Generation In Quantum Wells Research Articles

Quantum transport simulation of hot carrier photocurrent generation in quantum well solar cells

Hot carrier photocurrent generation in a selectively contacted semiconductor quantum well is simulated using a quantum transport approach including the effects of electron–photon, electron–phonon, and electron–electron interaction. Consideration of electron–electron scattering on equal footing with photogeneration and phonon-mediated carrier relaxation is achieved via the implementation of a GW self-energy in the non-equilibrium Green’s function formalism. Spectral resolution of carrier density, scattering rates and current flow reveals the microscopic mechanism responsible for hot carrier generation and extraction in quantum wells. The existence of a hot carrier population in the system subject to carrier–carrier scattering is verified via evaluation of luminescence spectra as obtained consistently within the same formalism. This development thus represents a major advancement towards a comprehensive microscopic theory of hot carrier solar cell operation in nanostructure device architectures.

Nonlinear spin-current generation in quantum wells with arbitrary Rashba-Dresselhaus spin-orbit interactions

The problem of the nonlinear spin-currents generated by an electric field $\mathbf{E}$ and a temperature gradient $\mathbf{\ensuremath{\nabla}}T$ in spin-orbit coupled systems is revisited in a different formalism. Here, the second-order correction to the particle distribution function $\ensuremath{\delta}{f}^{(2)}$ is derived in a semiclassical approximation that takes into account the local change in the equilibrium distribution function induced by the external fields. Our approach departs significantly from the present theory, where $\ensuremath{\delta}{f}^{(2)}$ is written as an iterative solution to the Boltzmann transport equation in the relaxation-time approximation. As we show, such an expression does not actually satisfy the collision term of the equation, and therefore it is not self-consistent. We apply our formalism to the case of a quantum well with arbitrary values of the linear Rashba $\ensuremath{\alpha}$ and Dresselhaus $\ensuremath{\beta}$ interactions. For the whole range of $\ensuremath{\alpha}$ versus $\ensuremath{\beta}$ values, we obtain analytic results for all the spin currents that can be driven in the system, proportional with ${\mathbf{E}}^{2}$, $\mathbf{\ensuremath{\nabla}}{T}^{2}$, or with $\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{\ensuremath{\nabla}}T$. The magnitude of these currents is smaller than previously anticipated.

Supercontinuum generation in quantum wells facilitated by electromagnetically induced transparency

In this paper, employing numerical simulation, we examine the possibility of generation of optical supercontinuum (SC) in a multiple quantum well (MQW) system facilitated by electromagnetically induced transparency. Within the transparency window created by a control laser beam in a MQW, another probe pulse experiences negligible loss and large optical non-linearity that has been exploited to obtain the optical SC. With 1.0 pico-second ‘sech’ probe pulses of 500 W peak power at wavelength 739.7 nm, the numerically simulated SC spectrum at the end of the 9.377 μm long MQW extends from 550 to 1000 nm. The spectrum is asymmetric, contains large number of oscillations which is attributed to the nonlinearity induced self-phase modulation and modulation instability. This investigation may lead to the fabrication of on-chip SC sources that can be easily integrated with other semiconductor devices.

Simultaneous time and wavelength resolved spectroscopy under two-colour near infrared and terahertz excitation

Time and wavelength resolved spectroscopy requires optical sources emitting very short pulses and a fast detection mechanism capable of measuring the evolution of the output spectrum as a function of time. We use table-top Ti:sapphire lasers and a free-electron laser (FEL) emitting ps pulses as excitation sources and a streak camera coupled to a spectrometer for detection. One of the major aspects of this setup is the synchronization of pulses from the two lasers which we describe in detail. Optical properties of the FEL pulses are studied by autocorrelation and electro-optic sampling measurements. We discuss the advantages of using this setup to perform photoluminescence quenching in semiconductor quantum wells and quantum dots. Carrier redistribution due to pulsed excitation in these heterostructures can be investigated directly. Sideband generation in quantum wells is also studied where the intense FEL pulses facilitate the detection of the otherwise weak nonlinear effect.

Frequency and amplitude modulation in terahertz-sideband generation in quantum wells

A narrow-band optical beam incident on a quantum well modulated at terahertz frequencies results in the formation of terahertz sidebands, i.e., optical signals at the input optical frequency plus integer multiples of the modulation frequency. The terahertz sidebands originate in the modulated complex optical susceptibility. In this study, we analyze theoretically terahertz-sideband generation in the context of frequency and amplitude modulations of the incident optical beam. We find that, at the optimum bias field, the frequency and amplitude modulations contribute almost equally to the maximum first-order sideband. The theoretical approach could offer simple yet useful tools for further engineering and optimization of modulator-type sideband-generation or wavelength-conversion devices.

