Asymmetric Quantum Dot Research Articles

Properties of a Strong Coupling Bipolaron in an Asymmetric Quantum Dot

We study a strong coupling bipolaron’s vibrational frequency, self-trapping energy and potential induced by the electron–longitudinal optical (LO) phonon interaction in an asymmetric quantum dot (AQD). The effects of the electron–phonon coupling strength, the transverse and longitudinal effective confinement lengths are taken into account by using linear combination operator and unitary transformation methods. It is found that the vibrational frequency is an increasing function of the electron–phonon coupling strength, whereas it is a decreasing one of the transverse and longitudinal effective confinement lengths. The absolute values of the self-trapping energy and the potential induced by the electron–LO phonon interaction will increase with increasing coupling strength or decreasing effective confinement lengths.

Nanoparticle Probes of Cell Surface Molecule Rotation

Rotation of membrane proteins is a sensitive measure of their aggregation state and their interactions with other membrane species. We have used nanoparticles including asymmetric quantum dots as non-bleaching probes of the rotation of individual cell surface proteins. Invitrogen QD655s conjugated to A2 DNP-specific IgE allow examination of slow rotation of the Type I Fce receptor (FceRI) on RBL-2H3 cells. Fluorescence from cell-bound quantum dots is excited by illumination polarized at 45 deg and a Princeton Instruments Dual View equipped with a polarizing beam splitter allows recording image sequences containing simultaneous x- and y-polarized sub-images in each frame. For individual quantum dots, we calculate the time-autocorrelation functions for fluorescence polarization fluctuations. Decays of these fluctuations extend down to the ms timescale, as implied by time-resolved phosphorescence anisotropy results. Treatment effects suggest that such slow decay may be a property of the membrane itself, perhaps reflecting large-scale fluctuations of mesoscale membrane regions. To examine rotational correlation function decays faster than imaging measurements permit, we have applied time-tagged single photon counting to individual quantum dot fluorescence and analyzed data down to correlation times below 1μs. These measurements should thus include the 80μs hydrodynamic rotation of single FceRI molecules, but significance of current data is limited by quantum dot emission rates. Other probes are therefore being explored as are the consequences of the two-dimensional transition dipoles exhibited by quantum dots. Supported by NSF grant MCB-1024668.

Control of indirect exciton population in an asymmetric quantum dot molecule

Abstract We analyze the problem of coherent population transfer to the indirect exciton state in an asymmetric double semiconductor quantum dot molecule that interacts with an external electromagnetic field. Using the controlled rotation method, we obtain analytical solutions of the time-dependent Schrodinger equation and determine closed-form conditions for the parameters of the applied field and the quantum system that lead to complete population transfer to the indirect exciton state, in the absence of decay effects. Then, by numerical solution of the relevant density matrix equations we study the influence of decay mechanisms to the efficiency of population transfer.

Effects of probe field intensity in nonlinear optical processes in asymmetric semiconductor quantum dots

We study nonlinear optical absorption and nonlinear optical rectification in an asymmetric semiconductor quantum dot structure under a strong probe field excitation. We apply a form of the rotating wave approximation for asymmetric quantum systems, solve the relevant density matrix equations under steady state conditions, and derive the formulae for nonlinear optical absorption and nonlinear optical rectification under the interaction with a strong probe field. The differences between our formulae and those of a previous study are also presented for the case of an electron confined in an asymmetric double quantum dot nanostructure.

Spectral line narrowing via spontaneously generated coherence in quantum dot molecules

We theoretically demonstrate that it is possible to simulate spontaneously generated coherence in an asymmetric double quantum dot system, a quantum dot molecule. With the tunneling coupling, the system emulates to large degree a three-level system with quantum interference and the fluorescence spectrum can display ultranarrow lines. In our system the coupling field is not necessary and the degree of quantum interference can be controlled. The dressed states for this system are identified, and the spectral features are interpreted in terms of transitions among these dressed states. The system studied here is more practical and can permit the observation of spontaneously generated coherence without the need for closely lying levels and parallel dipole moments.

Nonlinear optical rectification in asymmetric quantum dots with an external static magnetic field

The optical rectification (OR) coefficient in asymmetric quantum dots (QDs) with an external static magnetic field is theoretically investigated. Using the effective-mass approximation, we obtain the confined wave functions and energies of electrons in QDs. We also obtain the OR coefficient by the compact-density-matrix approach and the iterative method. The results of numerical calculations for the typical GaAs/AlGaAs QDs show that the OR coefficient depends strongly on the external static magnetic field, parameters of the asymmetric potential and radius of the QD. Moreover, the peak of the OR coefficient shifts with the magnetic field or the radius of the QD changing.

