Superposed picosecond luminescence kinetics in lithium niobate revealed by means of broadband fs-fluorescence upconversion spectroscopy.

Various manifestations of small polarons strongly affect the linear and nonlinear optical properties of the oxide crystal lithium niobate ([Formula: see text], LN). While related transient absorption phenomena in LN have been extensively studied in recent decades, a sound microscopic picture describing the blue-green (photo)luminescence of lithium niobate single crystals is still missing. In particular, almost nothing is known about: (i) the luminescence build-up and (ii) its room temperature decay. We present here the results of our systematic experimental study using nominally undoped and Mg-doped LN crystals with different Mg concentration. Picosecond luminescence was detected by means of femtosecond fluorescence upconversion spectroscopy (FLUPS) extended to the inspection of oxide crystals in reflection geometry. Two distinct luminescence decay components on the picosecond time scale are revealed. While a short exponential decay is present in each sample, a longer non-exponential decay clearly depends on the crystal composition. Since transient absorption spectroscopy excludes geminate small polaron annihilation as microscopic cause of the luminescence, both decay components are discussed in the context of self-trapped exciton (STE) transport and decay.

Excitonic hopping-pinning scenarios in lithium niobate based on atomistic models: different kinds of stretched exponential kinetics in the same system.

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 NbLi 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.

The role of self-trapped excitons in polaronic recombination processes in lithium niobate.

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.

Self-diffraction from two-photon absorption gratings in Sn2P2S6.

Self-diffraction with the appearance of higher diffraction orders is discovered when writing a grating with a single sub-100 fs pulse in a nominally undoped Sn(2)P(2)S(6) sample. The short time of grating development, dependence of diffraction efficiency on the recording light intensity, correlation of wavelength dependence of efficiency with the spectrum of the two-photon absorption (TPA) constant, and a π phase shift of the diffracted beam allow for attributing the recorded grating to a dynamic amplitude grating of TPA.

Small-polaron based holograms in LiNbO3 in the visible spectrum.

Diffraction efficiency, relaxation behavior and dependence on pump-beam intensity of small-polaron based holograms are studied in thermally reduced, nominally undoped lithium niobate in the visible spectrum (λ = 488 nm). The pronounced phase gratings with diffraction efficiency up to η = (10.8 ± 1.0)% appeared upon irradiation by single ns-laser pulses (λ = 532 nm) and are comprehensively assigned to the optical formation of spatially modulated densities of small bound NbLi4+ electron polarons, NbLi4+:NbNb4+ electron bipolarons, and O⁻ hole polarons. A remarkable quadratic dependence on the pump-beam intensity is discovered for the recording configuration K || c-axis and can be explained by the electro-optic contribution of the optically generated small bound polarons. We discuss the build-up of local space-charge fields via small-polaron based bulk photovoltaic currents.

Hologram recording via spatial density modulation of Nb(Li)(4+/5+) antisites in lithium niobate.

Hologram recording is studied in thermally reduced, nominally undoped lithium niobate in the time domain from 10 ns to 100 s by means of intense ns pump laser pulses (λ = 532 nm) and continuous-wave probe light (λ = 785 nm). It is shown that mixed absorption and phase gratings can be recorded within 8 ns that feature diffraction efficiencies up to 23 % with non-exponential relaxation and lifetimes in the ms-regime. The results are explained comprehensively in the frame of the optical generation of a spatial density modulation of Nb(Li)(4+/5+) antisites and the related optical features, i.e. absorption as well as index changes mutually related via the Kramers-Kronig-relation. Implications of our findings, such as the electrooptical properties of small bound Nb(Li)(4+) polarons, the optical features of Nb(Li)(4+):Nb(Nb)(4+) bipolarons, Nb(Nb)(4+) free polarons and O-hole-polarons, the impact of light polarization of pump and probe beams as well as of the polaron density are discussed.

Absorption cross sections and number densities of electron and hole polarons in congruently melting LiNbO(3).

