Synergy between pyroelectric and photovoltaic effects for optoelectronic nanoparticle manipulation.

The combined action of the pyroelectric (PY) and photovoltaic (PV) effects, exhibited by z-cut LiNbO3:Fe substrates, has been investigated for particle trapping and patterning applications. The novel hybrid procedure provides new possibilities and versatility to optoelectronic manipulation on LiNbO3 substrates. It has allowed obtaining periodic and arbitrary 2D patterns whose particle density distribution is correlated with the light intensity profile but can be tuned through ΔT according to the relative strength of the PV and PY effects. A relevant result is that the PY and PV contributions compete for a ΔT range of 1-20 °C, very accessible for experiments. Moreover, the synergy of the PY and PV has provided two additional remarkable applications: i) A method to measure the PV field, key magnitude for photovoltaic optoelectronic tweezers. Using this method, the minimum field needed to obtain a particle pattern has been determined, resulting relatively high, E~60 kV/cm, and so, requiring highly doped crystals when only using the PV effect. ii) An strategy combining the PY and PV to get particle patterning in samples inactive for PV trapping when the PV field value is under that threshold.

Diffractive optical devices produced by light-assisted trapping of nanoparticles.

One- and two-dimensional diffractive optical devices have been fabricated by light-assisted trapping and patterning of nanoparticles. The method is based on the dielectrophoretic forces appearing in the vicinity of a photovoltaic crystal, such as Fe:LiNbO3, during or after illumination. By illumination with the appropriate light distribution, the nanoparticles are organized along patterns designed at will. One- and two-dimensional diffractive components have been achieved on X- and Z-cut Fe:LiNbO3 crystals, with their polar axes parallel and perpendicular to the crystal surface, respectively. Diffraction gratings with periods down to around a few micrometers have been produced using metal (Al, Ag) nanoparticles with radii in the range of 70-100 nm. Moreover, several 2D devices, such as Fresnel zone plates, have been also produced showing the potential of the method. The diffractive particle patterns remain stable when light is removed. A method to transfer the diffractive patterns to other nonphotovoltaic substrates, such as silica glass, has been also reported.

Swift heavy ion damage to sodium chloride: synergy between excitation and thermal spikes.

Systematic data on the effect of irradiation with swift ions (Zn at 735 MeV and Xe at 929 MeV) on NaCl single crystals have been analysed in terms of a synergetic two-spike approach (thermal and excitation spikes). The coupling of the two spikes, simultaneously generated by the irradiation, contributes to the operation of a non-radiative exciton decay model as proposed for purely ionization damage. Using this scheme, we have accounted for the π-emission yield of self-trapped excitons and its temperature dependence under ion-beam irradiation. Moreover, the initial production rates of F-centre growth have also been reasonably simulated for irradiation at low temperatures ( < 100 K), where colour centre annealing and aggregation can be neglected.

Nonlinear optical waveguides generated in lithium niobate by swift-ion irradiation at ultralow fluences.

A novel method to produce optical waveguides is demonstrated for lithium niobate (LiNbO(3)). It is based on electronic excitation damage by swift ions, i.e., with energies at approximately 1 MeV/amu or above. The new technique uses high-energy medium-mass ions, such as Cl, with electronic stopping powers above the threshold value for amorphization (5-6 keV/nm), reaching the maximum value a few micrometers inside the crystal. At the ultralow fluence regime (10(12)-10(13) cm(-2)) an effective nanostructured medium is obtained that behaves as an optical waveguide where light propagates transversally to the amorphous nanotracks created by every single impact. The method implies a reduction of 4 orders of magnitude with respect to He implantation. The optical waveguides present reasonable losses (~10 dB/cm) and significant second-harmonic generation (SHG) and electro-optic (EO) responses (>50% bulk) for the lowest fluences.

Isotropic versus anisotropic modeling of photorefractive solitons.

The question of the isotropic versus anisotropic modeling of incoherent spatial screening solitons in photorefractive crystals is addressed by a careful theoretical and numerical analysis. Isotropic, or local, models allow for an extended spiraling of two interacting scalar solitons, and for a prolonged propagation of vortex vector solitons, whereas anisotropic, nonlocal, models prevent such phenomena. In the context of Kukhtarev's material equations, the difference in behavior is traced to the continuity equation for the current density. We further show that neither an indefinite spiraling of two solitons nor stable propagation of vortex vector solitons is generally possible in both isotropic and anisotropic models. Such systems do not conserve angular momentum, even in the case of an isotropic change in the index of refraction.

