Andreas F. Terzis

Electronic structure and nonlinear optical rectification in a quantum dot: effects of impurities and external electric field

The electronic structure of a spherical quantum dot with parabolic confinement that contains a hydrogenic impurity and is subjected to a DC electric field is studied. In our calculations we vary the position of the impurity and the electric field strength. The calculated electronic structure is further used for determining the nonlinear optical rectification coefficient of the quantum dot structure. We show that both the position of the impurity and the strength of the electric field influence the nonlinear optical rectification process.

Size-dependent band gap of colloidal quantum dots

The size-dependent band gap of semiconductor quantum dots is a well-known and widely studied quantum confinement effect. In order to understand the size-dependent band gap, different theoretical approaches have been adopted, including the effective-mass approximation with infinite or finite confinement potentials, the tight-binding method, the linear combination of atomic orbitals method, and the empirical pseudopotential method. In the present work we calculate the size-dependent band gap of colloidal quantum dots using a recently developed method that predicts accurately the eigenstates and eigenenergies of nanostructures by utilizing the adiabatic theorem of quantum mechanics. We have studied various semiconductor (CdS, CdSe, CdTe, PbSe, InP, and InAs) quantum dots in different matrices. The theoretical predictions are, in most cases, in good agreement with the corresponding experimental data. In addition, our results indicate that the height of the finite-depth well confining potential is independent ...

Linear and nonlinear optical properties of ZnO/ZnS and ZnS/ZnO core shell quantum dots: Effects of shell thickness, impurity, and dielectric environment

In the present work, we investigated theoretically the linear, nonlinear, and total absorption coefficients and refractive index changes associated with intersubband transitions in ZnO/ZnS core shell quantum dot (CSQD) and ZnS/ZnO inverted CSQD (ICSQD), emphasizing on the influence of the shell thickness, impurity, and dielectric environment. The effect of the polarization charges due to the possible existence of the dielectric mismatch between the system and its surrounding matrix is considered. The electronic structures are numerically calculated by employing the potential morphing method in the framework of effective mass approximation. We find that in both impurity-free CSQD and ICSQD, increasing the shell thickness red shifts significantly the threshold energy and enhances drastically the nonlinear absorption coefficients and all the refractive index changes, independently on the dielectric environments. Similar behaviour has also been observed in most of the cases studied when the impurity is displa...

Rabi oscillations in a strongly driven semiconductor quantum well

We study the interaction of an ac electric field with a semiconductor quantum well by using the effective nonlinear Bloch equations. Only the first two electron subbands in the well are considered. We apply the rotating wave approximation and derive analytical solutions for the Bloch equations for two different values of the detuning. At exact resonance we find a critical value of the Rabi frequency around which the dynamics of the system changes abruptly. Above this critical value one obtains electron oscillations with complete inversion in the two-subband system, while below this value we obtain electron oscillations without complete inversion and with the majority of the electron population on average in the lower subband. We also present numerical calculations for a specific quantum well structure and assess the limits of validity of the analytical results.

Stability of an exciton bound to an ionized donor in quantum dots

Abstract Total energy, binding energy, recombination rate (of the electron–hole pair) for an exciton ( X ) bound in a parabolic two-dimensional quantum dot by a donor impurity located on the z -axis at a distance d from the dot plane, are calculated by using the Hartree formalism with a recently developed numerical method (PMM) for the solution of the Schrodinger equation. As our analysis indicates there is a critical dot radius R c such that for R R c the complex is unstable and with an increase of the impurity distance this critical radius increases. Furthermore, there is a critical value of the mass ratio σ=m ∗ e /m ∗ h such that for σ σ c the complex is stable. The appearance of this stability condition depends both on the impurity distance and the dot radius, in a way that with an increase of the impurity distance we have an increase in the maximum dot radius where this stability condition appears. For dot radii greater than this maximum dot radius (for fixed impurity distance) the complex is always stable.

Optical response of a quantum dot?metal nanoparticle hybrid interacting with a weak probe field

We study optical effects in a hybrid system composed of a semiconductor quantum dot and a spherical metal nanoparticle that interacts with a weak probe electromagnetic field. We use modified nonlinear density matrix equations for the description of the optical properties of the system and obtain a closed-form expression for the linear susceptibilities of the quantum dot, the metal nanoparticle, and the total system. We then investigate the dependence of the susceptibility on the interparticle distance as well as on the material parameters of the hybrid system. We find that the susceptibility of the quantum dot exhibits optical transparency for specific frequencies. In addition, we show that there is a range of frequencies of the applied field for which the susceptibility of the semiconductor quantum dot leads to gain. This suggests that in such a hybrid system quantum coherence can reverse the course of energy transfer, allowing flow of energy from the metallic nanoparticle to the quantum dot. We also explore the susceptibility of the metal nanoparticle and show that it is strongly influenced by the presence of the quantum dot.

Quantum interference induced sub-wavelength atomic localization

We investigate how observation of upper state atomic populations localize atomic position distributions for three-level atoms within a classical standing wave light field. We consider a three-level atom, with the standing wave near-resonantly coupling one transition and a probe laser field near-resonantly coupling the second transition. Two different cases of localization are identified and we explore the dependence of localization on the parameters of the field–atom interaction.

Strongly modified four-wave mixing in a coupled semiconductor quantum dot-metal nanoparticle system

We study the four-wave mixing effect in a coupled semiconductor quantum dot-spherical metal nanoparticle structure. Depending on the values of the pump field intensity and frequency, we find that there is a critical distance that changes the form of the spectrum. Above this distance, the four-wave mixing spectrum shows an ordinary three-peaked form and the effect of controlling its magnitude by changing the interparticle distance can be obtained. Below this critical distance, the four-wave mixing spectrum becomes single-peaked; and as the interparticle distance decreases, the spectrum is strongly suppressed. The behavior of the system is explained using the effective Rabi frequency that creates plasmonic metaresonances in the hybrid structure. In addition, the behavior of the effective Rabi frequency is explained via an analytical solution of the density matrix equations.

Optical susceptibilities in singly charged ZnO colloidal quantum dots embedded in different dielectric matrices

Within the two-level system approximation, analytical expressions for the linear, third-order nonlinear and intensity-dependent susceptibilities in quantum dots (QDs) embedded in a dielectric matrix are developed by using density matrix equations, considering the local field effect due to the presence of dielectric mismatch. Based on the derived expressions, we perform a comparative study of the optical susceptibilities in singly charged zinc oxide QDs embedded in various dielectric matrices. Three commonly adopted matrices are considered. The electronic structure of the system is numerically calculated. In general, our results indicate that the optical susceptibilities are highly affected by the capped matrices. For example, QDs embedded in the matrix with the largest dielectric constant but the smallest energy band gap exhibit the largest linear and nonlinear optical susceptibilities, while that dispersed in a matrix with the largest energy band gap show the highest threshold energy. It is also found that the third-order nonlinear susceptibility exhibits a stronger dependence on the nature of the capped matrix as compared to its linear counterpart. Finally, we find that the total susceptibility in charged QD immersed in a matrix with a higher dielectric constant is more sensitive to the applied radiation intensity.

Coherent single-electron transfer in coupled quantum dots

We theoretically investigate the coherent transfer of one electron between the ground states of a double coupled quantum dot structure. The coherent transfer of the electron is externally controlled by applied electromagnetic fields with on- or close-resonance driving frequencies and various shapes and duration. We derive the analytical expressions for the parameters of the external fields by approximating the quantum dot system as a three-level Λ-type system. The analytical solutions are compared with numerical results and good agreement is found. The control methods developed here are applicable in symmetric and asymmetric quantum dot nanostructures.