The number densities and absorption cross sections of both optically generated and reduction-induced small electron and hole polarons in LiNbO(3) are determined by means of time-resolved pump-multiprobe spectroscopy. The data are obtained for free (Nb(Nb)(4+)) and bound (Nb(Li)(4+)) electron polarons, bound Nb(Li)(4+):Nb(Nb)(4+) electron bipolarons, and bound O(-) hole polarons. The peak absorption cross sections are in the range of σ(pol)≈(4-14) × 10(-22) m(2), comparable to that for Fe(2+). In all cases the ratio of occupied to unoccupied polaronic sites is less than 10(-2).

Electron small polarons and bipolarons in LiNbO(3).

An overview of the properties of electron small polarons and bipolarons is given, which can occur in the congruently melting composition of LiNbO(3) (LN). Such polarons influence the performance of this important optical material decisively. Since coupling to the lattice strongly quenches the tunnelling of free small polarons in general, they are easily localized at one site even by weak irregularities of a crystal. The mechanism of their optical absorptions is thus shared with those of small polarons localized by binding to selected defects. It is shown that the optical properties of free electrons in LN as well as those bound to Nb(Li) antisite defects can be attributed consistently to small polarons. This is extended to electron pairs forming bipolarons bound to Nb(Li)-Nb(Nb) nearest neighbours in the LN ground state. On the basis of an elementary phenomenological approach, relying on familiar concepts of defect physics, the peak energies, lineshapes, widths of the related optical absorption bands as well as the defect binding energies induced by lattice distortion are analysed. A criterion universally identifying small polaron absorption bands in oxide materials is pointed out. For the bipolarons, the dissociation energy, 0.27 eV, derived from a corresponding study of the mass action behaviour, is shown to be consistent with the data on isolated polarons. Based on experience with simple O(-) hole small polaron systems, a mechanism is proposed which explains why the observed small polaron optical absorptions are higher above the peak energies of the bands than those predicted by the conventional theory. The parameters characterizing the optical absorptions are seen to be fully consistent with those determining the electrical conductivity, i.e. the bipolaron dissociation energy and the positions of the defect levels as well as the activation energy of mobility. A reinterpretation of previous thermopower data of reduced LN on the basis of the bipolaron model confirms that the mobility of the free polarons is activated by 0.27 eV. On the basis of the level scheme of the bipolarons as well as the bound and free polarons the temperature dependence of the electronic conductivity is explained. The polaron/bipolaron concept also allows us to account for the concentrations of the various polaron species under the combined influence of illumination and heating. The decay of free and bound polarons dissociated from bipolarons by intense short laser pulses of 532 nm light is put in the present context. A critical review of alternative models, being proposed to explain the mentioned absorption features, is given. These proposals include: single free polarons in the (diamagnetic) LN ground state, oxygen vacancies in their various conceivable charge states, quadpolarons, etc. It is shown why these models cannot explain the experimental findings consistently.

Parametric hybrid scattering from light-induced ferroelectric and photorefractive structures.

Processes of photo-induced light scattering are studied in single crystals of LiNbO(3):Fe to uncover the origin of a new part of the entire scattering pattern which can be observed on a viewing screen. The new scattering manifests itself as two arcs enclosing the directly transmitting pump beam. It is shown that this type of scattering is due to a parametric wave-mixing process of coherent optical noise and a pump beam on a combination of photorefractively recorded phase-gratings and photo-induced ferroelectric structures. Phase-matching conditions corresponding to the new scattering are introduced. All photo-induced scattering phenomena contributing to the total scattering pattern are discussed, compared and classified.

Insight to UV-induced formation of laser damage on LiB(3)O(5) optical surfaces during long-term sum-frequency generation.

Microscopic investigations of UV-induced formation of laser damage on LiB(3)O(5) optical surfaces during long-term sum-frequency generation (SFG) uncovers a significant growth of a SiO(2)-amorphous layer spatially limited to the illuminated area. The layer gives rise to a catastrophic break-down of the LiB(3)O(5)-output surface upon long-term laser operation even at intensities far below the laser-induced damage threshold. The interaction of UV laser light, LiB(3)O(5) surface and foreign atoms in the ambient atmosphere is discussed in the frame of a two-step process for surface-damage formation.

Evidence for two-path recombination of photoinduced small polarons in reduced LiNbO(3).

