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.

We gain hitherto missing access to the spatio-temporal evolution of lattice distortions caused by carrier self-trapping in the class of oxide materials - and beyond. The joint experimental/theoretical tool introduced combines femtosecond mid-infrared probe spectroscopy with potential landscape modeling and is based on the original approach that the vibration mode of a biatomic molecule is capable to probe strongly localized, short-lived lattice distortions in its neighborhood. Optically generated, small, strong-coupling polarons in lithium niobate, mediated by OH− ions present as ubiquitous impurities, serve as a prominent example. Polaron trapping is found to result in an experimentally determined redshift of the OH− stretching mode amounting to Δνvib = −3 cm−1, that is successfully modeled by a static Morse potential modified by Coulomb potential changes due to the displacements of the surrounding ions and the trapped charge carrier. The evolution of the trapping process can also be highlighted by monitoring the dynamics of the vibrational shift making the method an important tool for studying various systems and applications.

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.

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Energy redistribution between two subpicosecond laser pulses of 2.5 eV photon energy is observed and studied in congruent, nominally undoped LiNbO, aiming to reveal the underlying coupling mechanisms. The dependences of pulse amplification on intensity, frequency detuning and pulse duration point to two different contributions of coupling, both based on self-diffraction from a recorded dynamic grating. The first one is caused by a difference in pulse intensities (transient energy transfer) while the second one originates from a difference in pulse frequencies. The latter appears when chirped pulses are mutually delayed in time. A quite high coupling efficiency has been observed in a 280 µm thin crystal: one order of magnitude energy amplification of a weak pulse and nearly 10% net energy enhancement of one pulse for the case of equal input intensities.

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