Optical properties of impurity-bound polaron in a parabolic quantum dot

The linear and nonlinear optical properties of an electron, which is bounded to a Coulomb impurity in a polar semiconductor quantum dot with parabolic confinement in both two and three dimensions, are studied by using the Landau-Pekar variational method and the compact density-matrix approach. With typical semiconducting GaAs-based materials, the linear, third-order nonlinear, total optical absorption coefficients and refractive indexes have been examined. We find that the all absorption spectra and refractive index changes are strongly affected by the electron-LO-phonon interaction. The results also indicate that the polaron effect increases with decreasing dimensionality of a quantum dot.     

Hybrid stable-chaotic states in coupled quantum well stadia

We use tunnel current spectroscopy to investigate the quantum states of two GaAs quantum wells coupled by a low (100 meV) (AlGa)As tunnel barrier. A high tilted magnetic field is used to generate strongly chaotic electron motion in the two wells which act as coupled chaotic ‘stadia'. The effect of the tunnel barrier on the dynamics of the system depends on the magnitude of the applied bias voltage V. For V375 mV, the central potential barrier acts as a perturbation which modifies the trajectories of selected periodic orbits in the quantum well. Scattering off the central barrier also generates new periodic orbits involving multiple collisions on all three barriers. These orbits ‘scar' distinct sets of eigenstates which generate periodic resonant peaks in the current–voltage characteristics of the device. When the device is biased such that the injected electrons just surmount the central barrier, our calculations reveal novel hybrid scarred states with both stable and chaotic characteristics.     

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).     

Magnetoresistance and magnetization studies through a Cu interlayer in spin valves of NiFe/Cu/NiFe/FeMn

The influence of the Cu layer thickness on the magnetic and magnetotransport properties has been investigated in Ta/NiFe/Cu/NiFe/FeMn spin valves. The magnetization and magnetoresistance measurements were carried out for magnetic field applied along the easy-axis direction. A phenomenological model, which assumes formation of a planar domain wall at the anti-ferromagnetic side of the interfaces as well as bilinear coupling between the ferromagnetic layers, was used to derive the anisotropy characteristics and orientation of each NiFe layer magnetization. The anisotropy and spin valve magnetoresistance were simulated numerically and compared with the experiment. It was found that the anisotropy magnetoresistance is negligible and that there is a poor agreement for the spin-valve one, which was attributed to the model (valid for ferromagnetic layers in single-domain state only) used for its calculation. It was found that the increase of the Cu layer thickness provokes a decrease of the interdiffusion between the NiFe and FeMn layers, and, as consequence, changes of the uniaxial anisotropy of the pinned NiFe layer, of the exchange interaction between the pinned NiFe layer and the FeMn ones, as well as of the exchange-bias field of the pinned NiFe layer.     

Antiresonance of electron tunneling through a quantum dot array

Electronic transport through a one-dimensional quantum dot array is theoretically studied. In such a system both electron reservoirs of continuum states couple with the individual component quantum dots of the array arbitrarily. When there are some dangling quantum dots in the array outside the dot(s) contacting the leads, the electron tunneling through the quantum dot array is wholly forbidden if the electron energy is just equal to the molecular energy levels of the dangling quantum dots, which is called as antiresonance of electron tunneling. Accordingly, when the chemical potential of the reservoir electrons is aligned with the electron levels of all quantum dots, the linear conductance at zero temperature vanishes if there are odd number dangling quantum dots; Otherwise, it is equal to 2e²/h due to resonant tunneling if the total number of quantum dots in the array is odd. This odd–even parity is independent of the interdot and the lead–dot coupling strength.     

Spin valve effect and negative tunnel magnetoresistance in a quantum dot transistor

We study the spin polarized transport through a quantum dot transistor. It is shown that the interplay of large Coulomb interaction and optically induced spin accumulation gives rise to the spin valve effect over a range of bias. We also find negative tunnel magnetoresistance for system with ferromagnetic electrodes.     

Optical absorptions of a biexciton quantum dot

A investigation of the linear and nonlinear optical properties for intersubband electronic transitions associated with a biexciton in a quantum dot has been performed by using the method of few-body physics. The optical absorption coefficients and the refractive index changes have been examined based on the computed energies and wave functions. It is over two orders of magnitude higher than that obtained in an exciton quantum dot. The results show that the optical absorption saturation intensity can be controlled by the confinement potential frequency and the relaxation time.     

Temperature dependence of magnetic properties in Fe/Fe-O nanoparticles dispersed in water

Commercial water dispersions of Fe/Fe-O nanoparticles (∼50-80 nm diameter) were prepared bare and treated with a biodegradable polymer to stabilize the suspension. Hysteresis loops and FC-ZFC curves of samples in liquid and dried form were measured by means of a vibrating sample magnetometer from 77 to 300 K. A marked dependence on the conditioning field is observed in all samples that display the Verwey transition at about 120 K. The role of magnetic interactions on dried samples was also investigated by means of magnetoresistance measurements as a function of temperature.     

Multi-exciton spectroscopy on a self-assembled InGaAs/GaAs quantum dot

We report about spatially resolved magneto-optical experiments on a self-assembled InGaAs quantum dot. Using electron beam lithograpy for patterning a metal shadow mask we can isolate a single dot. This allows us to study the optical response of a single dot as a function of excitation power and magnetic field. We investigate the influence of many body interaction in the emission spectra for different exciton occupation numbers of the dot. The diamagnetic/orbital shift as well as Zeeman splitting in a magnetic field can be fully resolved and are used to identify the observed emission lines. Further we report on absorption properties of the quantum dot as a function of magnetic field. We analyse in detail the phonon-assisted absorption process connected with the GaAs LO-phonon 36 meV above the single-exciton ground state.     

