Biased Quantum Wells Research Articles

Electron-phonon interaction within an unbiased and biased quantum well

This paper addresses the electron-phonon interaction when a perpendicular electric field is applied across a quantum well (QW). An applied perpendicular electric field redistributes the one-dimensional density of states in position and energy within a QW. The redistribution of the one-dimensional density of states modifies the calculation of the initial and final states in the electron-phonon interaction. The overall result is to increase the scattering rate between the electrons and phonons while adding additional characteristics to the shape of the phonon scattering time.

Tunnel transfer of excitons due to a Fano disintegration and an optical phonon emission in biased double quantum wells

We consider theoretically and experimentally the tunnel transfer of excitons in an asymmetric double quantum well structure. We interpret quantitatively the measured tunnel transfer time in terms of two intrinsic mechanisms for excitons: an inter-well Fano disintegration for excitons photocreated in an excited state and the inter-well emission of optical phonons.

Interwell radiative recombination in the presence of random potential fluctuations in GaAs/AlGaAs biased double quantum wells

The interwell radiative recombination from biased double quantum wells (DQW) in pin GaAs/AlGaAs heterostructures is investigated at different temperatures and external electrical fields. The luminescence line of interwell recombination of spatially separated electron-hole pairs exhibits systematic narrowing with temperature increase from 4.5 to 30 K. A theoretical model is presented which explains the observed narrowing in terms of lateral thermally activated tunneling of spatially separated e-h pairs localized by random potential fluctuations in the quantum wells.

Electron escape via polar optical-phonon interaction and tunneling from biased quantum wells

In this study, the escape of electrons from a biased quantum well through polar optical-phonon interaction is addressed for an ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum-well system. Unlike previous models, the redistribution of the density of states resulting from the applied electric field is included. The calculation of the density of states, wave functions, and energy levels is performed by solving the Schr\"odinger equation through the logarithmic derivative of the wave function. A comparison of the escape time of electrons through a polar optical-phonon interaction and through tunneling out of the quantum well is made. It is demonstrated that for shallow quantum wells, escape via tunneling dominates for fields greater than 20 kV/cm, whereas, with increasing Al concentration, the escape via polar optical phonons dominates over a larger range of electric fields. The temperature dependence of the tunneling and polar optical-phonon assisted escape time is addressed, demonstrating an exponential dependence.

Quasi-half-cycle terahertz pulse generation via optical rectification in quantum wells using shaped optical pulses

It is shown that optical rectification in biased quantum wells using specially shaped optical pulses can be used to generate quasi-half-cycle THz electromagnetic pulses. Namely, we investigate THz generation by pulses incorporating a rapid pi phase shift. We further explore the potential of this scheme for high-repetition-rate quasi-half-cycle THz pulse generation.

Electronic g factor in biased quantum wells

A theory of the Zeeman splitting of electronic spin states in biased quantum-well structures has been developed. The g factor tensor has been calculated as a function of the applied electric field and the well width. The longitudinal g factor has been shown to be more sensitive to the field than the transverse, or in-plane, component. For a fixed electric field and increasing the well width, the g factor components saturate towards the corresponding values for a single heterojunction. The theoretical results has been compared with available experimental data.

Localization and delocalization of an electron in biased and unbiased quantum wells driven by a mono- and bichromatic laser field.

Localization (delocalization) of an electron driven by laser in a biased quantum well is considered. The initially trapped electron can be delocalized by the field if the bias energy is close but not equal to an integer number of a photon energy. When the bias energy is exactly equal to an integer number of photon quanta, dynamical localization occurs. A general analytical solution for a population difference is obtained. When the electron is driven by a bichromatic field, localization regions are strongly dependent on the field incommensurability. The topology of localization exhibits stable and unstable regions originating from nonanalytical behavior of the population with respect to the small incommensurability. This phenomenon resembles phasetransition instabilities in the solid state. A low-frequency spectrum of electron oscillations consists of a low-frequency mode and split lines. We also show frequency controlling of the time evolution of an electron dipole moment on a phase shift between the two lasers. The low frequency can be increased or decreased, or it oscillates with the phase shift depending on the laser intensity.

