Interplay of collective excitations in quantum-well intersubband resonances

Intersubband resonances in a semiconductor quantum well (QW) display fascinating features involving various collective excitations such as Fermi-edge singularity (FES) and intersubband plasmon (ISP). Using a density matrix approach, we treated many-body effects such as depolarization, vertex correction, and self-energy consistently for a two-subband system. We found a systematic change in resonance spectra from FES- to ISP-dominated features, as QW width or electron density is varied. Such an interplay between FES and ISP significantly changes both line shape and peak position of the absorption spectrum. We found that a cancellation of FES and ISP undresses the resonant responses and recovers the single-particle features of absorption for semiconductors with a strong nonparabolicity such as InAs, leading to a dramatic broadening of the absorption spectrum.     

Built-in electric field effect on the linear and nonlinear intersubband optical absorptions in InGaN strained single quantum wells

Considering the strong built-in electric field (BEF) induced by the spontaneous and piezoelectric polarizations and the intrasubband relaxation, we investigate the linear and nonlinear intersubband optical absorptions in In_xGa_1-xN/Al_yGa_1-yN strained single quantum wells (QWs) by means of the density matrix formalism. Our numerical results show that the strong BEF is on the order of MV/cm, which can be modulated effectively by the In composition in the QW. This electric field greatly increases the electron energy difference between the ground and the first excited states. The electron wave functions are also significantly localized in the QW due to the BEF. The intersubband optical absorption peak sensitively depends on the compositions of In in the well layer and Al in the barrier layers. The intersubband absorption coefficient can be remarkably modified by the electron concentration and the incident optical intensity. The group-III nitride semiconductor QWs are suitable candidate for infrared photodetectors and near-infrared laser amplifiers.     

Occurrence of intersubband polaronic repellons in a two-dimensional electron gas

Counteracting electron-phonon and electron-electron interaction in the optical absorption spectra of a quantum well intersubband system can have a significant impact on the line shape and line width of optical transitions at low temperatures. In particular, polaronic repellons can be formed due to an attractive intersubband polaron-polaron interaction. At low temperatures and for properly chosen densities; this effect can lead to a line broadening of the intersubband transition, even in a weak coupling regime.     

Intersubband plasmons of a semiconductor superlattice

Collective intersubband plasmon-like excitations are predicted to exist for a semiconductor superlattice. These modes arise because the single quantum well depolarization shifted intersubband excitation couples via the long-range Coulomb interaction with the corresponding excitations of the other quantum wells of the superlattice. The dispersion relation for these intersubband plasmons is obtained.     

Optical properties of -doped semiconductors: Plasmon–phonon coupling and Raman spectra

Collective excitations and their coupling to optical phonons have been studied for a two-dimensional electron gas in -doped polar semiconductors within the random-phase approximation. The inelastic light scattering spectrum due to the coupled plasmon–phonon modes are calculated for the multisubband two-dimensional electron systems. Our calculation shows that, due to the high electron density in these systems, both intrasubband and intersubband plasmons are strongly coupled to the optical-phonons. On the other hand, due to the high impurity concentration, level broadening modifies the inelastic light scattering spectrum significantly.     

On the propagation characteristics of metal clad waveguides with a quantum well

The electromagnetic modes of planar metal clad dielectric waveguides containing an n-doped quantum well (QW) are studied theoretically. Special attention is paid on the coupling between metal surface plasmons and intersubband plasmons and the manifestation of this coupling in the propagation characteristics of metal/QW/dielectric and multimode metal/QW/dielectric/metal waveguide structures. The results obtained indicate that the modification of the propagation characteristic induced by the above-mentioned coupling is substantial only in the case of metal/QW/dielectric waveguide structures.     

Intersubband collective excitations in a quasi-two-dimensional electron system in external magnetic field

The spectrum of collective excitations in a quasi-two-dimensional electron system was studied by the method of Raman scattering spectroscopy. In an applied magnetic field, such systems exhibit collective excitations related to the electron transitions between dimensionally quantized subbands with a change in the Landau level index (intersubband Bernstein modes). It is shown that these modes interact with the fundamental intersubband excitations of the charge and spin densities, the interaction energy being determined by the excitation quasimomentum. Interaction of the intersubband Bernstein modes and the fundamental intersubband excitations with quasi-two-dimensional LO phonons was studied. Behavior of the new branches of collective excitations in a quasi-two-dimensional electron system possessing more than one occupied Landau level was studied and the nature of these branches was determined.     

