Hydrogenic impurities in quantum dots under intense high-frequency laser field

The effects of intense high-frequency laser field on photoionization cross-section and binding energy of shallow-donor impurities in GaAs/GaAlAs quantum dots are calculated using variational method with the effective-mass approximation. From these calculations, it has been concluded that the dependences of the impurity binding energy and photoionization cross-section on the intense laser field are very significant.     

The effect of intense laser field on the Electronic Raman Scattering of shallow donor impurities in quantum dots

The laser-field dependence of energy levels and donor-related Electronic Raman Scattering is investigated by a quasi-analytical approach. The differential cross section involved in this process is calculated as a function of secondary radiation photon energy. We find that the laser field amplitude and confinement strength have an important influence on the Raman scattering. And the transitions between lower electronic energy states are more sensitive to the laser field.     

The effects of intense laser on nonlinear properties of shallow donor impurities in quantum dots with the Woods-Saxon potential

A theoretical study of the effects of intense laser fields on the nonlinear properties of donor impurities in a quantum dot with Woods-Saxon potential is performed within the matrix diagonalization method with the use of the effective mass approximation. The great advantage of our methodology is that it enables confinement regimes by varying two parameters in the model potential. The intense laser effects are included through the Floquet method, by modifying the confining potential associated to the heterostructure. Based on the computed energies and wave functions, the optical absorption coefficients and the refractive index between the ground state (L=0) and the first excited state (L=1) have been examined. Several configurations of the barrier height, the dot radius, the barrier slope of the confinement potential and the incident intense laser radiation have been considered. The outcome of the calculation suggests that all the factors mentioned above can influence the nonlinear optical properties strongly.     

Laser field effect on the nonlinear optical properties of donor impurities in quantum dots with Gaussian potential

A detailed investigation of optical properties of donor impurities in quantum dots under the influence of laser field with Gaussian potential is performed by using the matrix diagonalization method within the effective mass approximation. Based on the computed energies and wave functions, the dependence of the nonlinear optical properties on the dot size and the potential depth is investigated. The outcome of the calculation suggests that all the factors mentioned above can influence the nonlinear optical properties strongly. We also note that the increase of the laser-dressing parameter leads to important effects on the electronic and optical properties of a quantum dot. This gives a new degree of freedom in various device applications based on the intersubband transition of electrons.     

Polaronic effects on laser dressed donor impurities in a quantum well

The effect of laser field on the binding energy in a GaAs/Ga1_1−xAl_xAs quantum well within the single band effective mass-approximation is investigated. Exciton binding energy is calculated as a function of well width with the renormalization of the semiconductor gap and conduction valence effective masses. The calculation includes the laser dressing effects on both the impurity Coulomb potential and the confinement potential. The valence-band anisotropy is included in our theoretical model. The 2D Hartree–Fock spatial dielectric function and the polaronic effects have been employed in our calculations. We investigate that reduction of binding energy in a doped quantum well due to screening effect and the intense laser field leads to semiconductor–metal transition.     

Stark shift and dissociation process of an ionized donor bound exciton in spherical quantum dots

The effect of an electric field on the ground state energy of an exciton bound to an ionized donor (D⁺, X) was studied in CdSe spherical quantum dots where quantum confinement is described by an infinitly deep potential. Calculations have been performed in the framework of the effective mass approximation using a variational method by choosing an appropriate sixty-terms wave function taking into account different interparticles correlations and symetry distorsion induced by the electric field. It appears that the Stark shift is significant even for low fields and depends strongly of spherical dot sizes. The competition between the confinement effect and the Stark effect is discussed as function of the spherical dot size and the applied electric field strength. The (D⁺, X) Stark shift is estimated and its behavior is discussed as a function of the dot radius and electric field strength. The electron and hole average distances have also been calculated and the role of the ionized donor in the excitonic dissociation is established.     

Intersubband transitions in quantum wells under intense laser field

An intense laser field effect on the intersubband transitions in quantum wells is theoretically investigated within the framework of the effective-mass approximation. Results obtained show that intersubbband optical transitions and energy levels in quantum wells can be significantly modified and controlled by an intense laser field. PACS 71.55.Eq; 73.21.Fg; 78.67.De     

Polaron effect on the optical rectification in spherical quantum dots with electric field

The polaron effect on the optical rectification in spherical quantum dots with a shallow hydrogenic impurity in the presence of electric field is theoretically investigated by taking into account the interactions of the electrons with both confined and surface optical phonons. Besides, the interaction between impurity and phonons is also considered. Numerical calculations are presented for typical Zn_(1-x)Cd_xSe/ZnSe material. It is found that the polaronic effect or electric field leads to the redshifted resonant peaks of the optical rectification coefficients. It is also found that the peak values of the optical rectification coefficients with the polaronic effect are larger than without the polaronic effect, especially for smaller Cd concentrations or stronger electric field.     

