Fe-doped Lithium Niobate Crystals Research Articles

Bessel beam approach for photovoltaic trapping of micro- and nanoparticles on Fe-doped lithium niobate crystal

The non-diffracting Bessel beam approach was used for the first time for the elaboration of photovoltaic tweezers. We report the trapping of dielectric microparticles of chalk (CaCO3), as well as silver nanoparticles in glycerin suspension via dielectrophoretic forces on the surface of Fe doped LiNbO3 (LN:Fe) crystal with recorded volume holographic grating by the non-diffracting Bessel beam technique. The photorefractive Y-cut LN:Fe crystal was illuminated by 20–40 mW power Bessel beam at 532 nm wavelength which generates refractive Bessel lattice with ~40 μm periodicity via induction of photovoltaic space-charge fields. The Bessel beam approach provides the induction of high contrast 2D quasi-periodic distribution of space-charge electric field in the crystal volume and proximity of the crystal surface and high quality 2D patterning of the particles. Experiments showed that CaCO3 microparticles are trapped by attractive forces on the crystal surface in the areas of refractive index maxima of the Bessel lattice, while the trapping of Ag nanoparticles dispersed in glycerin takes places at the borderlines of lattice rings due to the repulsive forces. A physical model was developed to explain the experimental results. The photovoltaic approach of trapping and manipulation of micro- and nanoparticles is promising for applications in photonics, integrated optics and biotechnology.

Polaron approach for nondestructive readout of holograms in photorefractive LiNbO3:Fe crystal in an external magnetic field

A novel method of magnetic field assisted nondestructive optical probing of photonic lattice structures in photorefractive crystals is demonstrated experimentally. High contrast annular symmetry micrometric scale refractive lattice structures were recorded by nondiffracting Bessel beam technique with the use of 532 nm, 17 mW power laser beam in Fe doped lithium niobate crystal. The stability of recorded lattices against erasure during readout by probe Gaussian beam at the recording wavelength has been studied. Time evolutions of the diffraction efficiency of the lattices for readout beam power of 17 mW in zero magnetic field and in external magnetic field of 0.85 T were measured. The measurements showed up to 3.4 times increase of erasure constant for magnetic field assisted readout of stored lattices. The physical model based on the approach of small polarons in external magnetic field to explain the experimental results is discussed.

Photogalvanic/pyroelectric power conversion and self-pulsing of electro-wetting of LC droplet on lithium niobate (LN)-crystal

Photogalvanic current and voltage photoinduced by CW laser illumination of Fe-doped LN reveals quasi-periodic pulsing due to microplasmas discharges. This transformation of constant external influence into periodic response is interesting as example of self-excited systems, resembling pulsations of biological systems. Microplasmas discharges are also interesting for applications in various fields, such as surface treatment, sterilization, water splitting. We analyze self-pulsing of photogalvanic current visualized by dynamic electrowetting in a simple LC-cell, formed by a nematic LC droplet placed on Fe-doped LN crystal. In modeling contributions from both photogalvanic (bulk photovoltaic) and pyroelectric (charging by heating-cooling) effect are included. The analysis of dynamic regime reveals that a current pulse (in microsecond range) has asymmetric shape with extremely sharp rise with longer decay time. Reflected interference patterned was modulated by the dynamic electro-wetting effect. For high light intensity power conversion efficiency is enhanced due to nonlinear photogalvanic effect and microplasmas discharges.

Study on the partial self-recovery process in Fe-doped lithium niobate crystals

We study the temperature and pump-intensity dependence of the temporal evolution of photoinduced light scattering in congruent, hafnium-codoped and near-stoichiometric LiNbO3:Fe crystals. A partial self-recovery process is found in the evolution trend of photoinduced light scattering in all crystals at room temperature while it disappears at −190 °C or above 120 °C. Moreover, a “scattering acceleration effect” is found for both Hf-doped and near-stoichiometric crystals. Basing the saturation refractive-index modulation the partial self-recovery process is explained by the slow recombination of light-induced space charges of the noise gratings with high wave vectors. The “scattering acceleration effect” is connected with the activation of the protons located around Fe ions and their impact on the photoconductivity.

