The Nonlinear Optical Rectification In Semiconductor Quantum Dots

Nonlinear optics is a very important branch of modern optical fields which is developed quickly after the invention of the laser.With the increasing maturity of the research,nonlinear optics provides important applications for optical fiber communication,optical computing,spectroscopy,laser technology and analysis of material structure.Besides,it promotes the development in other areas of science.A great deal of scientific research proved that artificial low dimensional semiconductor materials are one of the ideal nonlinear optical materials which have fast response time,very large nonlinear susceptibility and small threshold power.The nonlinear optical effect in low dimensional semiconductor materials is quite obvious due to the existence of quantum confinement effect.Hence,it is significant for investigations on the nonlinear optical properties in low dimensional semiconductor materials.As the group ?-? and ?-? low-dimensional semiconductor materials show the special characteristics in nonlinear optical properties,the nonlinear optical rectification(OR)in a zinc-blende GaN/AlxGa1-xN ellipsoidal quantum dot(QD)with electric field is studied firstly in the thesis by using the variational procedure within the effective mass approximation.Secondly,we investigate the polaron effects on the nonlinear OR in a spherical QD in the presence of electric field,taking into account the interactions between electron and impurity with both confined and surface optical phonons.Finally,the nonlinear OR of an exciton in a spherical QD under an applied electric field is further studied.Numerical results indicate that the OR in an ellipsoidal quantum dot is obvious due to the existence of quantum confinement effect.The dot shape,size,the electric field and aluminium concentrations have an important influence on the nonlinear OR.The peak values of the OR coefficients decrease and shift to lower photon energy with the increasing ellipticity constant.That is to say,the prolate shape reduces the peak values of the OR coefficients and shifts its peak positions to lower photon energy side compared with the oblate case.Moreover,we find that the increase of the electric field and QD radius enhances the peak values of the OR coefficients and shifts its peaks to lower photon energy side.The electric field has a more pronounced influence on the peak enhancement of the OR coefficients for the prolate shape compared with the oblate shape.Furthermore,we find that the increasing Al component reduces the peak values of the OR coefficients and shifts its peak positions to higher photon energy side.Secondly,we investigate the polaron effects on the nonlinear OR in a Zn1-xCdxSe/ZnSe spherical quantum dot in the presence of electric field by taking into account the interactions between electron and impurity with both confined and surface optical phonons and find that the polaron effect has a very significant impact on the nonlinear OR.The numerical results show that the peak values of the OR coefficients with the polaronic effect are larger than that without the polaronic effect.It also can be found that the polaronic effect gives rise to the redshifts of the positions of the resonant peaks of the OR coefficients,especially for the smaller QD radius and Cd concentrations.In addition,whether the polaronic effect is considered or not,the peak values of the OR coefficients are enhanced and move towards lower photon energy side with increasing dot size and electric field.The peak values of the OR coefficients decrease and move towards higher photon energy side with increasing Cd concentrations.Furthermore,the difference in the OR peaks with and without the polaronic effect is enhanced with increasing electric field,but gradually becoming small with Cd concentrations increasing.On the other hand,different optical phonon modes have different effects on the nonlinear OR.Compared with the confined phonons,the surface phonons have more obvious effects on the nonlinear OR.Finally,by studying the nonlinear OR of an exciton in a GaAs/AlxGa1-xAs spherical QD under an applied electric field,we find that the OR coefficients with exciton effects taken into account are greatly enhanced compared with the case of single electron state,but the positions of its peak values are not changed.The peak values of the OR coefficients of an exciton increase and shift towards higher photon energy side with considering Coulomb interactions.Moreover,with the increase of the electric field intensity,the peak values of the OR coefficients first increase and then decrease,and the peak positions move towards lower photon energy direction.As the QD radius increases,the peak values of the OR coefficients increase and move towards lower photon energy direction.By considering exciton effects,the peak values of the OR coefficients first increase and then decrease with Al concentrations increasing,and the peak positions move towards higher photon energy side.