Second-harmonic Generation Spectroscopy from Time-dependent Density-functional Theory

Nonlinear Optics is one of the most active fields for fundamental and applied research in physics. The interest for nonlinear optical phenomena is becoming extremely strong, because of their versatile and innovative properties and technological applications. Very recently we have developed a first-principles theory, based on the Time-Dependent Density-Functional Theory approach, for the calculation of the second-order susceptibility χ2. We find a general expression for χ2 valid for any fields, containing the ab initio relation between the microscopic and macroscopic formulation of the second-order responses. We consider the long wavelength limit and we develop our theory in the Time-Dependent Density-Functional Theory framework. This allows us to include straightforwardly many-body effects such as crystal local-field and excitonic effects. We apply this formalism to the calculation of the Second-Harmonic Generation spectra for different type of materials: cubic semiconductors, hexagonal SiC polytypes finding good agreement with experiments. We are also exploring with this formalism the effects of anysotrophies for more complex systems like CaF2/Si multi quantum-well and silicon surfaces.