The recombination of photoinduced free Nb(4+)(Nb) and bound Nb(4+)(Li) small polarons toNb(4+)(Li): Nb(4+)(Nb) bipolarons is investigated in nominally pure, reduced LiNbO(3) single crystals by means of excited-state-absorption spectroscopy. We discovered a two-component decay of the light-induced absorption alpha(li)(t) for probe light at lambda=785 nm and moderate pump beam intensities (I(p) << 670 GW/m(2)). These experimental results give strong evidence for the existence of a two-path recombination of the photoinduced polarons. A corresponding model taking into account hopping charge transport and trapping is presented.

Evidence for light-induced hole polarons in LiNbO3.

Transient light-induced absorption in LiNbO3 is observed in the blue-green spectral range after pulsed illumination with 532 nm. Its buildup and decay in Fe-doped LiNbO3 is satisfactorily described by a sum of two stretched exponential functions. For undoped LiNbO3, however, only one stretched exponential decay is observed. These experimental results are explained by the formation of both small Nb(Li)4+ electron polarons and O- hole polarons. The mechanism is discussed on the basis of a proposed band scheme.

Comment on "Theoretical study of the photoinduced transfer among the ground state and two metastable states in [Fe(CN)5NO]2-" [J. Chem. Phys. 122, 074314 (2005)].

We discuss the computational results of the "Theoretical study of the photoinduced transfer among the ground state and two metastable states in [Fe(CN)5NO]2-" [J. Chem. Phys. 122, 074314 (2005)] with respect to our previously reported polarized absorption study on the metastable states SI and SII in Na2[Fe(CN)5NO]2H2O [D. Schaniel, J. Schefer, B. Delley, M. Imlau, and Th. Woike, Phys. Rev. B 66, 085103 (2002)].

Holographic recording with reduced intermodulation noise in periodically poled lithium niobate.

We show that the use of periodically poled lithium niobate doped with Fe and Y ensures a considerable improvement in the quality of reconstructed images compared with the use of single-domain crystals. This improvement is due to inhibition of intermodulation noise and elimination of optical damage.

Change from 3D-Ising to random field-Ising-model criticality in a uniaxial relaxor ferroelectric.

Pyroelectric measurements of polarization have been used to determine the temperature dependence of the polarization in strontium barium niobate, SBN:Ce, close to its phase transition temperature T(c) approximately 317 K. A gradual increase of the critical exponent from beta approximately 0.13 to beta approximately 0.30 is observed when decreasing the initial polarization from 100% to 0.8% of the saturation value. A change from three-dimensional random-field Ising to pure Ising model behavior is conjectured and explained by a gradual compensation of quenched random electric fields by those emerging from charged fractal nanodomain walls.

New parametric scattering in photorefractive Sr0.61Ba0.39Nb2O6:Cr.

A new class of light-induced parametric scattering, not included in the conventional model, has been discovered in photorefractive Sr(0.61)Ba(0.39)Nb(2)O(6):Cr illuminated by two coherent beams. A novel model of multiwave mixing of coherent noise and transmitted light is developed to explain the new scattering phenomena. The model includes all known types and predicts a multitude of new types of parametric scattering. Generalized phase-matching conditions for parametric scattering in photorefractive crystals are proposed.

Polarization-based adjustable memory behavior in relaxor ferroelectrics.

The irreversible decay of the spontaneous polarization above the phase-transition temperature is a limiting factor in any application of ferroelectric crystals. Here we show that electric fields applied at high temperatures induce a preferred direction in the crystal which is stable even after repeated heating and cooling through the phase transition. This preference in direction leads to a reorientation of domains in the ferroelectric phase. We use pyroelectric measurements to show that the directional preference originates from internal charge carriers interacting with domain walls.

Light-induced extinction originating from holographic scattering.

We present extinction spectra of sodium nitroprusside exhibiting holographic light scattering after irradiation with coherent light. A characteristic extinction band appearing in the vicinity of the wavelength of the pump beam was discovered as well as an increase of the extinction coefficient over the whole spectral range. These features are proved to originate from diffraction of the probe beam from parasitic holograms and can be explained within the framework of a simple Ewald construction.