Quantum dot semiconductor laser with optoelectronic feedback

The optoelectronic feedback (OEF) in quantum dot semiconductor lasers (QD SLs) is studied theoretically where a model includes wetting layer ground state and excited state for QDs are included separating electrons and holes in their dynamics. Both positive and negative OEF are studied. The time series of photon density, the phase portraits of carriers in the states are studied. The parameters affecting OEF are examined where an excitability is seen. The QD SL is found to be more sensitive to the changes in time delay compared with other SLs and a complicated routs are seen in the behavior of QD SL.     

Single-shot readout of electron spins in a semiconductor quantum dot

We report on a method for single-shot readout of spin states in a semiconductor quantum dot that is robust against charge noise and can be used even when the electron temperature exceeds the energy splitting between the states. The spin states are first correlated to different charge states using a spin dependence of the tunnel rates. A subsequent fast measurement of the charge on the dot then reveals the original spin state. The method is analyzed theoretically, and compared to a previously used method. We experimentally demonstrate the method by performing readout of the two-electron spin states, achieving a single-shot visibility of more than 80%. We find very long triplet-to-singlet relaxation times (up to several milliseconds), with a strong dependence on in-plane magnetic field.     

Bandgap calculation in Si quantum dot arrays using a genetic algorithm

In this work we present a fast and accurate genetic algorithm to determine the envelope functions and eigenenergies of the ground states of electrons and holes in low-dimensional complex semiconductor structures. We have developed the theoretical formalism of the algorithm in a general way in order to make it easy to include arbitrary nonparabolic and anisotropic band profiles in the calculations. From these results, calculation of the bandgaps of nanostructures can be carried out efficiently.Besides presenting and testing the algorithm, we calculate the ground state of electron and holes in two-dimensional quantum dot arrays, taking nonparabolicity and anisotropy into account.     

Lateral electric field effects on quantum size confinement in cylindrical quantum dot under parabolic potential

Within the effective mass approximation, we investigated theoretically the ground-state energy of a single particle and the binding energy of the neutral donor impurity (D⁰) affected by a lateral electric field in a parabolic quantum dot (QD). The results show that the electron and the hole ground-state energy and the band to band transition energies shift to lower values (red shift) by increasing the field intensity. The quantum Stark shift (QSS) for the electron increases rapidly in the quasi spherical QD (QSQD) by increasing the lateral field, whereas for the hole it increases monotony. In the cylindrical QDs (CQDs), we found that the QSS for electron and hole increase monotonically. The quantum size, lateral electric field and impurity position effect on the binding energy of neutral donor (D⁰) is studied. Unexpected behavior of D⁰ in quantum well limit (QW), the binding energy of D⁰ is increasing (blue shift) with increasing QD radius R$R$ at the presence of a lateral electric field. It appears that for a fixed size of the QD, the off-center binding energy decreases when the impurity ion is displaced from the center to the QD borders, while it is shifted to lower energy with increasing the field.     

External cavity quantum dot tunable laser through 1.55 μm

The optical performance of InAs/InGaAsP quantum dot (QD) lasers grown on (1 0 0) InP was studied for three different material structures. The most efficient QD laser structure, having a threshold current of 107 mA and an external differential quantum efficiency of 9.4% at room temperature, was used to form the active region of a grating-coupled external cavity tunable laser. A tuning range of 110 nm was demonstrated, which was mainly limited by the mirror and internal losses of the uncoated laser diode. Rapid state-filling of the QDs was also demonstrated by observing the evolution of the spectra with increasing injected current.     

Incoherent control of optical bistability in an InGaN/GaN quantum dot nanostructure

The manipulation of bistable curve in the infrared (IR) region has been investigated theoretically in a unidirectional ring cavity doped by a four-level InGaN/GaN quantum dot nanostructure. The four-level quantum dot nanostructure is designed numerically by using the Schrödinger and Poisson equations. By controlling the size of the quantum dot and external voltage, one can design a four-level quantum dot with appropriate energy levels which can be suitable for interaction with IR signals. It is realized that the incoherent pumping fields play an essential role in controlling the intensity threshold of optical bistability. Decoherence effects such as the dephasing rate and electron density of the quantum dot are also analyzed at the threshold of optical bistability. Our proposed model due to its important application in all-optical systems may be favorable for real experimental evolution in infrared regions.     

Optical field distribution in ZnO/MgZnO quantum dot nanostructure at 375-nm wavelength

Computer analysis of near and far-field intensities in zinc oxide based quantum dot nanostructure has been carried out for the optimization of smaller active region volume of quantum dot to achieve desired modal gain. Near and far-field intensity distribution along junction plane has been studied as a function of mole fractions of Mg and for varying quantum dot thickness. Solutions to the wave equations have been obtained for the transverse electric mode. Effective index method has been used to deduce the propagation constants in various regions of optical wave-guide. Excellent confinement of near field intensity for greater values of Mg composition has been attributed to the increased values of refractive index step between dot and barrier regions. For the thicker dots, greater fraction of the optical field has been confined to the dot region. Near field intensity spread has been deduced and articulated in terms of full width of half maximum (FWHM) as a function of Mg mole fraction and a dot thickness. It clearly shows nonlinear decrease with increase of both the Mg mole fraction and a dot thickness. The surface image clearly reveals a bright spot in the center corresponding to far-field emitted from the dot region at 375-nm wavelength. Far-field intensity reveals divergence of 16°.     

