arXiv subjects

Tobias Hehemann

Publications and source records attributed to Tobias Hehemann.

4 recordsLinked to original sources

arXiv2026

Self-selected phase-matched second harmonic generation in nonlinear optical materials: from phenomenon to applications

Self-selected phase-matched second harmonic generation is introduced as an all-optical probe of refractive-index dispersion in birefringent nonlinear optical materials. Rather than requiring wavelength or angular tuning, the exposure with a spectrally broad, intense ultrashort pulse allows the material to self-select the fundamental spectral component that satisfies the type-I noncritical phase-matching condition. This produces a narrow peak in the second harmonic spectrum whose position is governed by the refractive indices and is therefore highly sensitive to material parameters that affect the optical dispersion. We demonstrate the application of this phenomenon for the optical inspection of stoichiometry and temperature gradients in technologically relevant lithium niobate, as well as composition inhomogeneities in newly grown lithium niobate-tantalate solid solutions. These results establish self-selected phase-matched second harmonic generation as a rapid, non-contact method for inspecting nonlinear optical materials, with potential relevance for bulk crystals, wafers, and thin-film platforms.

physics.optics↗

arXiv2024

Ground- and excited-state properties of LiNb$_{1-x}$Ta$_x$O$_3$ solid solutions

LiNb$_{1-x}$Ta$_x$O$_3$ solid solutions are investigated from first principles and by optical spectroscopy. The ground- and excited-state properties of the solid solutions are modelled within density functional theory as a function of the Ta concentration using special quasirandom structures spanning the entire composition range between LiNbO$_3$ and LiTaO$_3$. Deviations from a Vegard behavior are predicted for the lattice parameters, the heat capacity, the electronic bandgap, and consequently the absorption edge. The latter is measured for crystals of different composition by low temperature optical spectroscopy, qualitatively confirming the theoretical predictions. The LiNb$_{0.11}$Ta$_{0.89}$O$_3$ composition is found to be a highly unusual crystal with a permanent macroscopic electric polarization and nonetheless zero birefringence.

cond-mat.mtrl-sci↗

arXiv2024

Small electron polarons bound to interstitial tantalum defects in lithium tantalate

The absorption features of optically generated, short-lived small bound electron polarons are inspected in congruent lithium tantalate, ${\rm LiTaO}_3$ (LT), in order to address the question whether it is possible to localize electrons at interstitial ${\rm Ta_V}$:${\rm V_{Li}}$ 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 ${\rm Ta}_{\rm Li}^{4+}$ (antisite) and ${\rm Ta}_{\rm V}^{4+}$:${\rm V_{Li}}$ (interstitial site) electron polarons. As a result, two energetically well separated ($ΔE \approx 0.5\,{\rm eV}$) absorption features with a distinct dependence on the probe light polarization and peaking at $1.6\,{\rm eV}$ and $2.1\,{\rm eV}$ are discovered. We contrast our results to the interpretation of a single small bound ${\rm Ta}_{\rm Li}^{4+}$ electron state with strong anisotropy of the lattice distortion and discuss the optical generation of interstitial ${\rm Ta}_{\rm V}^{4+}$:${\rm V_{Li}}$ small polarons in the framework of optical gating of ${\rm Ta}_{\rm V}^{4+}$:${\rm Ta}_{\rm Ta}^{4+}$ bipolarons. We can conclude that the appearance of carrier localization at $\mathrm{Ta_V}$:${\rm V_{Li}}$ must be considered as additional intermediate state for the 3D hopping transport mechanisms at room temperature in addition to ${\rm Ta_{Li}}$, 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.

cond-mat.mtrl-sci↗

arXiv2024

Long-lived, pulse-induced absorption in $\mathrm{LiNb}_{1-x}\mathrm{Ta}_x\mathrm{O}_3$ solid solutions: the case of three intrinsic defect sites for electron localization with strong coupling

Femto-/nanosecond pulse-induced, red and near-infrared absorption is studied in $\mathrm{LiNb}_{1-x}\mathrm{Ta}_{x}\mathrm{O}_3$ (LNT) solid solutions with the goal to probe the intrinsic defect structure via the formation, transport and recombination of optically generated small bound electron polarons with strong coupling to the lattice. As a result, long-lived transients are uncovered for LNT which exceed lifetimes of LN and LT by a factor of up to 100 over the entire range of investigated compositions. At the same time, the starting amplitude varies in the range of $α_\mathrm{li}^0\approx10-100\,\mathrm{m}^{-1}$ as a function of $x$ and exceed the ones of LN and LT by a factor of up to ten. The results are interpreted in the model of three-dimensional small polaron hopping transport considering the simultaneous presence of three different types of small bound polarons, in particular of small electron $\mathrm{Nb}_\mathrm{Li}^{4+}$ and $\mathrm{Ta}_\mathrm{Li}^{4+}$ antisite polarons, and of small electron $\mathrm{Ta}_\mathrm{V}^{4+}$ interstitial polarons. We conclude that the differences between LNT, LN, and LT may point to model systems that consist of one (LN), two (LT) and three (LNT) intrinsic defect centers for electron localization.

cond-mat.mtrl-sci↗