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Photonic Materials from ab-initio Theory

Figure 1: Potassium titanyl phosphate (KTP) is a nonlinear optical material which is intensively investigated in our project. Its unit cell is shown above.
Figure 2: Optically induced bond breaking in silicon substrate-stabilized indium nanowire.
Figure 3: Time-resolved electron diffraction enables measuring the time dynamics of optically induced phase changes.

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.


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Beyond the molecular movie: Dynamics of bands and bonds during a photoinduced phase transition

C.W. Nicholson, A. Lücke, W.G. Schmidt, M. Puppin, L. Rettig, R. Ernstorfer, M. Wolf, Science (2018), pp. 821-825

<jats:p>Ultrafast nonequilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photoinduced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a nonequilibrium population of excited electronic states. Using femtosecond photoemission, we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photoholes in shaping the potential energy landscape and enables a combined momentum- and real-space description of PIPTs, including the ultrafast formation of chemical bonds.</jats:p>

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