Observation of intersubband excitations in a multilayer two dimensional electron gas

We report the observation, by resonant inelastic light scattering, of intersubband excitations of the multilayer two dimensional electron gas, in modulation doped GaAsAlGaAs heterojunction superlattices. These are the first measurements of these transitions by any technique, and furnish intersubband energies in good agreement with calculated values. The spectral bands are broad, and nearly Lorentzian in shape: the implied relaxation rates scale linearly with band energy and are significantly faster than transport relaxation rates. Finally, the polarized spectra reveal differences between spin-flip and non spin-flip excitations which are unique to multilayer two dimensional electron gases.     

Microscopic Many-Body Effects on Intersubband Resonance in Quantum Well

Considering the Coulomb many-body interactions, we investigate the intersubband optical processes of the quantum well by using the semiconductor Bloch equations. We calculate the evolution of intersubband absorption spectral line shape as a function of lattice temperature and electron density. It is found that the coupling of intersubband plasmons can reduce and red-shift the lower energy resonance, simultaneously enhance and blue-shift the higher energy resonance. The dependence of cascading resonances on temperature and electron density is also discussed.     

On the possibility of designing interacting semiconductor quantum wells with zero perturbation coupling

The paper describes the possibility of designing matched interacting semiconductor quantum wells. It is shown that for a given eigenstate of a quantum well (QW), it is always possible to find another QW in such a way that the coupling leaves the original eigenstate of the host QW unperturbed irrespective of the strength of interaction. For rectangular QWs, the condition is met with whenever the second QW has appropriate width and depth so that phase travelled by an electron wave through it is an integral multiple of π.     

Minigap plasmons in a two-dimensional electron gas subjected to a one-dimensional periodic potential

We investigate the plasmon excitations in a two-dimensional electron gas subjected to a one-dimensional weak periodic potential. We derive and discuss the dispersion relations for both intrasubband and intersubband excitations within the framework of Bohm-Pines' random-phase approximation. For such an anisotropic system with spatially modulated charge density, we observe a splitting of the 2D plasmon dispersion. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum. We also discuss how the tunneling and the potential amplitude affect the plasmon excitations.     

Intersubband spectroscopy of two dimensional electron gases: Coulomb interactions

We have made a direct determination of resonant screening (the depolarization field effect) in the collective intersubband excitations of a dense two dimensional electron gas. The effect was observed, for both odd and even parity transitions, in polarized inelastic light scattering spectra of a modulation-doped GaAs-AlGaAs superlattice. We offer a quantitative interpretation in terms of the Coulomb matrix elements for the transitions. Final state, or exciton-like, many-body effects are considered briefly.     

Femtosecond mid-infrared spectroscopy of low-energy excitations in solids

Recent progress in the generation and spectroscopic application of femtosecond pulses in the wavelength range between 3 and 25 m has led to new insight into low-energy excitations of solids, in particular semiconductors, semiconductor nanostructures, and high-T_C superconductors. The ultrafast nonequilibrium dynamics of such excitations has been observed in real-time and the underlying microscopic interactions have been analyzed. This article gives a brief overview of this exciting field of ultrafast science with the main emphasis on pulse generation and applications in semiconductor research where femtosecond intersubband Rabi flopping in quantum wells and quantum coherent electron transport in quantum cascade devices have been observed. PACS 78.67.De; 73.21.Fg; 07.57.Hm; 42.65.Re     

Hyperentanglement source by intersubband two-photon emission from semiconductor quantum wells

We propose an efficient hyperentanglement source emitting photon pairs entangled in both energy and polarization. The compact electrically driven room-temperature source, based on intersubband two-photon emission from semiconductor quantum wells (QWs) exhibits pair generation rates several orders of magnitude higher than alternative conventional schemes. A theoretical formalism is derived for the calculation of photon pair generation spectra and rates. The results are presented for superlattice structures similar to quantum cascade lasers of GaAs/AlGaAs QWs emitting in the mid-IR and far-IR and for InN/AlN QW structures suitable for telecommunication wavelengths.     

Interaction between an electron and optical phonons in polar semiconductor heterostructures

Interface phonons and bulk-like longitudinaloptical (LO) phonons and their interaction with an electron are studied for a finite four-layer heterostructure (FFLHS). An analysis of the field eigenvectors shows that, in the vicinity of the Brillouin-zone center, an interface transverse-optical (TO) mode oscillates at the bulk LO frequency, and an interface LO mode oscillates at the bulk TO frequency. Analytic expressions and numerical illustrations for dispersion relations of interface modes and for electron-phonon coupling functions and scattering rates are obtained for finite, semi-infinite and infinite quantum well (QW) structures which are important special cases of an FFLHS. It is shown that the scattering rates depend strongly on the well width of a QW structure, and that interface modes are much more important than bulk LO modes when the well width is small. The calculated results also show that the usual selection rules for intersubband and intrasubband transitions break down in asymmetric heterostructures. Moreover, we have found an interesting result. That is, in comparison with the negligibly small interaction between an electron and the lowest-frequency interface-mode in symmetric single QWs and commonly used step QWs, this interaction may be very large in asymmetric single QWs and general step QWs.     

