Theoretical Study On Optical Absorption Properties Of Low-dimensional Semiconductor Materials

Due to their unique crystals and electronic structures,low-dimensional materials,such as two dimensional(2D)materials and two dimensional topological insulators,exhibit very peculiar physical properties.2D materials play an important role in nano-photon devices,since they have a wide range of material properties and can be combined together for different layers and compositions.The 2D topological insulator will form a unique band structure after the topological phase transition,such as Dirac point,edge state and so on.These changes will give rise to unique transport properties of materials,and the optical properties that can be altered greatly.This paper focuses on the optical properties of low-dimensional materials.The optical absorption properties of the 2D topological insulators such as HgCdTe/CdTe quantum wells and the general two-dimensional materials are studied by using k·p theory.This thesis consists the following five chapters:In Chapter 1,we briefly review the research background of typical two-dimensional materials and two-dimensional topological insulators.In Chapter 2,we introduce the k-p theory,and then derive the expression of optical absorption coefficient by using Fermi's golden rule and density matrix equation,respectively.In Chapter 3,we investigate the electric-field-driven Hg1-xCdxTe/CdTe quantum well topological phase transition and the corresponding optical absorption by Kane eight-band k·p model.Moreover,we analytically calculate optical absorption through Bernevig-Hughes-Zhang(BHZ)model.The results show that the electric field can drive the topological phase transition.Further increasing the electric field will distort the lowest conduction band and the highest valence band into a mexican hat shape.The special energy band shape will lead to a significant enhancement of the joint density of states as well as the optical absorption,which are consistent with the analytical results.Meanwhile,the absorption curve will form two-humped structure for the transverse electric mode(TE).Our results can provide new design guidance for infrared photodetectors,lasers and frequency selectors.In Chapter 4,we investigate the optical absorption and its universality in general 2D materials by introducing a general two-band k p effective Hamiltonian.We demonstrate that the absorptance at the direct band-edge can manifest an universal value in three cases,i.e.,the isotropic 2D systems with(i)zero masses;(ii)nearly equal remote-bands couplings of the conduction and valence band;(iii)zero band-gap.However,for general 2D materials,the absorptance become nonuniversal,since its value is found to be dependent on the light-frequency and the band parameters.We investigate the dependencies of absorptance on a variety of band parameters,including the band-gap,remotebands couplings,band-anisotropies and band-warping.Interestingly,we find the coherent interband coupling and the band warping are responsible for the occurrence of the saddle-point type of Van Hove singularity,which leads to strong light-matter interactions of 2D materials.In Chapter 5,we give a brief summary and consider the future prospects of this thesis.