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 LiNbO:Fe substrates, has been investigated for particle trapping and patterning applications. The novel hybrid procedure provides new possibilities and versatility to optoelectronic manipulation on LiNbO 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.

Massive ordering and alignment of cylindrical micro-objects by photovoltaic optoelectronic tweezers.

Optical tools for manipulation and trapping of micro- and nano-objects are a fundamental issue for many applications in nano- and biotechnology. This work reports on the use of one such method, known as photovoltaic optoelectronics tweezers, to orientate and organize cylindrical microcrystals, specifically elongated zeolite L, on the surface of Fe-doped LiNbO crystal plates. Patterns of aligned zeolites have been achieved through the forces and torques generated by the bulk photovoltaic effect.

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.

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.

Photovoltaic versus optical tweezers.

The operation of photovoltaic (PV) tweezers, using the evanescent light-induced PV fields to trap and pattern nano- and micro-meter particles on a LiNbO(3) crystal surface, is discussed. The case of a periodic light pattern is addressed in detail, including the role of particle shape and the modulation index of the light pattern. The use of a single Gaussian light beam is also considered.

Tumour cell death induced by the bulk photovoltaic effect of LiNbO3:Fe under visible light irradiation.

This work reports a pioneer application of the bulk photovoltaic effect in the biomedical field. Massive necrotic cell death was induced in human tumour cell cultures grown on a bulk photovoltaic material (iron-doped lithium niobate, LiNbO(3):Fe) after irradiation with visible light. Lethal doses (≈100% cell death) were obtained with low-intensity visible light sources (10-100 mW cm(-2) irradiances) and short exposure times of the order of minutes.

Thick optical waveguides in lithium niobate induced by swift heavy ions (approximately 10 MeV/amu) at ultralow fluences.

Heavy mass ions, Kr and Xe, having energies in the approximately 10 MeV/amu range have been used to produce thick planar optical waveguides at the surface of lithium niobate (LiNbO3). The waveguides have a thickness of 40-50 micrometers, depending on ion energy and fluence, smooth profiles and refractive index jumps up to 0.04 (lambda = 633 nm).

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.

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.

Structural modulation of the dipolar-octupolar contributions to the NLO response in subphthalocyanines.

Second order nonlinear optical properties of a series of trinitrosubphthalocyanine (SubPc) isomers were studied experimentally by electric field induced second harmonic (EFISH) generation and hyper Rayleigh scattering (HRS). These experimental values were compared to the ones obtained theoretically employing both sum over states (SOS) and finite field (FF) methods. From these studies, it was shown that the dipolar contributions to the beta tensor are very much dependent on the substitution pattern at the periphery of the subphthalocyanine macrocycle, whereas the octupolar contributions remain mostly unchanged.

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.

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.

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.

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.