Abstract
In this review article, we discuss photoluminescence phenomena mediated by polarons in lithium niobate (LNO). At first we present the fundamentals on polaron states in LNO and their energy levels, i.e., on free and bound electron polarons, on hole polarons as well as on bipolarons. We discuss the absorption measurements on reduced as well as on doped LNO that made the characterization of the formed polaron states possible by their absorption bands. Next, we proceed by reporting on the two polaron-mediated photoluminescence bands that have been observed in LNO: (1) A near-infrared luminescence band in the range of 1.5 eV shows a mono-exponential decay and a strong dependence on iron doping. This luminescence is emitted by bound polarons returning from an excited state to the ground state. (2) A luminescence band at visible wavelengths with a maximum at 2.6 eV shows a stretched-exponential decay and is strongly enhanced by optical damage resistant doping around the doping threshold. This luminescence stems from the recombination of free electron and hole polarons. The next major topic of this review are domain contrasts of the visible photoluminescence that have been observed after electrical poling of the substrate, as singly inverted domains show a slightly reduced and faster decaying luminescence. Subsequent annealing results in an exponential decrease of that domain contrast. We show that this contrast decay is strongly related to the mobility of lithium ions, thus confirming the role of polar defect complexes, including lithium vacancies, for these domain contrasts. Finally we discuss the extension of our investigations to lithium tantalate (LTO) samples. While the results on the domain contrast and its decay are similar to LNO, there are remarkable differences in their luminescence spectra.
Highlights
Introduction and OverviewPolarons play an important role in the optical and electrical properties of lithium niobate (LNO), lithium tantalate (LTO) and other ferroelectrics
Excited electrons can occur in free polaron states on NbNb sites with an absorption maximum at 1 eV, in bound polaron states on NbLi anti-sites with an absorption maximum at 1.6 eV, or in bound pairs of bound and free electron polarons, called bipolarons, with an absorption maximum at 2.5 eV
To obtain the absorption spectra of these electron polarons, electrons have been brought into these polaron states by reduction of the crystal or by optical excitation of Fe2+ -doped samples
Summary
Polarons play an important role in the optical and electrical properties of lithium niobate (LNO), lithium tantalate (LTO) and other ferroelectrics. They are quasiparticle states formed by excited charge. Crystals 2018, 8, 214 carriers and the deformation of the surrounding crystal lattice due to their Coulomb forces [1]. Despite LNO to be exceptionally transparent over a wide wavelength range, reduction, doping and optical excitation can increase the absorption. This emergent absorption in the visible range is commonly associated with the formation of polarons. The governing physics of polarons in oxides and especially LNO has already been the subject of several review articles, yet solely discussing the induction of absorption [2,3,4,5,6,7]
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