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Several aspects of the theory of plasmon resonant DC photoconduction are discussed here, in connection with recent observations involving a THz-irradiated grid-gated double-quantum-well FET.¹ In this, we construct a classical model of nonlinear polarizability to second order in the THz field using a “hydrodynamic” type formulation including the roles of a stress-tensor and friction/viscosity. The resulting second order polarizability exhibits resonant behavior when the THz frequency matches plasmon frequencies of the system, sharply reducing the effectiveness of screened impurity scattering potentials which can admit resonant DC photoconduction. Furthermore, we also show that an asymmetric double-quantum-well system with lateral periodicity can mix optical and acoustic plasmons, giving rise to an interlayer THz field which becomes very strong when tuned by gate voltage into the “mode-mode-repulsion” regime wherein the optical and acoustic modes equally share amplitude. This can enhance interlayer electron tunneling and may contribute to photoconductivity.

Precise scanning of the (B_⊥,B_‖)-plane while measuring magnetoresistance of the n-InGaAs/GaAs double quantum well (DQW) reveals a number of peculiarities connected with intricate DQW energy spectrum, which are analyzed on the basis of quasiclassical calculations. Magnetic breakdown effects are also considered. Peaks due to the latter mechanism reveal spin-splittings (in spite of lower mobilities as compared with the traditional n-GaAs/AlGaAs DQWs) corresponding to an enhanced effective Lande g-factor.

We present far-infrared cyclotron resonance measurements on a strongly coupled symmetric GaAs/GaAlAs double quantum well sample. Cyclotron resonance is measured at several discrete wavelengths and tilt angles of the sample with respect to the magnetic field. The width and strength of the resonance peaks depend strongly on the tilt angle and the laser wavelength, demonstrating the complexity of this system.

We have studied the quantum Hall effect in Al_xGa_1-xAs-double well structure with vanishing g-factor. We determined the density-magnetic field n_s - B diagrams for the longitudinal resistance R_xx. In spite of the fact that the n_s - B diagram for conventional GaAs double wells shows a striking similarity with the theory, we observed the strong difference between these diagrams for double wells with vanishing g-factor. We argue that the electron-electron interaction is responsible for unusual behavior of the Landau levels in such a system.

The bulk energy gap of two-dimensional (2D) topological insulators (TIs) is generally small, leading to their study being limited to extremely low temperatures, thus increasing the energy gap of 2D TIs is an urgent challenge. It has been reported that the compressive strain could enhance the band gap of the 2D TI to about 55 meV. Based on this result, this paper further calculated the band structures of 2D TIs with a special structure, i.e., the double quantum well, and especially evaluated their band gaps. We found that by choosing different internal barrier thicknesses, asymmetry factors and electric fields, numerical results show that the energy gap of the TIs with double quantum well (DQW) structure can further reach over 70 meV. This result might be significant for the possible applications of 2D TIs.

The nonlinear second-harmonic generation (SHG) and third-harmonic generation (THG) in “12–6” tuned GaAs/GaAlAs double quantum wells (QWs) are calculated in the presence of applied electric field under the effective mass approximation. The influence of the structural parameter of the double QW is also considered, which is the main means to adjust its nonlinear optical properties. The eigenvalues and corresponding eigenfunctions of the double QWs system are obtained via the finite difference technique. The nonlinear SHG and THG are presented as a function of the photon energy, structural parameter, and applied electric field. It is shown that the SHG and THG are sensitive to both the applied electric field and the structure parameters, which can tune the nonlinear optical properties efficiently by adjusting the symmetry and confined potential of the double QWs.

In this study, we have calculated theoretically the effects of the electric field and doping concentration on the sub-band energies, the electron population, and total charge density in modulation-doped symmetric and asymmetric GaAs/Al_0.33Ga_0.67As double quantum wells. Electronic properties of the system are determined by the solving the Schrödinger and Poisson equations self-consistently in the effective-mass approximation. The application of an electric field in the growth direction of the system causes a polarization of the carrier distribution and shifts the sub-band energies, which may be used to control and modulate intensity output devices. In an asymmetric double-quantum-well structure, the effects mentioned above appear more clearly.

Magnetic field dependence of the excitonic spectrum and the intensity of the optical transitions for excitons in the double quantum well heterostructures based on semimagnetic semiconductors of various compositions are studied. The calculations carried out for (Zn, Mn)Se-based double quantum well structures showed that in the weak magnetic fields, the lowest energy states are the single-well states (direct excitons) for which both the electron and the hole are predominantly localized in the same well. At some values of magnetic field, the crossing of the direct exciton with indirect exciton formed by an electron and a hole, situated predominantly in the different wells, occurs. In the magnetic field exceeding some critical value, the lowest energy level belongs to the indirect exciton. According to the estimates, the lifetime of the indirect exciton is by several orders of magnitude larger than that of a single-well exciton. The exciton lifetime depends significantly on the width and the material of the barrier between the wells.

The basic technique of stimulated Raman adiabatic passage for laser-induced adiabatic population transfer between discrete quantum states of an asymmetric double quantum well has been used in our study. The results show that the proper time-delay, overlap, and detuning of two pulses allows the coherent transfer between the states of a double quantum well system, leading to the possibility of implementation of semiconductor–based quantum logic gates and high efficiency optical switches. The impact of phase relaxation on the population transfer efficiency is also studied.

Several aspects of the theory of plasmon resonant DC photoconduction are discussed here, in connection with recent observations involving a THz-irradiated grid-gated double-quantum-well FET.¹ In this, we construct a classical model of nonlinear polarizability to second order in the THz field using a “hydrodynamic” type formulation including the roles of a stress-tensor and friction/viscosity. The resulting second order polarizability exhibits resonant behavior when the THz frequency matches plasmon frequencies of the system, sharply reducing the effectiveness of screened impurity scattering potentials which can admit resonant DC photoconduction. Furthermore, we also show that an asymmetric double-quantum-well system with lateral periodicity can mix optical and acoustic plasmons, giving rise to an interlayer THz field which becomes very strong when tuned by gate voltage into the “mode-mode-repulsion” regime wherein the optical and acoustic modes equally share amplitude. This can enhance interlayer electron tunneling and may contribute to photoconductivity.