Processes In Quantum Wells Research Articles

Wurtzite quantum well structures under high pressure

Quantum well systems based on semiconductors with the wurtzite crystalline structure have found widespread applications in photonics and optoelectronic devices, such as light-emitting diodes, laser diodes, or single-photon emitters. In these structures, the radiative recombination processes can be affected by (i) the presence of strain and polarization-induced electric fields, (ii) quantum well thickness fluctuations and blurring of a well–barrier interface, and (iii) the presence of dislocations and native point defects (intentional and unintentional impurities). A separate investigation of these phenomena is not straightforward since they give rise to similar effects, such as a decrease of luminescence efficiency and decay rate, enhancement of the Stokes shift, and strong blueshift of the emission with increasing pump intensity. In this Perspective article, we review the usefulness of measurements of the quantum well luminescence as a function of the hydrostatic pressure for both scientific research and the development of light-emitting technologies. The results presented here show that high-pressure investigations combined with ab initio calculations can identify the nature of optical transitions and the main physical factors affecting the radiative efficiency in quantum well systems. Finally, we will discuss an outlook to the further possibilities to gain new knowledge about the nature of recombination processes in quantum wells using high-pressure spectroscopy.

Systematic investigation of effects of exciton–acoustic-phonon scattering on photoluminescence rise times of free excitons in GaAs/Al0.3Ga0.7As single quantum wells

We have systematically investigated the photoluminescence (PL) dynamics of free excitons in GaAs/Al0.3Ga0.7As single quantum wells, focusing on the energy relaxation process due to exciton–acoustic-phonon scattering under non-resonant and weak excitation conditions as a function of GaAs-layer thickness from 3.6 to 12.0 nm and temperature from 30 to 50 K. The free exciton characteristics were confirmed by observation that the PL decay time has a linear dependence with temperature. We found that the free exciton PL rise rate, which is the reciprocal of the rise time, is inversely linear with the GaAs-layer thickness and linear with temperature. This is consistent with a reported theoretical study of the exciton–acoustic-phonon scattering rate in the energy relaxation process in quantum wells. Consequently, it is conclusively verified that the PL rise rate is dominated by the exciton–acoustic-phonon scattering rate. In addition, from quantitative analysis of the GaAs-layer thickness and temperature dependences, we suggest that the PL rise rate reflects the number of exciton–acoustic-phonon scattering events.

Quasiphase matching of second-harmonic generation in quantum cascade lasers by Stark shift of electronic resonances

We demonstrate a quasiphase matching scheme for second-harmonic generation in quantum cascade lasers with integrated resonant nonlinearity. Modulation of the nonlinear susceptibility is achieved by the periodic modulation of the bias voltage along the ridge waveguide leading to a periodic shift of electronic resonances and a change in the electron population in different energy levels. An up to tenfold enhancement of the conversion efficiency is observed. This technique is applicable to any resonant nonlinear optical process in quantum wells.

Population inversion and IR amplification induced by intersubband electron transitions and resonant Auger processes in quantum wells

A mechanism for electron population inversion and mid-IR amplification is proposed for the case of the current or optical injection of electron-hole pairs into the undoped region of a heterojunction with quantum wells. The presence of an upper long-lived size-quantization level and resonant Auger recombination in the well are crucial features of the mechanism. A long electron lifetime at the upper level or a relatively low probability of electron scattering to the other subbands is achieved by choosing the shape of the well and its parameters in such a way as to provide weak overlap between the upper level and two lower level electronic wave functions. Resonant Auger recombination plays a positive role. It stabilizes the electron and hole concentrations at lower levels and makes a substantial contribution to the excitation of the upper electronic level and the population inversion. The degree of population inversion and the gain coefficient are estimated.

Studies of carrier relaxation in low dimensional structures

We have made theoretical and experimental studies of the carrier relaxation process in quantum wells and quantum wires. Monte Carlo studies of the relaxation process in wells and wires indicate that the relaxation times can be very large in narrow structures. Femtosecond differential transmission spectra measurements on similar structures grown by molecular beam epitaxy give relaxation times in good agreement with the theoretically calculated values.

Recombination processes and photoluminescence intensity in quantum wells under steady-state and transient conditions.

We present a theoretical and experimental study of recombination processes in quantum wells. Our model calculations, which include free-excitonic, free-carrier, and defect-mediated nonradiative recombination, describe the dependence of the photoluminescence intensity on excitation density under steady-state conditions and on time under transient conditions. For the former conditions, it is not, in principle, possible to distinguish between excitonic and free-carrier contributions to the photoluminescence intensity. However, an accurate determination of the relative weight of nonradiative and radiative contributions can be made. For transient conditions, on the other hand, excitonic contributions may be identified, while the discrimination between radiative and nonradiative contributions is---though possible---possessed with ambiguities. We finally apply our model to a set of experimental data, taken at 300 K from a single ${\mathrm{In}}_{0.1}$${\mathrm{Ga}}_{0.9}$As/${\mathrm{Al}}_{0.33}$${\mathrm{Ga}}_{0.67}$As quantum well under both steady-state and transient conditions. The results of this analysis demonstrate the consistency of our model and its potential for the quantitative understanding of the recombination dynamics in quantum wells.

Influence of disorder and exciton‐exciton interactions four‐wave mixing in quantum wells

AbstractThe interplay of disorder effects and exciton‐exciton interactions in the four‐wave‐mixing process in quantum wells is investigated. Experiments are performed employing femtosecond and picosecond pulses to have either good temporal or good spectral resolution. It is shown that the localisation of excitons is of primary importance in the generation of the nonlinear signal. It is shown which states contribute and how to the FWM process.