The effect of electric field on an asymmetric quantum dot qubit

On the condition of strong electron---LO phonon coupling in an asymmetric quantum dot (QD), we study the eigenenergies and eigenfunctions of the ground and the first excited states under an applied electric field by using variational method of Pekar type. This QD system may be used as a two-level qubit. When the electron is in the superposition state of the ground and the first excited states, we obtain the time evolution of the electron probability density, which oscillates in the QD. It is found that due to the presence of the 3-D anisotropic harmonic potentials in the transverse and longitudinal directions of the QD, the electron probability density shows double-peak configuration, whereas there is only one peak if the confinement is 2-D symmetric in the x- and y-directions. The oscillation period is an increasing function of the transverse and longitudinal effective confinement lengths of the QD, and decreases with respect to the electron---phonon coupling strength and the electric field.

Temperature and impurity effects of the polaron in an asymmetric quantum dot

Temperature and impurity effects of the ground state energy and the ground state binding energy in an asymmetric quantum dot are studied here by using the linear combination operator method. It is found that the ground state energy and the ground state binding energy increase with increasing temperature. The ground state energy is a decreasing function of the Coulomb bound potential, whereas the ground state binding energy is an increasing one.

Effects of the magnetic field direction and anisotropy on the interband light absorption of an asymmetric quantum dot

In this paper, the direct interband transition and the threshold frequency of absorption in a two-dimensional anisotropic quantum dot are studied under the influence of a tilted external magnetic field. We first calculate the analytical wave functions and energy levels using a transformation to simplify the Hamiltonian of the system. Then, we obtain the analytical expressions for the light interband absorption coefficient and the threshold frequency of absorption as a function of the magnetic field, magnetic field direction, and anisotropy of the system. According to the results obtained from the present work, we find that (i) the absorption threshold frequency (ATF) increases when the magnetic field increases for all directions. (ii) When anisotropy is increased, ATF increases. (iii) At small anisotropy, the magnetic field direction has no important effect on the ATF. In brief, the magnetic field, magnetic field direction, and anisotropy play important roles in the ATF and absorption coefficient.

Optical absorption of an asymmetric quantum dot in the presence of an uniform magnetic field

We theoretically investigate the optical absorption coefficient (OAC) of an asymmetric quantum dot (QD) in the presence of an uniform magnetic field. Using the effective-mass approximation, we study the electronic structure of the QD. We obtain the linear, nonlinear and total OAC by the compact-density-matrix approach and iterative method. The results of numerical calculations for the typical GaAs/AlGaAs QD show that the OAC depend strongly on the radius of the QD, parameters of the asymmetric potential, external magnetic field and incident optical intensity. Moreover, the peak of the OAC shifts with the magnetic field or the radius of the QD changing.

Simultaneous positive and negative photocurrent response in asymmetric quantum dot infrared photodetectors

We study the influence on the photocurrent of the final state in bound-to-quasibound transitions in self-assembled quantum dot infrared photodetectors. We investigate two structures designed to explore different mechanisms of carrier extraction and therefore achieve a better insight on these processes. We observe photocurrent in opposite directions, with positive and negative sign, for different incident frequencies at the same applied external electric field. This phenomenon is attributed to the asymmetry of the potential barriers surrounding the quantum dots.

Electric Field Effect on State Energies and Transition Frequency of a Strong-Coupling Polaron in an Asymmetric Quantum Dot

We study the ground and the first excited states’ energies and the corresponding transition frequency of a strong-coupling polaron in an asymmetric quantum dot (AQD). The effects of the electric field, the transverse and the longitudinal effective confinement lengths and the electron-phonon coupling strength are taken into account by using a variational method of the Pekar type. It is found that the ground and the first excited states’ energies and the transition frequency are decreasing functions of the electric field. They will increase rapidly with decreasing the transverse and longitudinal effective confinement lengths. The transition frequency is an increasing function of the electron-phonon coupling strength, whereas the energies are decreasing ones of it.

库仑场对非对称量子点中弱耦合极化子激发能量的影响

The influence of coulomb field to excitation energy on the weak-coupling polaron in an asymmetric quantum dot has been studied by making use of the linear combination operator and the unitary transformation method.It is obtained that the transition frequency and the excited energy of the weakcoupling polaron change with the effective confinement length of the quantum dot and the Coulomb bound potential in an asymmetric quantum dot.It is shown that the transition frequency and the excited energy increase rapidly with decreasing effective confinement length when the Coulomb bound potential is a constant value.And the transition frequency and excited energy increase with the increasing Coulomb bound potential.

