Terahertz photoresponse of quantum wires in magnetic fields

We have investigated the terahertz photoresponse of quantum wires in high magnetic fields, employing intense far-infrared (FIR) radiation from the UCSB Free-Electron Lasers. Both GaAs-based and InAs-based quantum wires, with widths ranging from 50 nm to 1 μm, were studied. At high FIR power we observed Shubnikov–de Haas type oscillations in photoresponse versus magnetic field,B, resulting from non-resonant electronic heating the oscillations were much more pronounced than those in resistance versusB. At low FIR power we observed resonant peaks due to magnetoplasmon excitations, whose strength shows strong polarization-dependence and whose energy extrapolates to a finite value at zeroB. These results provide a powerful tool for characterizing 1D electronic states in quantum wires.     

Photoluminescence of single quantum wires and quantum dots

The results of a study into the photoluminescence spectra of a set of quantum dots based on GaAs enclosed in AlGaAs nanowires are presented. The steady state and time resolved spectra of photoluminescence under optical excitation both from an array of quantum wires/dots and a single quantum wire/dot have been measured. In the photoluminescence spectra of single quantum dots, emission lines of excitons, biexcitons and tritons have been found. The binding energy of the biexciton in the studied structures was deduced to be 8 meV.     

Correlation energy and excitation spectra of two-dimensional electron-hole systems in high magnetic fields

The correlation phenomena in two-dimensional semiconductors in high transverse magnetic fields are considered. A diagram expansion convergent at any temperatures T is obtained due to the special choice of an initial approximation. The expansion parameter appears equal to (E_o/T)exp(-E_o/2T) where E_o is the exciton binding energy. The expansion parameter of the usual Matsubara diagram technique would be proportional to 1/T, if the noninteracting electron-hole system were choosen as the initial approximation, due to the infinite degree of degeneracy of the ground state. The system proves equivalent to the slightly non-ideal exciton gas with the same long-wave properties as the two-dimensional Bose gas. The Hartree-Fock approximation is exact at T = 0, but there is no phase transition with a long-range order creation at T≠0 in contrast to the HFA results (although there remain some properties of the transition such as the maximum of specific heat etc.). The Berezinsky-Kosterlitz-Thouless transition is predicted in the system for very low temperatures. The diagram technique developed may be employed to treat the thermodynamics of other infinitely-fold degenerated systems.     

Optical spectroscopy of a single InAs/GaAs quantum dot in high magnetic fields

We report on the measurements of the photoluminescence from the s-shell of a single InAs/GaAs quantum dot in magnetic fields up to 23 T. The observed multiline emission is attributed to different charge states of a single dot. Characteristic anticrossing of emission lines is explained in terms of hybridization of final states of a triply charged exciton (X⁻³).     

Third-harmonic generation in cylindrical parabolic quantum wires with static magnetic fields

The third-harmonic generation (THG) and its conversion efficiency in Al_xGa_1-xAs/GaAs cylindrical parabolic quantum wires with static magnetic fields are studied in detail. The calculated results show that the parabolic confining potential and the static magnetic field have evident influence on the THG and its conversion efficiency. In addition, the conversion efficiency of the THG is also related to the input optical intensity. It is noted that very high conversion efficiency of the THG can be obtained by increasing properly the input optical intensity and choosing an optimized parabolic confining potential and applied static magnetic field.     

Semiconductor quantum dots in high magnetic fields

We review and extend the composite fermion theory for semiconductor quantum dots in high magnetic fields. The mean-field model of composite fermions is unsatisfactory for the qualitative physics at high angular momenta. Extensive numerical calculations demonstrate that the microscopic CF theory, which incorporates interactions between composite fermions, provides an excellent qualitative and quantitative account of the quantum dot ground state down to the largest angular momenta studied, and allows systematic improvements by inclusion of mixing between composite fermion Landau levels (called Λ levels).     

Evolution of the energy structure of n-InGaAs/GaAs double quantum wells in tilted magnetic fields

Complex variations of a magnetoresistance oscillation pattern with a tilted magnetic field angle are found in a n-In_0.2Ga_0.8As/GaAs double quantum well. These variations reflect the nontrivial behavior of gaps in the calculated magnetic level patterns. Fourier spectra of oscillations in tilted fields also exhibit a complex structure, and the sum of frequencies of peaks is not constant. It is assumed that this is associated with the magnetic breakdown effect.     

Photoluminescence spectra of doped GaAs films

We have studied the dependence of the photoluminescence (PL) spectrum on the doping level and the film thickness of n-GaAs thin films, both experimentally and theoretically. It has been shown theoretically that modification of the PL spectrum of p-type material by p-type doping is very small due to the large valence-band hole effective mass. The PL spectrum of n-type material is affected by two factors: (1) the electron concentration which determines the Fermi level in the material; (2) the thickness of the film due to re-absorption of the PL signal. For the n-type GaAs thin films under current investigation, the doping level as well as the film thickness can be very well calibrated by the PL spectrum when the doping level is less than 2×10¹⁸ cm^-3 and the film thickness is in the range of the penetration length of the PL excitation laser. PACS 78.20.-e; 78.55.Cr; 78.66.Fd     

Studies on the third-harmonic generation of double-layered quantum wires in magnetic fields

The third-harmonic generations of two vertically stacked quantum wires in magnetic fields are investigated. The analytic formula for the third-harmonic generation of double-layered quantum wires is derived by means of density matrix treatment. The numerical results are presented for GaAs/Al _x Ga_1–x As double quantum wires. Finally, the calculated third-harmonic generations are plotted versus the magnetic field B, the photon energy h and the interlayer distance D.     

Polaron effects on the binding energy of a hydrogenic impurity in parabolic quantum wires in perpendicular electric and magnetic fields

Using a variational approach, the binding energy of shallow hydrogenic impurities in a parabolic quantum wire is calculated within the effective mass approximation. The polaron effects on the ground-state binding energy in electric and magnetic fields are investigated by means of the Pekar–Landau variation technique. The results for the binding energy as well as a polaronic correction are obtained as a function of the applied fields and the impurity positions.     

Size dependence of a magnetopolaron in cylindrical quantum wires at arbitrary magnetic fields

With the use of variational method to solve the effective mass equation, we have studied the cyclotron resonance of a magnetopolaron in cylindrical quantum wires at arbitrary magnetic fields. The interaction of the electron with surface-optical (SO) phonons is used. Having calculated the ground state energy and the excited state energy of the magnetopolaron, we obtain the hybrid frequency of the magnetopolaron in quantum wires.