Optical transitions in parabolic quantum dot

Direct optical absorption of light is theoretically investigated in a spherical quantum dot from GaAs. The confinement potential of the dot is approximated as parabolic. Three regimes of size quantization are discussed: weak, strong, and intermediate. The corresponding threshold frequencies of absorption are determined. A comparison with the case of a spherical quantum dot with rectangular infinitely high confinement potential is performed.     

Influence of N cluster states on band dispersion in GaInNAs quantum wells

We use a modified band-anticrossing (BAC) model to investigate the band dispersion in a GaN_xAs_1-x/AlGaAs quantum well (QW) as a function of hydrostatic pressure. The band edge mass increases considerably more quickly with pressure than in the case of a GaAs/AlGaAs QW, and the subband separation also decreases significantly. We predict that the strong anticrossing interaction between the GaAs host conduction band and isolated N levels will inhibit tunnelling through the QW for a range of energy above the isolated N levels. The energy of N resonant states depends strongly on details of the local environment, giving a broader calculated distribution of N states in GaInNAs compared to GaNAs.     

Optical properties of a GaAs cone-like quantum dot: Second and third-harmonic generation

In the present work, we have studied numerically the second harmonic generation (SHG) and third harmonic generation (THG) in a cone-like quantum dot. For this purpose, we firstly obtained the energy levels and wave functions of this system. Then, we have used an expression for the SHG and THG by a compact density matrix approach and an iterative procedure. The SHG and THG enhance and shift toward lower energy by increasing the height h and vertex angle α. According to the results it is found that the structural parameters have great influence on the THG in this system.     

Optical properties in a two-dimensional quantum ring: Confinement potential and Aharonov-Bohm effect

Optical properties of a two-dimensional quantum ring with pseudopotential in the presence of an external magnetic field and magnetic flux have been theoretically investigated. Our results show that both of the pseudopotential and magnetic field can affect the third non-linear susceptibility and oscillator strength. In addition, we found that the oscillator strength and the absolute value of the resonant peak of the linear, non-linear and total absorption coefficient demonstrates the Aharonov-Bohm oscillation with magnetic flux, moreover, changes in confinement potential can influence the Aharonov-Bohm oscillation in peak while the resonant peak value of the linear, non-linear and total refractive index changes decreases as magnetic flux increases.     

Optical properties of semiconductor quantum dots in magnetic fields

Optical properties of semiconductor quantum dots in magnetic fields are reviewed. A theory is described based on a multi-band effective-mass approximation with a nonparabolic conduction electron dispersion, the direct Coulomb interaction, and the electron-hole exchange interaction taken into account. The transition from the quantum-confined Zeeman effect for a weak magnetic field to the quantum-confined Paschen-Back effect to a strong magnetic field is discussed in comparison with atomic spectra in magnetic fields. Experimental results of the optical properties of isolated CuCl, CdSSe, and Si quantum dots in magnetic fields are also discussed in conjunction with the theoretical results.     

Optical properties of an off-center hydrogenic impurity in a spherical quantum dot with Gaussian potential

Optical absorption coefficients and refractive index changes associated with intersubband transition of an off-center hydrogenic impurity in a spherical quantum dot (QD) with Gaussian confinement potential are theoretically investigated. Our results show that the optical absorption coefficients in a spherical QD are 2–3 orders of magnitude higher than those in quantum wells and are 2–3 orders smaller than those in a disk-like QD. It is found that the optical absorptions and the optical refractive index are strongly affected not only by the confinement barrier height, dot radius but also by the position of the impurity.     

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.     

Optical solitons in semiconductor quantum dot waveguides

A theory of self-induced transparency for a TM-mode propagating in a planar semiconductor waveguide sandwiched between two dielectric media is developed. A transition layer between the waveguide and one of the connected media is described using a model of a two-dimensional sheet of quantum dots. Explicit analytical expressions for the optical soliton in the presence of single-excitonic and biexcitonic transitions are obtained with realistic parameters which can be reached in current experiments.     

Multi-color tunneling quantum dot infrared photodetectors operating at room temperature

Quantum dot structures designed for multi-color infrared detection and high temperature (or room temperature) operation are demonstrated. A novel approach, tunneling quantum dot (T-QD), was successfully demonstrated with a detector that can be operated at room temperature due to the reduction of the dark current by blocking barriers incorporated into the structure. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunneling, while the dark current is blocked by AlGaAs/InGaAs tunneling barriers placed in the structure. A two-color tunneling-quantum dot infrared photodetector (T-QDIP) with photoresponse peaks at 6 μm and 17 μm operating at room temperature will be discussed. Furthermore, the idea can be used to develop terahertz T-QD detectors operating at high temperatures. Successful results obtained for a T-QDIP designed for THz operations are presented. Another approach, bi-layer quantum dot, uses two layers of InAs quantum dots (QDs) with different sizes separated by a thin GaAs layer. The detector response was observed at three distinct wavelengths in short-, mid-, and far-infrared regions (5.6, 8.0, and 23.0 μm). Based on theoretical calculations, photoluminescence and infrared spectral measurements, the 5.6 and 23.0 μm peaks are connected to the states in smaller QDs in the structure. The narrow peaks emphasize the uniform size distribution of QDs grown by molecular beam epitaxy. These detectors can be employed in numerous applications such as environmental monitoring, spectroscopy, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.