TRANSPORT AND OPTICAL PROPERTIES FOR NANOSTRUCTURES UNDER THE INFLUENCE OF RASHBA SPIN ORBIT INTERACTION

Abstract

Transport properties and non-linear processes via multiphoton transitions have been investigated in atomic, molecular, and bulk states theoretically and experimentally. Quantum nanostructures like quantum wells, wires and dots have been playing a key role in the investigation of transport properties and multi-photon processes as well as non-linear optical properties. These nanostructures have many important applications in optoelectronics, such as optical switching, THz multi-photon quantum well infrared photodetectors fusion of frequency up-conversion for bioimaging among other useful purposes. Much focus has recently been directed toward spin-dependent features because of the possible applications to novel generation and multifunction devices, in particular - towards spintronic type systems such as a Spin transistor or a Quantum computer. The study of spin-orbit interaction directly through this is used to perform the analysis on the basis of device functionality in a semiconductor nanostructure. To investigate the influence of this interaction on non-linear optical processes in such nanostructures, understanding is a must. It is, therefore, imperative to understand how quantum nanostructures couple and interact with strong fields in the presence of spin-orbit interaction for further developments at fundamental level as well scaling up novel devices based on spins. Moreover, parameters such as static magnetic and electric fields, the photon energy of laser field are crucial in investigating of electronic dispersion curve, ballistic conductance, linear and non-linear optical characters (referred to inter-band transitions in nanostructures). PRIYANKA vii Chapter 1 offers a concise introduction to nanostructures, external perturbations, as well as their linear and non-linear properties & transport properties, with an emphasis on multiphoton processes. It includes a brief discussion on the desired characteristics of nanostructures, specifically quantum wires and quantum dots, along with a focus on the selected material, highlighting its distinct properties. Additionally, this chapter provides key definitions and explanations of various optical and transport properties. Optical properties describe how materials interact with light, involving processes such as absorption, reflection, and transmission. In contrast, nonlinear optical properties refer to the material's response to intense light, where the relationship between the input and output light is non-linear. Whereas, transport properties give the Ballistic conductance by the help of Landuer-Büttiker formalism. In Chapter 2, we investigate the combined impacts of Rashba spin orbit-interaction, external electric field, magnetic field, and Aluminum concentration on energy dispersion and conductivity in a Ga1-xAlxAs quantum wire. The Energy eigenvalues and eigenvectors are quantified using the diagonalization method and the transport properties are computed by Landuer-Büttiker formalism. It is noticed that the external electric field, magnetic field, Rashba spin-orbit interaction, and Aluminum concentration (impurity factor x) alter the energy spectra and conductivity. Hence, these parameters significantly affect the physical and transport properties. Chapter 3 focuses on the electron quantum transport of a GaAs quantum wire at the nanoscale under the influence of hydrostatic pressure and temperature. The existence of spin-orbit coupling in the quantum wire has set up a propitious stage for the development of apparatus for electron transportation. Here, we analyze the impact of hydrostatic pressure and PRIYANKA viii temperature on the energy band structure as well as on the ballistic conductivity. The Energy eigenvalues and eigenvectors are found by using the diagonalization method and the ballistic conductance is computed by Landuer-Büttiker formalism. Also, we have studied the behaviour of energy concerning an external electric field, magnetic field and temperature. The system is expressed by parabolic confinement to the normal intense magnetic field and RSOI causes the collaborative impact of interior and exterior agents leading to downward/upward and lateral/vertical shifts in the dispersion. The oddity of the energy subbands results in oscillatory patterns in the ballistic conductance. Chapter 4 provides the study of the impact of impurities on optical absorption coefficients, refractive index changes, second-harmonic generation, and third-harmonic generation for inter subband transitions between electronic states in a Ga1-xAlxAs quantum wire, driven by a symmetric parabolic potential. The system is influenced by an intense electric field, magnetic field, and Rashba spin-orbit interaction. The analytical expressions for linear and nonlinear optical absorption coefficients, refractive index changes, and both second and third harmonic generation are derived using the compact density-matrix approach. The numerical results demonstrate that the optical properties are highly sensitive to impurity concentration and can be controlled through impurity. The shifts in peak magnitude and position due to the impurity factor reveal potential for manipulating optical non-linearity within the quantum wire and offering opportunities for tuning optical non-linearities with practical applications in devices. Chapter 5 describes the optical properties of InxGa1-xAs quantum dot. First, we calculate the energy levels and wavefunctions in the presence of an impurity. Then, we examine how the impurity affects the absorption coefficients, refractive index changes, and third-harmonic PRIYANKA ix generation. The results show that as the impurity concentration increases, the absorption coefficients, refractive index changes, and third-harmonic generation peaks shift from their original positions and decrease in magnitude. This highlights the significant influence of impurity concentration on the optical properties of the nanostructure. Chapter 6 delves into the the effect of both Rashba and Dresselhaus spin-orbit interactions in a system with double-well anharmonic confinement potential. We show that one can manipulate the energy band structure by tuning structural parameters of confinement potential and strengths of electric field, magnetic field, spin-orbit interactions. Moreover, we find that ballistic magnetoconductance oscillations are susceptible to spin-orbit induced modifications of the wire's energy dispersion in presence of magnetic and electric fields. Chapter 7 indicates that the thesis conclusion with a summary, a brief recapitulation of the research presented in previous chapters and possible future approaches for extending work addressed. References also form part along with bibliography at the end of each chapter.