[en] Light-induced metastable electronic states as observed e.g. in sodium nitroprusside are of fundamental interest for data storage and optical computing. Structural functionality and therefore the light-induced structure itself is of basic interest in such systems. As neutrons are sensitive to the position of the nucleus and non-destructive with respect to the metastable electronic states, neutron photocrystallography provides a useful method to determine light-induced structural changes. A photocrystallographic experimental setup has been built-up at SINQ and has been successfully used at the single-crystal instrument TriCS and the test facility TOPSI (now called MORPHEUS)

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[en] Conical light scattering is uncovered in poly- and mono-domain, nominally pure and Eu-doped strontium barium niobate (SBN) crystals over a wide temperature regime. The appearance of two scattering cones, a scattering line and a corona is observed and can be explained comprehensively within the Ewald sphere concept. Photorefraction, scattering from domain boundaries or from growth striations can be excluded from explaining the origin of the scattering. It is shown that the temperature-persistent scattering process is related to a growth-induced seeding rod, i.e. a composition inhomogeneity primarily localized at the centre of the SBN sample. The rod is directed parallel to the c axis and yields a refractive-index inhomogeneity with spatial frequencies on the micro-scale

[en] 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 LiNbO₃, 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 LiNbO₃, while restricted hopping of the pinned excitons is proposed to play a role in strongly delayed PL in LiNbO₃: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 LiNbO₃ 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 LiNbO₃:Fe. Weak intrinsic sub-80 ns luminescence in congruent LiNbO₃ 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 LiNbO₃ provides an intuitive picture of the underlying physical processes. The findings are relevant not only for holographic and non-linear optical applications of LiNbO₃ 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. (topical review)

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[en] Transient light-induced absorption changes α_li(t), caused by optically generated small polarons, are investigated in LiNbO₃:Mg below and above the optical-damage-resistance threshold (ODRT). The lifetime of α_li(t) is reduced by three orders of magnitude above the ODRT while a significantly enhanced amplitude α^max_li is observed in the infrared. Our observations are in full accordance with the predictions of microscopic models for the ODRT, namely the removal of Nb_Li antisite defects upon incorporation of Mg ions, and an enhanced dark conductivity. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (Abstract Copyright [2008], Wiley Periodicals, Inc.)