Dynamics of a spherical particle in an intense optical field

The evaporation rate of a spherical particle in an intense, isotropic optical field is calculated.     

Electronic and optical properties of asymmetric GaAs double quantum dots in intense laser fields

In the present work, we investigated the effect of an intense non-resonant laser field on the electronic structure and the nonlinear optical properties (the light absorption, the optical rectification) of a GaAs asymmetric double quantum dot under a strong probe field excitation. The calculations were performed within the compact-density matrix formalism under the steady state conditions with the use of the effective mass approximation. The obtained results show that: (i) the electronic structure and, consequently, the optical properties are sensitive to the dressed potential; (ii) the changes in the incident light polarisation lead to blue or redshifts in the intraband optical absorption spectrum; (iii) for specific values of the structure parameters and under an intense laser illumination, the asymmetric double quantum dots can be a good candidate for NOR emission of THz radiation.     

Geometrical effects on shallow donor impurities in quantum wires

We have studied theoretically the impurity binding energy for wires of different shapes (V-shaped quantum wire (V-QWR) and rectangular wire) with a variational procedure without using any coordinate transformation. The effective potential for V-QWR used in this work consists of a square well potential in the z-direction and full graded well potential in the x-direction. Our results are in good agreement with previous theoretical results, found by the coordinate transformation method. Furthermore, it is shown that the impurity binding energy in quantum wires is sensitive to the geometrical effects.     

Effect of an electric field on the carrier collection efficiency of InAs quantum dots

Individual and multiquantum dots of InAs are studied by means of microphotoluminescence in the case where, in addition to the principal laser exciting photoluminescence, second infrared laser is used. It is demonstrated that the absorption of the infrared photons effectively creates free holes in the sample, which leads to both a change in the charge state of a quantum dot and to a considerable reduction of their photoluminescence signal. The latter effect is explained in terms of effective screening of the internal electric field, facilitating carrier transport along the plane of a wetting layer, by the surplus holes from the infrared laser. It is shown that the effect of quenching of quantum dot photoluminescence gradually disappears at increased sample temperature (T) and/or dot density. This fact is due to the essentially increased value of quantum dot collection efficiency, which could be achieved at elevated sample temperatures for individual quantum dots or even at low T for the case of multiquantum dots. It is suggested that the observed phenomena can be widely used in practice to effectively manipulate the collection efficiency and the charge state of quantum-dot-based optical devices.     

Effect of the dielectric-constant mismatch and magnetic field on the binding energy of hydrogenic impurities in a spherical quantum dot

Within the effective mass approximation and variational method the effect of dielectric constant mismatch between the size-quantized semiconductor sphere, coating and surrounding environment on impurity binding energy in both the absence and presence of a magnetic field is considered. The dependences of the binding energy of a hydrogenic on-center impurity on the sphere and coating radii, alloy concentration, dielectric-constant mismatch, and magnetic field intensity are found for the GaAs–Ga_1−xAl_xAs–AlAs (or vacuum) system.     

Exchange-coupled donor dimers in nanocrystal quantum dots

Doping of semiconductor nanocrystals (NCs) is expected to enable the control of key NC properties, yet its practical exploitation requires an understanding of exchange interactions when multiple dopants are incorporated in a single NC. Here, we experimentally probe the exchange of donor dimers in NCs via a deviation of their triplet-state magnetic resonance from Curie paramagnetism. We show that the exchange coupling of the closely spaced donors can be well described by effective mass theory, which allows the consideration of statistical effects crucial in NC ensembles. While a dimer induces discrete states in a NC, their energy splitting differs by up to 3 orders of magnitude for randomly placed dimers in a NC ensemble, due to an enormous dependence of the exchange energy on the dimer configuration.     

Effect of an intense laser field on the magnon damping in free carrier magnetic semiconductors

The influence of an intense radiation fieldE₀ on the magnon damping in free carrier magnetic semiconductors is discussed. It is shown that the effect of the intense field is to give a drift velocity v₀ = eE₀/mΩ to the carriers such that when v₀ exceeds the phase velocity of the spin waves the electrons are more able to emit magnons than they absorb and as a consequence the magnon population grows with time.     

Confined electron and shallow donor states in graded GaAs/Al Ga As spherical quantum dots

A theoretical study is performed on the confined electron and shallow donor states properties in graded GaAs/Al_xGa_1-xAs spherical quantum dots. The two lowest energy levels of a confined electron are obtained taking into account the dependence of the electron effective mass on the spatial profile of the Al molar fraction. The ground state of a single Si shallow donor, which may be located at an arbitrary position in the structure, is calculated through a variational approach. Depending on the dot interface width and localization, we find that the energy levels of the electron and donor states for the system under study can be blue or red shifted appreciably in comparison to those calculated within the sharp interface picture. We show that it is necessary to have accurate information concerning the interface of semiconductor dots whose samples are used in the experiments, in order to achieve a better understanding of their optical properties. Received 31 May 1999