Prof. Dr. Wolf Gero Schmidt, Universität Paderborn
Accurate parameter-free calculations of optical response functions for real materials and nanostructures still represent a major challenge for computational materials science. Our project concentrates on the development and application of ab-initio methods which provide access to linear and nonlinear optical spectra. It explores, on the atomistic level, how the material structure, its composition and defects but also external parameters like stress, temperature or magnetic fields influence the optical response. In addition, we explore how optical excitations modify the material electronic and atomic structure as well as the time dynamics of optical excitation and de-excitation. The project thereby leads to a better understanding of existing materials and contributes to the design of new photonic materials.
Our calculations start from an accurate description of the structural and electronic ground-state properties within density-functional theory (DFT). Time-dependent DFT in conjunction with a Berry-phase formulation of the dynamical polarization accounts for many body effects in the optical response in an efficient way without recourse to virtual orbitals. More precise schemes based on many body perturbation theory, such as the GW approximation for the quasiparticle energies or the Bethe-Salpeter equation (BSE) for the linear optical response, are be used to benchmark the TDDFT results. Both zero-point vibrations and thermal lattice vibrations are included in the calculations. The computational methods developed in our project are applied to a wide range of II-VI, III-V and nitride semiconductors and nanostructures as well as ferroelectric materials such as potassium titanyl phosphate, which are shown below.