The Band Structure And Optical Nonlinearity Of Core-shell Quantum Dot

Quantum dot is a zero-dimensional material whose structure is confined in three-dimensional. Because of this, quantum dots have some unique characteristics, such as quantum size effect, surface effect, quantum tunneling effect, dielectric confinement effect, Coulomb blockade effects. Nonlinear optical properties of the quantum dots are one of the hot topics in recent years too.These properties of the quantum dots can be controlled in production process by adjusting production conditions. For that reason quantum dots can be widely used in electronic devices, biological medicine, laser preparation and many other fields. Based on the quantum dots which are commonly used including spherical quantum dot (CdS), the single-shell quantum dot quantum well (ZnS/CdSe), double-shell quantum dot quantum well (ZnS/CdSe/ZnS), we use the effective mass approximation, infinitely deep potential well model, perturbation method and the density matrix method to make the theoretical research on the energy band structure and nonlinear optical properties. Then we use chemical method to synthesis ZnS-Cu-ZnS core-shell quantum dots. Then the PL spectrum of the sample has been we measured and analysised. The nonlinear optical absorption coefficient of the sample has also beed measured by Zscan method. Specific content includes:1. Brief review of quantum dot research history including optical devices, biological medicine, solar battery, quantum dots has been made. Origin and the development of nonlinear optics in quantum dots have been introduced. The main conntent of this paper has also been introduced.2. Using the effective mass approximation, infinitely deep potential well model, the Schrodinger equation of of carriers in cylindrical core shell quantum dots has been set up. The Eigen energy and wave function of electron and hole are got by using Matlab computing and separation of variables. We find that the energies in the core-shell quantun dot can be divided into two forms. Then the energy levels of electron and hole in ZnS/CdSe quantum dot have been theoretical analyzed. When the shell radius is fixed ait10nm, we find that the energy will incresase with the increasing of core radius.The energy in the well increase slowly with fluctuation curve. The energy out of the well increase fast and smoothly. When the core radius is fixed at3nm, we find that the energy will decrease with the increasing of shell radius. The energy in the well increases fast with fluctuation curve. The energy out of the well increase slowly and smoothly. When the shell thickness is fixed, we find that the energy out of the well decrease fast with the increasing of quantum dot size. But the curves fluctuate greatly reduced. The energy out of the well remains unchanged with the increasing of quantum dot size. The Eigen energy and wave function of electron in ZnS/CdSe/ZnS quantum dot have also been solved. We find that the energy out of the well decrease with the increasing of quantum dot size, the energy curve also has fluctuations.3. Based on two level model, we use perturbation method to get the expression of nonlinear polarization coefficient. For ZnS/CdSe, we discuss the four wave mixing and three harmonic effect when energy is in the well, for ZnS/CdSe/ZnS. We discuss the Kerr effect when energy is out of the well.We first discuss the relationship between the nonlinear polarization and relaxation time, found that polarization rate coefficient of resonance peaks increases with the relaxation time increases, in order to obtain the resonance peaks higher. We choose relaxation time300fs for latter calculation. For the single-shell structure quantum, we find that as the inner radius decreases and the resonance peak blue-shift, along with the outer radius increases the positions of resonance peak increases and the red shift. For double shell structure We discuss the transition out of the well,we found that with the increase of the outer radius increases the resonance peak, but the positions of resonance peak sometimes red-shift sometimes blue-shift, but the general trend is red-shift.4. Then KP perturbation method and the effective mass method have been used to solve quantum dot energy band structure. TheLuttinger Hamiltonian matrix mode of effective mass method has been use to analyze valence band of spherical CdS quantum dot. The third order nonlinear polarization rat of the Kerr effect has been calculated. We found that with the quantum dot size increased, energy reducing, and the hole energies are referred to the top of the valence band.Peak of Kerr effect polarization move to the low-frequency as the dot size increasing5. We also discuss the exciton effect on quantum dots nonlinear polarizability. We introduces the general solution method of the exciton energy and wave function, the discussion of the two order nonlinear optical effect of spherical CdS parabolic quantum has been made under exciton conduction.The nonlinear coefficient of Second harmonic generation effect (SHG) and the optical rectification effect (OR) has been discuss when ω0=2x1012s-1and ω0=2×1014.And the results of finite element method by Comsol software were use to compare with the eralier result,we found when the tight limit the coulomb inter-action can been omitted when loose limit the potential can been omitted.Then we calculated third harmonic generation (THG) coefficients considering exciton effects, comparing with the result of single electron we fund that exciton effect has significant enhancement of the three harmonic coefficient.6. We introduce several methods of preparing of quantum dots such as sol-gel method, micro emulsion method, molecular beam epitaxial, and analyzes their respective advantages and disadvantages. Based on our present experimental conditions, we choose the chemical synthesis method to prepare ZnS-Cu-ZnS core-shell quantum dots who doped by copper ion. By changing the amount of CuCl2to obtain different Cu ion doping samples.By adjusting the amount of second addition of Zn (NO3)2to change the thickness of outer layer of ZnS.We tested PL spectra of different samples. And the ZScan method was used to determined nonlinear absorption rate of the samples.We found that with the doping of Cu ions increase the PL peak is red-shifted, but no obvious change in amplitude.Thus found with the ZnS thickness increases the PL peak increasing but the peak’ position does not move. The nonlinear absorption coefficient of the samples increases when the second addition of Zn (NO3)2increases.