Transient absorption and photoluminescence are experimentally investigated in the polaronic reference system lithium niobate, LiNbO 3 (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 dopantdependent 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 β (T) 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 5+ and O 2− 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.

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 from the value of the stabilization energy provided by the lattice distortion surrounding the charge carriers. This energy, normally not directly accessible for bound polarons by spectroscopic techniques, is here determined by a combination of experimental and numerical methods for the important case of small electron polarons bound to \mathrm{Nb}_{\mathrm{Li}} defects in the prototype ferroelectric oxide lithium niobate. Our findings provide an estimation of the \mathrm{Nb}_{\mathrm{Li}} polaron stabilization energy E_{GP}=\unit[(0.75\pm0.05)]{eV} and point out that in lithium niobate both free and bound polarons contributes to charge transport already at room temperature, explaining the fast decays of the light-induced bound polaron population observed by transient absorption spectroscopy.

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.

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 LiNbO 3 :Mg. We find a noninstantaneous 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 Nb 4+ Nb electron polaron formation along the sequence: (i) twophoton 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 electronphonon-coupling.

Transient absorption is studied in Fe-doped lithium niobate single crystals with the goal to control and probe a blue absorption feature related with excitonic states bound to Fe Li defect centers. The exciton absorption is deduced from the comparison of ns-pump, supercontinuumprobe spectra obtained in crystals with different Fe-concentration and Fe 2+/3+ Li-ratio, at different pulse peak and photon energies as well as by signal separation taking well-known small polaron absorption bands into account. As a result, a broad-band absorption feature is deduced being characterized by an absorption cross-section of up to σ max (2.85 eV) = (4 ± 2) • 10 −22 m 2. The band peaks at about 2.85 eV and can be reconstructed by the sum of two Gaussians centered at 2.2 eV (width ≈ 0.5 eV) and 2.9 eV (width ≈ 0.4 eV), respectively. The appropriate build-up and decay properties strongly depend on the crystals' composition as well as the incident pulse parameters. All findings are comprehensively analyzed and discussed within the model of Fe 2+ Li − O − − V Li excitonic states.