Solitonlike beam propagation along light-induced singularity of space charge in fast photorefractive media.

We investigate light beam propagation in a fast photorefractive medium placed in an alternating electric ac field to enhance the nonlinear response. It is shown that the joint action of the optical and material nonlinearities leads to formation of a narrow singularity of the light-induced space charge at the intensity maximum and to self-trapping of the light energy near the corresponding discontinuity of the index profile. Owing to the strong saturation of the material nonlinearity, the trapped beam propagates over long distances with only a weak loss of its power.

Degenerate parametric light scattering in periodically poled LiNbO3:Y:Fe.

The first observation of parametric light scattering patterns (rings, lines, and dots) in bulk periodically poled nonlinear media is reported. Development of the scattering patterns proves efficient photorefractive grating recording and considerable parametric gain for seed radiation in this new nonlinear material. Several novel phase-matched parametric processes, caused by the periodicity of the domain structure, are revealed.

Dipole-mode vector solitons in anisotropic nonlocal self-focusing media.

We demonstrate, theoretically and experimentally, that dipole-mode vector solitons created in biased photorefractive media possess a number of anisotropy-driven properties, such as stability of a selected orientation, wobbling, and incomplete rotation, owing to the anisotropic nonlocal response of the photorefractive non-linearity. Such features are found for higher-order (multipole) vector solitons, and they are carefully verified in an experiment.

Singular behavior of light-induced space charge in photorefractive media under an ac field

We show that a photoconductive crystal, placed in a rapidly alternating ac field and exposed to nonuniform light, exhibits singularities of the induced space charge and discontinuities of the corresponding space-charge field. The singularities appear at the local intensity maxima when the curvature of the intensity profile exceeds a certain (often very low) threshold value. We analyze the characteristic features of the singular ac response and consider its possible optical manifestations.

Nonperturbative analytical solution for steady-state photorefractive recording.

A nonperturbative analytical solution for steady-state photorefractive recording in the diffusion approximation has been obtained. It applies to any arbitrary modulation depth m and includes, as limit cases, the perturbation solutions reported by previous authors.

Holographic writing and erasure in unipolar photorefractive materials with multiple active centers: theoretical analysis.

A general formalism based on the Kukhtarev equations is developed to describe the kinetics of holographic recording and erasure in unipolar photorefractive materials containing multiple active centers. One primary relevant result is that the exchange of charge among the various centers and the holographic grating dynamics, involving charge transport, are uncoupled after appropriate linearization of the equations. The formalism is then applied to the simple but physically meaningful case of two active centers, an optical donor and a thermally active trap. Detailed computer simulations have been carried out to investigate the influence of trap energy depth, trap concentration, temperature, and light intensity on the holographic behavior. The results qualitatively account for a number of unexpected features in the kinetics of grating recording and erasure observed in several photorefractive materials.

Erasure of holographic gratings in photorefractive materials with two active species.

The erasure kinetics of holographic gratings has been theoretically studied for a material containing two photorefractive species. The approach is an extension of the method developed by Carrascosa and Agullo- Lopez for a simple photorefractive center. The erasure of the grating involves the transfer of electronic charge between the two photorefractive systems together with a spatial transport of the charge. Both processes may have, in general, comparable time constants leading to a more complicated formalism than that for a single species. The electronic exchange between two photoactive centers has been first solved analytically. Then, the erasure kinetics of a sinusoidal grating, including charge exchange, has been formulated under the short transport length approximation. The coupled equations governing the decay of grating amplitude and the velocity of fringes have been numerically solved (a) after neglecting diffusion and (b) in the general case. The solution for the time dependence of grating amplitude is nonexponential. The particular situation where the electronic exchange process is very fast in comparison to grating erasure has been solved assuming arbitrary transport lengths. The decay of grating amplitude consists of two exponential curves if the photovoltaic drift is ignored and it is nonexponential if it is included. For short transport lengths, the decay reduces to a single exponential.