Felix Sauerwein

Understanding and controlling the optical properties of lithium niobate tantalate solid solutions (LiNb _{1− x} Ta _x O ₃ , 0 ≤ x ≤ 1) is essential for their use in integrated quantum and nonlinear photonics. This material system allows for composition-dependent tuning of key optical parameters such as refractive indices and birefringence. While lithium niobate (LiNbO ₃ , x = 0) and lithium tantalate (LiTaO ₃ , x = 1) are well characterized, reliable dispersion data for intermediate compositions remain scarce, limiting accurate modeling and refractive index engineering. Here, we address this gap by experimentally determining the ordinary and extraordinary refractive indices of LiNb _{1− x} Ta _x O ₃ over the spectral range of 405 − 1550 nm using an interferometric technique. As a result, composition-dependent refractive index dispersion and Sellmeier coefficients are derived and discussed in relation to previous studies and structural aspects. In particular, a monotonic decrease of the ordinary refractive index with increasing Tantalum concentration is observed. The present Sellmeier coefficients further allow discussion of zero birefringence dispersion and its composition dependence in lithium niobate tantalate.

The polar oxide Lithium Niobate Tantalate is probed using time-resolved luminescence spectroscopy with the goal of revealing an initial structural insight into the solid solution by analyzing the spectral properties and dynamics of radiatively decaying self-localization phenomena. A blue-green luminescence band can be induced by ultraviolet nanosecond laser pulses with a temperature-dependent intensity and spectral width, pointing to the radiative decay of optically generated self-trapped excitons as its origin, i.e., electron–hole pairs with strong coupling to either the NbO₆- or TaO₆-octahedra. The luminescence decay takes place in the microsecond time range and deviates significantly from a single exponential behavior, so the determined lifetime constants of up to ≈70 μs and stretching factors (1/3–1/5) are validated in more detail using alternative evaluation methods. We discuss our findings, considering the interplay of radiative and non-radiative decay channels, the transition from self-trapped to free excitons, and the presence of a structural disorder of the oxygen octahedra in the solid solutions. Overall, our results suggest self-trapped excitons as local probes for an initial structural elucidation and provide essential information about further experimental and theoretical studies on the atomic structure of Lithium Niobate Tantalate, but also for improving the crystal quality in the framework of applications in photonics and quantum optics.

Femto-/nanosecond pulse-induced, red and near-infrared absorption is studied in LiNb 1 − x Ta x O 3 ( 0 ⩽ x ⩽ 1 , LNT) solid solutions with the aim of studying transient optical nonlinearities associated with the formation, transport and recombination of optically generated small bound electron polarons with strong coupling to the lattice. As a result, a pronounced, long-lived transient absorption is uncovered for LNT which exceeds lifetimes and starting amplitudes of LiNbO 3 (LN) and LiTaO 3 (LT) by a factor of up to 100 and 10, respectively. The transients reveal a stretched-exponential decay behavior and a thermally activated process which provide strong evidence for an underlying hopping transport mechanism of small bound polarons. All findings are discussed in comparison to the model systems LN and LT within the framework of appropriate band models and optical generation of polarons via two-photon excitation. To explain the significant differences, the simultaneous presence of Nb Li 5 + , Ta Li 5 + antisites, and Ta V 5 + interstitial defects, i.e. a mixture of the intrinsic defects widely established for LN and LT, is assumed for LNT.

The absorption features of optically generated, short-lived small bound electron polarons are inspected in congruent lithium tantalate, LiTaO₃ (LT), in order to address the question whether it is possible to localize electrons at interstitial Ta V :V Li defect pairs by strong, short-range electron–phonon coupling. Solid-state photoabsorption spectroscopy under light exposure and density functional theory are used for an experimental and theoretical access to the spectral features of small bound polaron states and to calculate the binding energies of the small bound Ta Li 4 + (antisite) and Ta V 4 + :V Li (interstitial site) electron polarons. As a result, two energetically well separated ( Δ E ≈ 0.5 eV) absorption features with a distinct dependence on the probe light polarization and peaking at 1.6 eV and 2.1 eV are discovered. We contrast our results to the interpretation of a single small bound Ta Li 4 + electron state with strong anisotropy of the lattice distortion and discuss the optical generation of interstitial Ta V 4 + :V Li small polarons in the framework of optical gating of Ta V 4 + :Ta Ta 4 + bipolarons. We can conclude that the appearance of carrier localization at Ta V :V Li must be considered as additional intermediate state for the 3D hopping transport mechanisms at room temperature in addition to Ta Li , as well, and, thus, impacts a variety of optical, photoelectrical and electrical applications of LT in nonlinear photonics. Furthermore, it is envisaged that LT represents a promising model system for the further examination of the small-polaron based photogalvanic effect in polar oxides with the unique feature of two, energetically well separated small polaron states.