bandgap renormalization (BGR) in an 8 nm GaAs/Al0.3Ga0.7As single QW will be given, which demonstrates a non-universal scaling behavior for quasi-two-dimension structures, due to size-dependent efficiency of screening. In addition, effective mass renormalization (EMR) due to momentum-dependent self-energy many-body correction, for both electrons and holes, is studied and serves as another manifestation of the many-body effects. Finally, the effects on carrier density dependence of the alpha factor is evaluated to assess the sensitivity of the full inclusion of momentum dependence.
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