The Applications And Optical Properties Of Nano Heterostructures

Recent years, with the development of nano scale technologies, the low-dimensional semiconductor nano heterostructures have been the focuses in material science due to the unique quantum size effect, quantum tunneling effect, quantum hall effect and nonlinear optical properties. The carriers in low-dimensional nano heterostructures could be confined along one or more directions, then the favorable photoelectric properties can be obtained compared to the bulk materials. Therefore, the optoelectronic devices and quantum devices based on the low-dimensional nano heterostructures will push high-speed optical communications and quantum information technologies forward. Supported by the National High Technology Research and Development Program of China (Grant No.2009AA03Z405) and the National Natural Science Foundation of China (Grant No.60644004), this dissertation focuses on the applications and the optical properties of quantum wires (QWRs) and quantum dots (QDs), in which the effects of strain and built-in electric field on QWRs and QDs are included. The main contents and innovative ideas are as follows:1. The strain distributions of semiconductor QWRs. Based on the continuous elastic theory, we have investigated the strain distributions in quantum wires with different shapes and sizes by using the Green functions and energy minimization principle. We also have analyzed the effect of strain on the energy band edges, which will modify the electronic structures and wave functions. The obtained results are viselike theory foundations to research the critical dimension and optoelectronic characteristics of QWRs.2. The strain distribution of semiconductor QDs. Based on the continuous elastic theory, we have investigated the strain distributions in semiconductor QDs with different materials (InAs/GaAs and GaN/AlN), shapes, crystal phases and strain compensational layers. The obtained results give an explanation for the nucleation position and alignment during the growth of QDs. Also, the strain engineering can flexibly change the wavelength of quantum dots devices, and broaden the applications of quantum dots in optoelectronic devices and quantum devices.3. The optical properties of semiconductor QWRs. The electronic structures are investigated under the frameworks of effective mass approximation. Also, the energy levels and wave functions are calculated by solving two dimensional Schrodinger functions with variational method. Based on the density matrix theory and Femi-Dirac distribution, the optical absorption spectra are analyzed for TE and TM linear polarization lights by taking strain into account.4. The optical properties of semiconductor QDs. Due to the accumulated strain during the growth, we mainly focus on the effects of strain, piezoelectric potential, spontaneous polarization and the crystal symmetry on the electronic structures of self-organized QDs. Based on the density matrix theory, we have investigated the effects of the nonlinear effect, counter-rotating components and an additional orthogonal polarization light on optical properties of QDs. Due to the strain and built-in electric field, the electrons and holes are separated away from each other, and the piezoelectric potentials will change the symmetry of confining potentials in QDs along the different orientations of growth such as [001] and [111] directions. The results provide theoretical guidance to investigate the fine structure splitting (FSS) in QDs and push the applications of QDs on polarization-entangled optical sources and quantum information science.5. The properties of exciton in QDs. Based on the Hartree self-consistent field theory, we have investigated the exciton energy and wave functions by solving the nonlinear Schrodinger equations with fast Fourier transform (FFT). It turns out that the exciton binding energy increases at the beginning and then declines with the increment of the sizes of QDs. The results are in accord with the literatures published previous. This method can be expanded to further study the exciton properties by taking exchange interaction into account, such as Hartree-Fock approximation and configuration interaction.6. The electromagnetically induced transparency (EIT) and negative refractive index in QDs. Based on density matrix theory and three-level system, we have investigated the EIT and the Aulter-Townes (AT) splitting in strained QDs. And the pulse propagation characteristics such as the factor of slow down, waveform distortion and absorption are investigated with respect to their dependences on the dephasing rates. Also, the negative refraction in QDs are analyzed theoretically combined with the strain engineering. All the researches lay a solid foundation for the applications of QDs on optical buffer, slow lights and left-hand materials.