Θεωρητική μελέτη μη-γραμμικών οπτικών διαδικασιών σε ημιαγώγιμα κβαντικά πηγάδια

Abstract

In this PhD thesis, we study analytically and numerically linear and nonlinear optical phenomena in intersubband transitions of a symmetric GaAs/AlGaAs double quantum well structure, with two energy subbands. In first chapter, a theoretical description of the semiconductor heterostructures is presented. This is accompanied with a brief analysis of the basic elements of statistical and quantum mechanics follows, as far as this kind of structures is concerned. In the second chapter, we derive the generalised optical Bloch equations in intersubband transitions of semiconductor quantum well structures, which constitute the basis of the analysis that follows. These equations contain terms which owe their presence to the electron-electron interactions, since the quantum structure is doped with electron carriers. In the two following chapters, we consider the interaction of intersubband transitions of a double quantum well structure with an electromagnetic field of varying frequency, derive analytical expressions for the first, third and fifth order optical susceptibility, and analyze the corresponding spectra, with respect to the frequency of the external field, for different values of electron sheet density of the structure. Furthermore, we identify the areas of values of the parameters used, in which the phenomenon of optical bistability arises. In the last three chapters, we consider that the two quantum well subbands are coupled to a strong pump electromagnetic field with fixed frequency and a weak probe electromagnetic field of varying frequency and study the spectra of various linear and nonlinear optical phenomena, due to the existence of the probe field. More specifically, we examine the spectra of four-wave mixing, absorption, dispersion and the nonlinear optical Kerr effect of the probe field as they evolve in time and in the steady state, as well. Both analytical expressions are derived and numerical results are presented by solving the nonlinear differential density matrix equations that govern the dynamics of the system. In the study of the different kinds of optical phenomena, the influence of the electron sheet density on the spectral shapes is carefully examined.