Nonlinear regimes of coherent optical phonon generation in quantum wells under electric current pumping

We present an analysis of nonlinear regimes of the coherent optical phonon generation under the electron drift in quantum wells. The phonon and electron subsystems are treated self-consistently. This allows us to find thesteady-state generation regimes with macroscopic populations of optical phonon modes and the electron transport controlled in part by the generated phonons. The generation regimes demonstrate a pronounced threshold character. At high electric fields above the threshold, practically single-mode generation occurs and the current-voltage characteristic is considerably changed. We demonstrate high efficiency generation of the coherent optical phonons by the electric current. The coherent macroscopic optical displacements and the amplitudes of oscillating electrostatic fields are evaluated. The proposed model based on the electron nonlinearities predicts a range of the pumping electric fields under which the steady state phonon generation is realized. Our results suggest that the phonon avalanche occurs beyond this field range.

Second-Harmonic Generation in Quantum Wells with Anisotropic and Tilted Valleys: Effect of Coulomb Interaction

Analytical expressions for the components of second-harmonic susceptibility tensor for the L-valley quantum well structures are derived employing the perturbative density-matrix formulation within the time dependent local density approximation (TDLDA). Indirect and direct Coulomb interactions are shown to affect the second-harmonic generation (SHG) spectra differently from each other depending upon the polarization of the pumped and generated light and upon the kind of many-subband system. It is found that the nonlinear dependence of the exchange-correlation potential upon the electron density influences the peak value of the spectrum connected only with the normal-incidence intersubband transitions. The effect of the Coulomb interaction, both direct and indirect, is shown to reveal itself strongly for a near-doubly resonant three-subband system even with a big intersubband gap (of more than 100 meV), while in case of a two-subband system it is best if the gap is of about 10 meV. The numerical calculations show that very fine near-doubly resonant step quantum well systems are required for the SHG and the TDLDA should be used to determine the near-doubly resonance condition.

Second-Harmonic Generation in Quantum Wells with Anisotropic and Tilted Valleys: Effect of Coulomb Interaction

Second-Harmonic Generation in Quantum Wells with Anisotropic and Tilted Valleys: Effect of Coulomb Interaction

Optical absorption and sideband generation in quantum wells driven by a terahertz electric field

Linear optical absorption and terahertz sideband generation in quantum wells in the presence of a strong growth-direction oriented terahertz electric field is investigated. It is shown that the electric field not only produces multiple optical absorption peaks due to resonant coupling between the valence subbands---the Autler-Townes splittings---but also produces splittings in the generated sidebands. Due to the proximity of subbands in typical quantum wells, the terahertz field can couple more than two levels simultaneously and particular effects of that coupling are discussed.

Second-harmonic generation in quantum wells: the role of many-body effects

We study theoretically the influence of the Coulomb interaction on the second-harmonic generation (SHG) connected with intersubband transitions in asymmetric quantum wells. Our approach is based on the time-dependent local density approximation. Numerical calculations are performed for asymmetric coupled double quantum well structure with small energy separation between ground and excited subbands. It was shown that the Coulomb interaction not only shifts the peak positions in SHG spectrum but also considerably modifies their height. The correct description of the above modification must take into account the nonlinear dependence of the exchange-correlation potential on the electron density.

Terahertz sideband generation in quantum wells viewed as resonant photon tunneling through a time-dependent barrier: An exactly solvable model

The propagation of light through quantum wells in the vicinity of an excitonic resonance subjected to a THz electric field is considered. The treatment makes explicit the relationship between THz sideband generation and resonant photon tunneling through a time-dependent resonant-tunneling structure. A method for the coherent control of excitons in quantum wells is also suggested.

Doubly resonant second-harmonic generation in quantum wells with anisotropic and tilted valleys: Resonant screening effects

Second-harmonic generation due to intersubband transitions in near-doubly resonant systems with anisotropic and tilted valleys is discussed theoretically. Calculations are performed for the L-valley quantum well structures employing the perturbative density-matrix formulation within the self-consistent field approximation. The results obtained show that modification of the second-harmonic generation spectrum induced by the depolarization effect depends strongly on the polarization of the pumped and generated light.

Influence of the depolarization effect on third-harmonic generation in quantum wells

The influence of the depolarization effect on the third-harmonic generation due to intersubband transitions in the near-triply-resonant quantum well structures is studied theoretically. Calculations are performed in the framework of the perturbative density-matrix formulation with relaxation-time approximation. The results obtained show that the depolarization effect not only shifts the peak in the third-harmonic-generation spectrum but also, due to the coupling between different intersubband transitions, can enhance its maximum value.