Non-linear response in extended systems: a real-time approach

TL;DR

This thesis introduces a real-time formalism for studying linear and non-linear responses in extended systems, emphasizing the importance of local-field effects, electron-hole interactions, and a density functional approach for periodic crystals.

Contribution

It presents a novel real-time Schrödinger equation approach combined with modern polarization theory and Green's functions, advancing the modeling of non-linear optical phenomena in low-dimensional and periodic systems.

Findings

Accurate prediction of second and third harmonic generation in low-dimensional structures.

Development of a real-time density functional method for periodic crystals.

Highlighting the importance of local-field effects and electron-hole interactions.

Abstract

In this thesis we present a new formalism to study linear and non-linear response in extended systems. Our approach is based on real-time solution of an effective Schr\"odinger equation. The coupling between electrons and external field is described by means of Modern Theory of Polarization. Correlation effects are derived from Green's function theory. We show that the inclusion of local-field effects and electron-hole interaction is crucial to predict and reproduce second and third harmonic generation in low dimensional structures, where strong bound excitons are present. Finally in the last part we introduce a real-time density functional approach suitable for infinite periodic crystals in which we work within the so-called length gauge and calculate the polarisation as a dynamical Berry's phase. This approach, in addition to the electron density considers also the macroscopic polarisation as a main variable to correctly treat periodic crystals in presence of electric fields within a density functional framework.