Photonic Materials from ab-initio Theory

Accurate parameter-free calculations of optical response functions for real materials and nanostructures still represent a major challenge for computational materials science. Our project focusses on the development and application of efficient but accurate ab-initio methods that give access to the linear and nonlinear optical spectra. We explore, 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. It thus 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). Many-body perturbation theory, i.e., the GW approximation for the quasiparticle energies and the Bethe-Salpeter equation (BSE) for the linear optical response, is used to address excitation energies. We focus in particular on ferroelectric materials such as lithium niobate (LN) or Potassium titanyl phosphate (KTP).

Polarons in dielectric crystals play a crucial role for applications in integrated electronics and optoelectronics. In Reference [1], we used DFT and Green's function methods to explore the microscopic structure and spectroscopic signatures of electron polarons in lithium niobate. Total-energy calculations and the comparison of calculated electron paramagnetic resonance data with available measurements reveal the formation of bound polarons at NbLi antisite defects with a quasi-Jahn-Teller distorted, tilted configuration. The defect-formation energies further indicate that (bi)polarons may form not only at NbLi antisites but also at structures where the antisite Nb atom moves into a neighboring empty oxygen octahedron. Based on these structure models (see Fig. 1), and on the calculated charge-transition levels and potentialenergy barriers, we proposed two mechanisms for the optical and thermal splitting of bipolarons, which provide a natural explanation for the reported two-path recombination of (bi)polarons. Optical-response calculations based on the Bethe-Salpeter equation, in combination with available experimental data and new measurements of the optical absorption spectrum, further corroborate the geometries proposed here for free and defect-bound (bi)polarons.

The magnetic signatures of Ti3+ centers in KTP were studied within DFT in Reference [2]. The hyperfine tensor elements are very sensitive to the structural surrounding. Therefore, the paramagnetic hyperfine splittings were used to evaluate the defect models. For each of the four experimentally observed electron paramagnetic resonance (EPR) spectra, we identified a defect model that reproduces the paramagnetic signature, see, e.g., Figure 2. All of them are electron donors, whereby one specific Ti atom can be identified, whose formal oxidation number is lowered from 4+ in the ideal crystal to 3+. The related charge redistribution leads to a strong polarization of the corresponding Ti3+ center. However, in three cases a second Ti atom, connected to the first by a mutual polarized O atom, is polarized too. Positively charged O vacancies at the lattice site O(10) are unique in leading to the formation of the only Ti3+ center that is stable at room temperature. This defect induces a defect state within the band gap, which may be excited during second harmonic generation (SHG) applications and thus is a plausible candidate to explain the so-called gray tracking, i.e., photochromic damage.

References
[1] F Schmidt, AL Kozub, T Biktagirov, C Eigner, C Silberhorn, A Schindlmayr, WG Schmidt, U Gerstmann «Free and defect-bound (bi)polarons in LiNbO3 : Atomic structure and spectroscopic signatures from ab initio calculations» Phys. Rev. Research 2, 043002 (2020).
[2] A Bocchini, C Eigner, S Silberhorn, WG Schmidt, U Gerstmann «Understanding gray track formation in KPT: Ti3+ centers studied from first principles» Phys. Rev. Materials 4,124402 (2020).