Small-polaron hopping involved in charge transport in Fe-doped congruent lithium niobate is investigated as a function of temperature and composition by means of light-induced transient absorption spectroscopy. The relaxation dynamics of the light-induced polaron population is characterized by individual activation energies within different temperature ranges. A numerical investigation carried out by Monte Carlo simulations reveals that these findings may be understood in terms of the varying abundance of the different types of hops that the polarons may perform among regular or defective lattice sites. The role of the temperature and of the sample composition on the distribution of the different hop types is thus explored for a wide range of parameters, allowing one to preview the charge transport properties for a given set of experimental conditions.

Charge transport due to small polarons hopping among defective (bound polarons) and regular (free polarons) sites is shown to depend in a non-trivial way on the value of the stabilization energy provided by the lattice distortion surrounding the charge carriers. This energy, normally not directly accessible for bound polarons using spectroscopic techniques, is determined here by a combination of experimental and numerical methods for the important case of small electron polarons bound to N b L i defects in the prototype ferroelectric oxide lithium niobate. Our findings provide an estimation of the N b L i polaron stabilization energy E G P = ( 0.75 ± 0.05 ) e V and demonstrate that in lithium niobate both free and bound polarons contribute to charge transport at room temperature, explaining the fast decay of the light-induced bound polaron population observed by transient absorption spectroscopy.

Femtosecond-pulse-induced (E_pump = 2.5 eV) picosecond infrared absorption is studied in the spectral region between 0.30 eV and 1.05 eV in LiNbO₃:Mg. We find a non-instantaneous mid-infrared absorption peak in the time domain up to 1 ps and a broad-band, long-lived absorption (maximum at 0.85 eV, width ≈ 0.5 eV), for t > 1 ps. The modelling succeeds by considering small ${\text{Nb}}_{\text{Nb}}^{4+}$ electron polaron formation along the sequence: (i) two-photon injection of hot electron-hole pairs at Nb-O-octahedra, (ii) dissociation and electron cooling by electron-phonon-scattering, and (iii) electron self-localization by strong electron-phonon-coupling.

Transient absorption and photoluminescence are experimentally investigated in the polaronic reference system lithium niobate, LiNbO[Formula: see text] (LN), with the aim to refine the microscopic model of small polaron dynamics in materials with strong electron-phonon coupling. As a unique feature, our study is performed by using two different spectroscopic methods, in crystals with dopants enhancing photorefraction or damage resistance, and over a broad temperature range from 15-400 K. Although being self-consistent for particular experimental conditions, the hitherto used microscopic polaronic models reveal inconsistencies when applied to this larger data set. We show that comprehensive modeling is unlocked by the inclusion of an additional type of polaronic state with the following characteristics: (i) strongly temperature- and dopant-dependent relaxation times, (ii) an absorption feature in the blue-green spectral range, and (iii) a Kohlrausch-Williams-Watts decay shape with a temperature-dependent stretching factor [Formula: see text] showing a behavior contrary to that of small, strong-coupling polarons. The hypothesis of self-trapped excitons (STEs, i.e. bound electron-hole pairs strongly coupled to Nb[Formula: see text] and O[Formula: see text] within a niobium-oxygen octahedron) and their pinning on defects as the microscopic origin of these characteristics is supported by a spectroscopic linkage of photoluminescence at low (15 K) and elevated (300 K) temperatures and explains the long-lifetime components in transient absorption as due to pinned STEs.

A strongly temperature-dependent photo-induced transient absorption is measured in 6.5 mol% magnesium-doped lithium niobate at temperatures ranging from 45 K to 225 K. This phenomenon is interpreted as resulting from the generation and subsequent recombination of oppositely charged small polarons. Initial two-photon absorptions generate separated oppositely charged small polarons. The existence of these small polarons is monitored by the presence of their characteristic absorption. The strongly temperature-dependent decay of this absorption occurs as series of thermally assisted hops of small polarons that facilitate their merger and ultimate recombination. Our measurements span the high-temperature regime, where small-polaron jump rates are Arrhenius and strongly dependent on temperature, and the intermediate-temperature regime, where small-polaron jump rates are non-Arrhenius and weakly dependent on temperature. Distinctively, this model provides a good representation of our data with reasonable values of its two parameters: Arrhenius small-polaron hopping’s activation energy and the material’s characteristic phonon frequency.

Based on a model of coupled processes with differently time-dependent decay kinetics we present a critical review on photoluminescence (PL) and transient absorption (TA) experiments in undoped and Mg or Fe-doped LiNbO ₃ , together with a comprehensive interpretation of visible radiative and parallel non-radiative decay processes on timescales ranging from 50 ns up to minutes. Analogies and peculiarities of the kinetics of mobile self-trapped and pinned excitons are investigated and compared with those of hopping polarons in the same system. Exciton hopping with an activation energy of ≈0.18 eV is shown to govern the lifetime and quenching of the short PL component above 100 K. Strong interaction between excitons and dipolar pinning defects explains the exorbitant lifetimes and large depinning energies characterizing delayed TA components in doped LiNbO ₃ , while restricted hopping of the pinned excitons is proposed to play a role in strongly delayed PL in LiNbO ₃ :Mg exhibiting a narrowed emission band due to locally reduced electron–phonon coupling. Atomistic models of pinned excitons are proposed corresponding to charge-compensated dipolar defects predicted by theories of dopant incorporation in LiNbO ₃ and are systematically assigned to absorption bands observed near the UV edge. Excitation in these bands is shown to lead directly to pinned exciton states confirming also the previously proposed two-step exciton-decay scenario in LiNbO ₃ :Fe. Weak intrinsic sub-80 ns luminescence in congruent LiNbO ₃ is explained as an opposite effect of enhanced electron–phonon coupling for excitons pinned on Nb _Li antisite defects. The comparison of the different observed stretching behaviors in the paradigmatic system LiNbO ₃ provides an intuitive picture of the underlying physical processes. The findings are relevant not only for holographic and non-linear optical applications of LiNbO ₃ but are of general interest also for the treatment of stretched exponential or other time-dependent kinetics in complex condensed systems ranging from nanocrystals and polymers to liquids and biophysical systems.