Second-order optical susceptibility of biased quantum wells in the interband regime.

We develop a theory within the effective-mass approximation of the second-order susceptibility of a quantum well subject to an applied electric field. We show that close to the midgap of the structure it is possible within that approximation to reduce the normally derived triple sums over subbands to a simple sum over all the optically allowed transitions. We have demonstrated that in realistic structures, higher-energy transitions play a strong role for quantitative results. This strong influence of the higher-energy transitions on the second-order susceptibility has the consequence that it is not generally possible to define a quantum-well susceptibility independent of the structure in which the well is inserted. Numerical results indicate that the quantum wells should not lead to spectacular enhancements of the second-harmonic-generation susceptibility.

Complex-energy analysis of intrinsic lifetimes of resonances in biased multiple quantum wells.

The electric-field-dependent intrinsic lifetimes of resonances in biased multiple qunatum wells are studied by using a complex-energy analysis based on an Airy-function transfer-matrix description of tunneling. Special attention is paid to the case when at some particular electric field two resonances belonging to different quantum wells resonantly align in energy. Two different characteristic behaviors were found in such electric-field sweeps: Long-lived resonances typically exhibit an anticrossing of their quasieigenenergies and a corresponding crossing of their lifetimes, usually associated with large changes in these lifetimes over many orders of magnitude, while short-lived resonances feature a crossing of their quasieigenenergies and an anticrossing of their lifetimes with less variation in these lifetimes. The parameters relevant for these two different regimes are discussed, and a simple model is derived to describe the crossover between these regimes.

Scheme for generation of sub-Poissonian photons: Antibunching of emission events by population-dependent spontaneous-emission lifetime in semiconductor microcavities.

An alternative scheme for the generation of sub-Poissonian photons based on population-dependent spontaneous-emission lifetime in semiconductor microcavities is proposed and discussed theoretically. In the proposed scheme, the lifetime decreasing with electron population antibunches the emission events and, as a result, expands remarkably the frequency range for the suppression of spontaneous photon number fluctuation noise far beyond the inverse of the lifetime. The negative derivative of the lifetime is caused by a resultant effect of field screening due to populated electron-hole pairs and of quantum-confined Stark effect in biased quantum wells embedded in the microcavities. The proposed scheme may open up a new opportunity in extremely low-power (several photons) high-speed photonics.

THz pulses from the creation of polarized electron-hole pairs in biased quantum wells

We report generation of coherent terahertz electromagnetic transients from GaAs/Al0.3Ga0.7As quantum wells in perpendicular fields. Although, at low temperature, the quantum well barriers suppress the transport current perpendicular to the layers by at least two orders of magnitude compared to bulk, we observe terahertz signals that are comparable in strength to those generated from bulk GaAs surfaces. This directly proves that the field-induced polarization of photoexcited electron-hole pairs is an important mechanism for the generation of terahertz radiation at semiconductor surfaces.

Simultaneous measurement of electron and hole escape times from biased single quantum wells

Picosecond pump probe measurements on waveguides containing a single GaAs/GaAlAs quantum well have enabled us to determine for the first time both electron and hole escape times from biased quantum wells. Contributions from excitonic saturation and field screening by photogenerated carriers can be clearly identified in the nonlinear transition signal and are quantitatively modeled. Preliminary analysis suggests that thermally assisted escape is dominant at room temperature, but discrepancies are found with conventional thermionic emission models.

Optically induced intersubband absorption in biased double quantum wells

We present a new all-optical infrared modulator, which is based on biased double quantum wells. The electric field separates the photogenerated electrons and holes and hence increases their lifetime τ: the photoinduced infrared absorption is therefore enhanced. An increase of the modulation efficiency by two or three orders of magnitude is predicted and an enhancement of at least 20 has been observed at 4 K. At high pump power (150 mW), the linear dependence of the absorption as a function of applied bias is observed and discussed. We have also observed photoinduced intersubband transitions in the valence band in both infrared polarizations.

Analysis of the ground-state properties of biased double quantum wells by a relaxation-variational method.