Exciton relaxation in self-assembled semiconductor quantum dots

The present study focuses on the effect of excited states on the exciton–polaron spectrum for self-assembled InAs/GaAs semiconductor quantum dots. The analytical model takes into account the Coulomb interactions between the electron and the hole as well as, each carrier, the coupling with the longitudinal optical phonon field. Furthermore, the key role played by the exciton continuum in the dot spectrum is also introduced. Such an approach is well fitted to analyze recent experimental findings about single-dot spectroscopy and allows peaks assignment, line width estimation, relaxation time evaluation, etc., necessary steps toward an understanding of the internal dynamics of quantum dots.     

Fabrication of quantum dot transistors incorporating a single self-assembled quantum dot

We report on the fabrication and the characterization of quantum dot transistors incorporating a single self-assembled quantum dot. The current–voltage characteristics exhibit clear staircase structures at room temperature. They are attributed to electron tunneling through the quantized energy levels of a single quantum dot.     

An integrated circuit subsystem of quantum dot semiconductor optical amplifier coupled with electro-absorption modulator and its application in wavelength conversion

A multi-section circuit model of quantum dot semiconductor optical amplifier is proposed by employing the standard rate equations. Using this model, gain spectra, saturation property, and occupation probability of quantum dot semiconductor optical amplifier are analyzed by PSPICE simulation. An integrated circuit subsystem of quantum dot semiconductor optical amplifier cascaded with electro-absorption modulator is also derived to investigate the patterning effect reduction in wavelength conversion.     

Finite-difference calculation of donor energy levels in a spherical quantum dot subject to a magnetic field

The ground and excited states of a donor impurity at the center of a spherical quantum dot subject to a magnetic field are calculated within the effective-mass approximation. The barriers are infinitely high and the differential equation is solved by combining the finite-difference method with the Richardson extrapolation. The binding and transition energies are more accurate than the available variational values, and excellent agreement is found with the hydrogen atom. The transition energies for a medium-size quantum dot are given.     

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.     

Third-order nonlinear optical properties of excitonic state in a semiconductor quantum dot

By considering usual matrix procedures we examine how the exciton affects the nonlinear optical properties of 3-D semiconductor GaAs quantum dot. We calculate the third-order optical susceptibility of the GaAs (well) Al_xGaAs_1?x (barrier), and consequently the refractive index and the absorption coefficient. By increasing the Al content (x) in barrier material, carrier relaxation time is enhanced and the susceptibility peaks and their positions showed a blue shift, which agrees with the existing experimental work. For an anisotropic QD, the third-order nonlinear absorption coefficient depends strongly on the quantum dot width.     

Double detected spin-dependent quantum dot

We study the dynamics of a spin-dependent quantum dot system, where an unsharp and a sharp detection scenario is introduced. The back-action of the unsharp detection related to the magnetization, proposed in terms of the continuous quantum measurement theory, is observed via the von Neumann measurement (sharp detection) of the electric charge current. The behavior of the average electron charge current is studied as a function of the unsharp detection strength γ$\gamma$, and features of measurement back-action are discussed. The achieved equations reproduce the quantum Zeno effect. Considering magnetic leads, we demonstrate that the measurement process may freeze the system in its initial state. We show that the continuous observation may enhance the transition between spin states, in contradiction with rapidly repeated projective observations, when it slows down. Experimental issue, such as the accuracy of the electric current measurement, is analyzed.     

Shot noise in a quantum dot with the coulomb interaction

We study the noise in a quantum dot which is coupled to metallic leads by using the non-equation of motion technique at the Kondo temperature T_K. We compute the out of equilibrium density of states, the current and the shot noise. We find that the shot noise exhibits a nonmonotonic dependence on the voltage when variation of ε_d values of the QD energy in the absence of the external magnetic field occurs. We also find that the amplitude of current exhibits a saturation behavior when driving field is increased.     

Theory of pulse propagation and four-wave mixing in a quantum dot semiconductor optical amplifier

A theory, combining the relations of pulse traveling into quantum dot (QD) semiconductor optical amplifier (SOA) with the four-wave mixing (FWM) theory in these SOAs, is developed. Carrier density pulsation (CDP), carrier heating (CH), and spectral hole burning (SHB) contributions on FWM efficiency are discussed. Effect of QD ground state and wetting layer are included. An additional parameter appears in the gain integral relation of QD SOAs. An equation formulating pulses in the QD SOAs is introduced. We have found that FWM in QD SOAs is detuning and is pulse width dependent. For short pulses, CH is dominant at high detunings (10–100 GHz) while at higher detunings (>100 GHz) the SHB is the dominant one. Undesired paunch behavior is shown in QD SOAs then, CDP must be reduced.     

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.     

Tunable supercurrent in a triangular triple quantum dot system

The supercurrent in a triangular triple quantum dot system is investigated by using the nonequilibrium Green's function method. It is found that the sign of the supercurrent can be changed from positive to negative with increasing the strength of spin-flip scattering, resulting in the π-junction transition. The supercurrent and the π-junction transition are also modulated by tuning the system parameters such as the gate voltage and the interdot coupling. The tunable π-junction transition is explained in terms of the current carrying density of states. These results provide the ways of manipulating the supercurrent in a triple quantum dot system.     

Tunable quantum dot resonator embedded in a quantum wire

Combined quantum wire and quantum dot system is theoretically predicted to show unique conductance properties associated with Coulomb interactions. We use a split gate technique to fabricate a quantum wire containing a quantum dot with two tunable potential barriers in a two-dimensional electron gas. We observe the effects of the quantum dot cavity on the electron transport through the quantum wire, such as Coulomb oscillations near the pinch-off voltage and periodic conductance oscillations on the first conductance plateau.