缓冲层对量子阱二能级系统中电子子带间跃迁光吸收的影响

由于ZnO缓冲层对纤锌矿ZnO/Mg_xZn_(1-x)O有限深单量子阱结构左垒的限制作用,导致阱和右垒的尺寸、Mg组分值等因素将影响系统中形成二能级.本文考虑内建电场、导带弯曲及材料掺杂对实际异质结势的影响,利用有限差分法数值求解Schr?dinger方程,获得电子的本征能级和波函数,探讨ZnO缓冲层对此类量子阱形成二能级系统的尺寸效应及三元混晶效应的影响;利用费米黄金法则探讨缓冲层、左垒、阱及右垒宽度和三元混晶效应对此类量子阱电子子带间跃迁光吸收的影响.计算结果显示:对于加入ZnO缓冲层的ZnO/Mg_xZn_(1-x)O有限深单量子阱二能级系统,左垒宽度临界值会随着阱宽和Mg组分值的增大而逐渐减小,随着右垒宽度和缓冲层厚度的增大而逐渐增大;量子阱中电子子带间跃迁光吸收峰会随着左垒、右垒尺寸以及Mg组分的增大发生蓝移,随着阱宽增大而发生红移.本文所得结果可为改善异质结器件的光电性能提供理论指导.     

Photoluminescence of “dark” excitons in CdMnTe quantum well, embedded in a microcavity

A diluted magnetic semiconductor (DMS) quantum well (QW) microcavity operating in the limit of the strong coupling regime is studied by magnetoptical experiments. The interest of DMS QW relies on the possibility to vary the excitonic resonance over a wide range of energies by applying an external magnetic field, typically about 30 meV for 5 T in our sample. In particular, the anticrossing between the QW exciton and the cavity mode can be tuned by the external field. We observe the anticrossing and formation of exciton polaritons in magneto-reflectivity experiments. In contrast, magneto-luminescence exhibits purely excitonic character. Under resonant excitation conditions an additional emission line is observed at the energy of the dark exciton. The creation of dark excitons is made possible due to heavy hole–light hole mixing in the QW. The emission at this energy could be due to a combined spin flip of an electron and a bright exciton recombination.     

Dynamical Theory of X-Ray Diffraction for Restricted Beams: I. Coherent Scattering by a Porous Crystal

The processes of electron spin dynamics in a hybrid nonresonance structure, which includes a layer of a diluted magnetic II–Mn–VI semiconductor and an asymmetric quantum well (QW) of a nonmagnetic III–V semiconductor, are experimentally studied. The nonresonance of the structure is determined by the fact that the level of the ground state of the magnetic layer falls into the range of the excited states of the nonmagnetic QW. The electron polarization in the ground thermalized state of QW is found not to depend on the magnetic part of the structure. However, the magnetic part affects the electron polarization in the excited state via spin injection from the magnetic semiconductor and the mixing of the electronic states of the magnetic and nonmagnetic subsystems of the structure. The possibility of controlling the polarization of an electron spin by carrier excitation toward the region of mixed states along with the absence of depolarizing influence of the magnetic semiconductor on carriers in the thermalized state of QW can be applied to design new spintronic devices along with those that use spin injection, optical orientation, and depolarization.     

Design of polarization independent InGaAs/InGaAlAs coupled quantum well structure in 1.55 μm wavelength region

A novel polarization independent InGaAs/InGaAlAs quantum well (QW) structure in the 1.55 m wavelength region is proposed. A coupled QW structure with tensile strain in the QW and/or barrier region is considered for the reduction of the optical gain difference between TE and TM modes in the wide spectral range. A triple-coupled QW structure with alternative strain (tensile/compressive/tensile) is found to be the most effective in reducing the polarization gain difference. This is because the transition strength difference of each polarization is reduced by energy states coupling. The optimized triple-coupled QW structure shows polarization independence for wide carrier density and wavelength range, which is suitable for polarization independent operation of QW based semiconductor devices, such as semiconductor optical amplifiers.     

Strong correlation of electronic and lattice excitations in GaAs/AlGaAs semiconductor quantum wells revealed by two-dimensional terahertz spectroscopy

Coulomb-mediated interactions between intersubband excitations of electrons in GaAs/AlGaAs double quantum wells and longitudinal optical phonons are studied by two-dimensional spectroscopy in the terahertz frequency range. The multitude of diagonal and off-diagonal peaks in the 2D spectrum gives evidence of strong polaronic signatures in the nonlinear response. A quantitative theoretical analysis reveals a dipole coupling of electrons to the polar lattice that is much stronger than in bulk GaAs, due to a dynamic localization of the electron wave function by scattering processes.