The ground-state energy and certain effects of the electron-hole Coulomb interaction in coupled double quantum wells are analyzed by evolving the pair envelope wave function in imaginary time, or equivalently relaxing the wave function in real time. The ground-state energies and envelope wave functions for noninteracting pairs are calculated by a pure relaxation method, and those for interacting electron-hole pairs by a combined relaxation-variational method. The 1s-exciton binding energies are estimated from the difference of the two sets of calculated energies. The excitonic system is described by a two-band effective-mass model in which a 1s variational ansatz is used to average out the in-plane relative motion. Total pair energies, binding energies, variational parameters, and pair probability densities are analyzed as functions of applied longitudinal electric field. All, except the probability, are compared for various barrier thicknesses as well. The characteristic redshift of the quantum-confined Stark effect is reproduced, and the expected decrease of binding energies with increasing field strength is observed for both heavy-hole and light-hole states. As functions of the barrier thickness, the binding energies and variational parameters pass through minima in the 15--25-\AA{} range. Finally, significant migration of carriers is predicted to occur in the formation of the dipole moment in coupled double quantum wells.

Positon expectation value and oscillator strength of a biased asymmetric quantum well

An asymmetric quantum well consisting of one narrow and one wide well separated by a thick barrier is investigated. We calculate the change in the position expectation values of an electron in the first three quasibound states of the structure as a biased voltage increases. The value in the first quasibound state, which is the ground state of the wide well, shifts toward the right wall of the well. It has been found earlier that for a certain applied voltage, the first and second states of this whole structure, which are actually the ground state of the narrow well and the first-excited state of the wide well, respectively, coincide to give rise to resonant tunneling. At this resonance voltage, we see that the most probable position to find an electron in the second and third quasibound states lies in the barrier separating the two wells. Furthermore, these two states interchange their identities, meaning that the ground state of the narrow well becomes the first-excited state of the wide well and vice versa. We also calculate the oscillator strengths for transitions from the ground state of the wide well to the ground state of the narrow well and the first-excited state of wide well, and from the ground state of the narrow well to the first-excited state of the wide well. A similar conclusion can be drawn regarding the interchange of identities between the second and third quasibound states of the structure from the changes in oscillator strengths as a function of applied voltage. The oscillator strength for the transition from the ground state of the narrow well to the first-excited state of the wide well peaks at the resonant voltage.

Resonances in the Electron, Hole and Exciton Transfer in Biased Double Quantum Wells

We present theoretical and experimental results bearing on the assisted transfer of electrons, holes and excitons in biased double quantum wells. We show that the exciton relaxation proceeds via a conversion of a "vertical" exciton into a "crossed" one.

Exciton saturation in electrically biased quantum wells

We have measured the heavy hole excitation saturation intensity in GaAs/AlGaAs quantum wells as a function of applied electric field and AlGaAs barrier design. We find that the saturation intensity increased with increasing applied field, and decreasing barrier thickness or height, because of increased carrier sweep-out rates. Time-resolved sweep-out time and temperature-dependent saturation intensity measurement point out the roles of both thermionic emission and tunneling in the field and barrier-dependent carrier escape time. By reducing the barrier Al composition from 30 to 20%, we achieved an increase in the saturation intensity by a factor of ∼6.

Resonances in the excitonic transfer in biased double quantum wells.

We report evidence of resonances in the assisted transfer of excitons in a biased asymmetrical GaAs-(Ga,Al)As double quantum well. The excitonic transfers, i.e., the conversions of a ``direct'' exciton into a ``crossed'' one, are such that the initial and final hole locations are identical but the electron locations are different before and after the transfer. The calculated excitonic energies and transfer times provide a good description of the energy locations and intensities of the photoluminescence lines.

Coherent Nonlinear Optical Processes in Semiconductors. Physics and Applications

AbstractThe influence of monochromatic nonresonant laser fields is discussed on the elementary excitation spectrum of semiconductors. It is shown that the different renormalizations in inorganic and onedimensional organic materials can be understood in the framework of one and the same theory. Applications, such as generation of ultrashort voltage pulses in biased quantum wells, are discussed.

Exciton-exciton interaction and optical nonlinearity in biased semiconductor quantum wells.

Exciton-exciton interaction and optical nonlinearity in biased semiconductor quantum wells.