Optical Nonlinearity Of Quantum Dot Molecules

Further development of nanofabrication processes not only improve the quality of the nanostructure but also extend the kinds of the low-dimension confined system, the appearance of the quantum dot molecules provide the tools for us to adjust the degree of energy confinement and the coupled strength by tuning components of partially structure and the distance between the two nanoparticles. Quantum dot molecules that are different from the natural molecules have attracted considerable attention in the optical fields such as the information storage, optical communication, optical measurement and the field of the quantum information due to the potentially synergistic properties, which become the hot in recent years. In this paper, the optical nonlinear effects of the quantum dot molecules that composed of two nanoparticles have been theoretically studied by using single bands effective mass approximation, rotating-wave approximation, Born-Oppenheimer approximation, transfer matrix formalism and local field theory. We give an overview of the development in this field in the introduction, meanwhile, the main lines of this paper are organized based on the existed works. Perturbation theory and density matrix calculation of the susceptibility are presented in chapter2, in addition, advantage and disadvantage of these methods and the conditions that can be accurate description using these methods are pointed out, typical nonlinear optical effects in parametric and nonparametric processes are detailed description. Theory model of quantum dot molecule and its calculation methods are given in chapter3. The main work in this paper as follows:1. Linear, second-order and third-order density matrix elements are obtained when four different frequencies wave mixing by introducing the transformation of the density matrix on the two-level approximation in asymmetric structure. The analytical expression of the third-order susceptibility are given finally according to the polarization theory, these expression display the functional form of the nonlinear optical susceptibility and hence show the predictions of the numerical values of the nonlinear optical susceptibilities and how the susceptibility depends on the material parameters.2. The vertically stacked double quantum dot molecules(DQDM) are subject to a single frequency incident laser field, eigensolutions of DQDM are obtained by employing the transfer matrix formalism in the framework of the effective mass and adiabatic approximations based on the principle of the single electron tunneling and dipole-dipole interaction. We start from the density matrix and focus on the parametric processes in the vertically coupled QDQM, that is, optical saturation intraband absorption corresponding to the molecular orbital transition between Is bonding sate and1s antibonding state when incident light intensity reach some value by neglecting higher-harmonic terms and the off-resonance terms. It is found that saturation absorption is strongly dependent on the geometrical parameters and on the incident light intensity, the generation of saturation absorption is relatively easier and saturation intensity is become larger for larger dot separation, what's more, the stronger the incident light intensity, the more remarkable the saturation. When the size radii of the upper dot and the lower dot approach to each other and the thickness of the disk is thinner, absorption peaks redshift, the absorption spectra appeared at THZ frequency-domain are bleached significantly and broadened under the high incident light intensity. Saturation absorption can be only observed in the'strongly'coupled regime of the two quantum dots. The'strongly'coupled regime is4.0nm-15.4nm characterized by the dot separation d that is an important parameter of the DQDM. Saturation intensity is found to be0.7?1.2MW/cm2within the range of the parameters. The related results of numerical calculation are agreement with the measurement from the experiment. Relaxation time which is not ignored has an important impact on the intensity of the absorption peak. Saturation absorption spectroscopy can eliminate the Doppler broadening, which has some reference values in aspects of information storage and optical measurements.3. Nonlinear optical properties of the hybrid structure molecules which interacted with the laser field are studied. Energy band of the carriers are obtained by solving the Schrodinger equation using the single bands effective mass theory. The field inside the SQD are amplified by the plamon resonance effect. In general, the effective field and the macroscopic electric field should be distinguished, so we introduce the local field theory to our research model. We treat the two-level model through useing of the optical Bloch equations by considering the multipole effect(N=10) of the MNP to give the algebraic expressions of the local field factors and the population inversions. The effects of the microscopic parameters and electric field polarization on the nonparametric process in the hybrid structure, i.e. the third harmonic generation (THG) and four-wave mixing (FWM), are detailed theoretically investigated,(a).The transition between E1state and E2state denoting the lowest p state inside the well and the lowest s state outside the well, respectively, of the CdSe/ZnS core-shell quantum dot are chosen for the theoretical analysis and numerical calculation to investigate the THG effect of the hybrid system. The study shows that the magnitude of and the position of the resonant peak significantly dependent on the direction of the electric field polarization compared to the case of the bare quantum dot, which main arises from the self-interaction. There exist an optimum distance R0that decreases with the increase of the quantum dots size in the process of the increasing of the distance between the two nanoparticle when the electric polarization parallel to the major axis of the system. Similarly, there is also an optimum radius of MNP that make the to reach the maximum value, besides, the optimum radius is decrease with the increase of the shell radius accompany with the increase of the susceptibility intensity. However, the situation is revers when the electric polarization perpendicular to the major axis of the system, where the optimum radius increase with the increase of the shell radius accompany with the decrease of the susceptibility intensity.(b).Secondly, FWM optical response of the hybrid system interacted with the laser field are detailed discussed by considering of the exciton-plasmon interaction under the condition of the local and nonlocal optical response of the MNP. The exciton ground state and exciton first excited state in the core-shell quantum dot are chosen for the transition energy levels. The study has shown that multipole effect has a great effect on the position of the resonant peak. By controlling the geometrical parameters of the hybrid structure and the direction of the electric field polarization, a significant enhancement of the nonlinear response can be achieved. Specifically, nonlinear effect enhance significantly, especially, a strong absorption band appearance in the FWM spectrum of the hybrid system when the electric field polarization is along z axis. Nonlinear effect is weaken when the electric field polarization perpendicular to the z axis. The ratio of the intensity is approximately100under the two case, which is agreement with the available experimental data. The redshift of the center frequency appear with the increase of the MNP-size, irrelevant to the direction of polarization of the electric field, moreover, the larger size of the SQD has an enhance absorption spectrum, by nearly a factor of4.75at the maximum value of the absorption peak, the interaction between SQD and MNP is almost absent at very large interparticle distance(80nm). The nonlocal effect should be considered in some conditions where the FWM response is presented more strong according to our calculations. Finally, the impacts of the detuning of the two incident light beams on the nonlinear response are investigated. Our research shows that the hybrid system not only plays an important role in the field of the quantum information but also has a huge potential in the field of optical switches and optical transmission. Furthermore, the studies play a guiding role for the designing and manufacturing of nanostructures, optimizing and prediction the nonlinear optical properties.