Spin-polarized current in double quantum dots

We analyze the transport through asymmetric double quantum dots with an inhomogeneous Zeeman splitting in the presence of crossed dc and ac magnetic fields. A strong spin-polarized current can be obtained by changing the dc magnetic field. It is mainly due to the resonant tunnelling. But for the ferromagnetic right electrode, the electron spin resonance also plays an important role in transport. We show that the double quantum dots with three-level mixing under crossed dc and ac magnetic fields can act not only as a bipolar spin filter but also as a spin inverter under suitable conditions.

EFFECTS OF SPIN ON THE GROUND-STATE ENERGY OF STRONG-COUPLED BOUND MAGNETOPOLARON IN AN ASYMMETRIC QUANTUM DOT

The properties of a strong-coupled bound magnetopolaron in an asymmetric quantum dot (QD) have been investigated by using the Tokuda modified linear combination operator and the unitary transformation methods on the basis of the Huybrechts' strong-coupled model. We derive the expressions of the ground-state energy as function of the transverse and longitudinal confinement lengths, the magnetic field. Numerical calculation is performed and the results show that the ground-state energy of the bound magnetopolaron splits into two branches, taking into account the spin influences. And the ground-state energy decreases with increasing the transverse and longitudinal confinement lengths and increases with the rising of the magnetic field.

OPTICAL PHONON EFFECT IN AN ASYMMETRIC QUANTUM DOT QUBIT

We investigate the eigenenergies and the eigenfunctions of the ground and the first excited states of an electron, which is strongly coupled to LO-phonon in an asymmetric quantum dot (QD) by using variational method of Pekar type. The present system may be used as a two-level qubit. When the electron is in the superposition state of the ground and the first excited states, the probability density of the electron oscillates in the QD with a certain period. It is found that the oscillation period is an increasing function of the transverse and the longitudinal effective confinement lengths of the QD, whereas it is a decreasing one of the electron–phonon coupling strength.

Tuning the dynamic properties of electrons between a quantum well and quantum dots

We present a study of the dynamic properties of electrons tunneling from an InGaAs quantum well to self assembled InAs quantum dots. The experiments were conducted on three highly asymmetric quantum dot infrared photodetectors, where the quantum well and quantum dots were separated by a composite GaAs/AlGaAs/GaAs barrier, which varied from 3.5 nm to 7.0 nm. We performed interband (photoluminescence) and intraband (photocurrent) measurements to characterize the spectral properties of the well and the dots. The photoluminescence measurements revealed that the two nanostructures are decoupled when the device is at zero bias. By intraband pump-probe experiments and photocurrent saturation experiments, we were able to extrapolate a refilling time τ from the well to the dots, which varied from a few μs for the thinnest barrier and hundreds of μs for the thickest one. The extracted values are in good agreement with characteristic tunneling times computed by using a model based on the theoretically predicted transmission coefficient of the electrons through the composite barrier.

Nonlinear optical rectification of a hydrogenic impurity in asymmetric lens-shaped quantum dots

The compact density matrix approach and an iterative method are used to calculate the nonlinear optical rectification coefficient (ORC) of a hydrogenic impurity in an asymmetric lens-shaped quantum dot (LSQD). Numerical results are presented for a typical GaAs LSQD. The effects of the size of LSQD, incident photon energy and asymmetry of QDs on the ORC are investigated. The results show that the ORC increases with decreasing the mirror symmetry of the QD. In addition, the ORC increases and its peak shifts to lower energies when the dot size is increased.

Second-harmonic generation in asymmetric quantum dots in the presence of a static magnetic field

The second-harmonic generation (SHG) coefficient in an asymmetric quantum dot (QD) with a static magnetic field is theoretically investigated. Within the framework of the effective-mass approximation, we obtain the confined wave functions and energies of electrons in the QD. We also obtain the SHG coefficient by the compact-density-matrix approach and the iterative method. The numerical results for the typical GaAs/AlGaAs QD show that the SHG coefficient depends strongly on the magnitude of magnetic field, parameters of the asymmetric potential and the radius of the QD. The resonant peak shifts with the magnetic field or the radius of the QD changing.

Electric-Field Switching of Exciton Spin Splitting in Asymmetrical Coupled Quantum Dots

The spin splitting of the exciton states in semiconductor asymmetrical coupled quantum dots (CQDs) containing a single magnetic ion is theoretically investigated. It is found that the spin splitting is not the largest when the electric field is zero. An electric field stronger than that in symmetrical CQDs is necessary to switch the splitting on/off. The mixing between the bonding and antibonding hole states consequently results in the bright-to-dark transition of the ground exciton in the photoluminescence spectrum.