Optical Processes in Quantum Wells

A review is given of the theoretical framework of exciton dynamics in quantum wells including the spin degrees of freedom. A study is made of various momentum, energy, and spin relaxation mechanisms including the effects of exciton-phonon interaction, the single-particle spin-flips by means of spin—orbit interaction and the exciton spin-ffip by means of the exchange interaction. All these competing mechanisms are taken into account in a set of equations governing the time evolution of the exciton spin populations. Solutions are then used to interpret observed time-resolved observations of polarized luminescence spectra. For excitons in a two-dimensional system such as a semiconductor quantum well, the breaking of the translational symmetry in the direction normal to the interface plane has been shown theoretically by Hanamura, by Andreani and Bassani, and by Citrin to result in a reckbination rate much faster than in a three-dimensional system. Yet, experiments show ckparable decay rates in two- and three-dimensional excitons. Recent experiments with high time resolutions show two decay times for the total luminescence intensity. The slower one agrees witl the one usually observed and interpreted as the radiative recombination time. We shall give an explanation for the fast decay as a ckbination of radiative recombination and single-particle spin-flip and for the slow decay as the radiative recombination slowed down by the presence of lower energy dark states for excitons with parallel spins. The ability to use the same theory to account for the polarization behavior confirms the importance of the exciton spin dynamics. Furthermore, the longitudinal electric field dependence is used to check our theory of exchange.

Exciton dynamics in GaAs quantum wells under resonant excitation.

We present a comprehensive investigation of the dynamics of resonantly excited nonthermal excitons in high-quality GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple-quantum-well structures on picosecond time scales. The dynamics was investigated using the luminescence upconversion technique with two independently tunable, synchronized dye lasers, which allowed measurements of the time evolution of polarized resonant luminescence with 4-ps time resolution. We show that the evolution of excitons from the initial nonthermal distribution to the thermal regime is determined by three different physical processes: (1) the enhanced radiative recombination of the metastable two-dimensional exciton polaritons, (2) the spin relaxation of excitons, and (3) the momentum relaxation of excitons. We also show that these three processes have comparable rates, so that a unified model accounting for all important processes is essential for a correct analysis of the experimental results. Using such a unified model, we have determined the rates of these processes contributing to the initial relaxation of excitons as a function of quantum-well width, temperature, and applied electric field. Quantum confinement strongly influences the radiative recombination and spin relaxation of excitons, and our study provides significant insights into these processes in quantum wells. The measured radiative recombination rate is about a factor of 2 smaller than calculated theoretically. The electric field reduces the electron-hole overlap and hence reduces the spin-relaxation rate of excitons between the optically allowed \ensuremath{\Vert}\ifmmode\pm\else\textpm\fi{}1〉 states. The measured variation is in good qualitative agreement with a recent theory, but somewhat slower than predicted by the theory.

Direct Creation of Excitonic Molecules by Two‐Photon Absorption in Quantum Wells

AbstractA theory of the direct creation of excitonic molecules (biexcitons) by direct two‐photon absorption in semiconductor quantum well structures is developed. An analytical expression of the biexciton two‐photon absorption coefficient is given for both photon polarizations. Excitonic‐, interband‐two‐photon, and exciton one‐photon processes in quantum wells (QW) are also investigated. The analytical forms and numerical values of the momentum matrix elements involved in each process are provided. The biexciton matrix element is calculated adopting a six‐parameter variational wave function. A numerical estimation for GaAs QW shows that the biexciton two‐photon absorption (BTPA) process dominates the other processes by different orders of magnitude. This enhancement essentially comes from the resonance effect and the structure of the matrix elements included in each process. Furthermore, the BTPA process in the QW for both polarizations is enhanced over its bulk value by about four and five orders of magnitude. This increase is interpreted as due to the spatial confinement of the QW.

Hole relaxation process in quantum wells

Hole momentum relaxation times for two channels: namely, conserving and reversing parity of hole subband states has been evaluated, taking into account the deformation potential and piezoelectric scattering by acoustic phonons. The quantitative evaluation, in which parity-conserving hole relaxation is dominant, is consistent with the empirical value of the relaxation time ratio, deduced in our theory of luminescence polarization.

Quantum confined one-dimensional electron-hole plasma in semiconductor quantum wires.

We report a study of the temporal evolution of the luminescence from an electron-hole plasma confned in GaAs quantum wires under high-power excitation. In the spectra 1D intersubband transitions with quantum numbers as high as n=5 are resolved. The spectral positions are in excellent agrement with Kohn-Luttinger calculations based on an active wire width of w=60\ifmmode\pm\else\textpm\fi{}5 nm, accurately determined by an x-ray diffraction method employed for 1D systems. The decay of the luminescence and the hot-carrier cooling rate, which is obtained from line-shape analysis, are found to be slower than the corresponding processes in quantum wells.

Recent developments in optical processes in quantum wells and superlattices: Many body effects and field-induced localization

We present recent optical investigations done in a modulation-doped GaAs-AlGaAs quantum well, which provide information on the band-gap renormalization due to many-body effects. In addition, we describe results, also obtained from optical studies, which show that the application of an electric field perpendicular to the layer plane of a GaAs-AlGaAs superlattice results in a localization of the electronic states leading to a blue shift of the usual optical transitions.