Photovoltaic effect in Bi2TeO5 photorefractive crystal

We report on the presence of a strong photovoltaic effect on nominally undoped photorefractive Bi2TeO5 crystals and estimated their Glass photovoltaic constant and photovoltaic field for λ = 532 nm illumination. We directly measured the photovoltaic-based photocurrent in this material under λ = 532 nm wavelength laser light illumination and compared its behavior with that of a well known photovoltaic Fe-doped Lithium Niobate crystal. We also show the photovoltaic current to strongly depend on the polarization direction of light. Holographic diffraction efficiency oscillation during recording and the behavior of fringe-locked running holograms in self-stabilized experiments are also demonstrated here as additional indirect proofs of the photovoltaic nature of this material.

Photo-assisted proton exchange and chemical etching on Fe-doped lithium niobate crystals.

We report the photo-assisted proton exchange and chemical etching on Fe-doped LiNbO(3) crystals. Selective proton exchange and chemical etching are realized through the 455nm-laser irradiation on the crystal surface in pyrophosphoric acid. Optical microscopy and Micro-IR spectroscopy analysis show that the hydrogen incorporation is confined spatially by the laser irradiation. Moreover, under the laser irradiation, + z surface is found to be more easily etched than -z surface. This unexpected etching anisotropy is attributed to the photogalvanic effect of the crystal.

Photo-conductivity decay properties of Fe-doped congruent lithium niobate crystals

Photo-conductivity transient processes of Fe:LiNbO3 congruent crystals are investigated by electro-chemical analyzer. The experiments are executed with Fe:LiNbO3 crystals of different Fe concentrations in the conditions of different laser intensities. The results show that the transient photo-conductivity of the Fe-doped lithium niobate crystal is formed through a complex process of electron transport; the decay of photo-conductivity can be fitted to an exponential function and a stretched-exponential function. The dependences of the fitting parameters on laser intensity and iron-doped concentration are measured. The values of amplitudes σ1max, σ2max, time constant τ2 and stretching factor β increase strongly at low intensities, and τ2 and β reach their saturation value for higher intensities; with the increase of the concentration of Fe ions, the values of σ1max, σ2max and τ2 incerease, but β decreases. With experimental results, we propose a charge transfer model which includes the migration of electrons in the conduction band and the jumping of electrons between small-polarons. The model better explains the main features of photo-conductivity decay for Fe-doped congruent lithium niobate crystals.

Talbot Effect from Rotational Symmetry Gratings: Application to 3D Refractive Grating Formation

We report the first observation of Talbot effect – diffraction grating images self-replications at regular distances, from 1D annular grating with the period of d = 30 ± 1 μm. Up to 20 high contrast revivals of the grating were observed by the green laser beam at λ = 532 nm with Talbot distance ZT = d2/λ = 1.57 ± 0.01 mm. The grating self imaging with half period of the original grating at ZT/2 was also observed. Using the annular grating as a mask, the recording of refractive index photonic lattice in 2 mm thick Fe doped lithium niobate crystal was performed by 532 nm laser beam. The replication of the grating images due to Talbot effect provides the recording of two refractive replicas of the grating with the periods of d and d/2 inside the 2 mm crystal checked by phase microscope depth scan observations. The obtained results show the possibility of the use of 2D masks for the creation of 3D photonic lattices in photorefractive materials, which can be used as photonic crystals.

Composition and temperature dependence of the light-induced scattering in Fe-doped lithium niobate

In this paper, Fe-doped lithium niobate crystals with different Li compositions are prepared. The composition and temperature dependence of the light-induced scattering in Fe-doped lithium niobate are studied. The results show that the crystals with the compositions lower than 49.0 mol% suffer from the serious light-induced scattering, but when the composition increases to 49.3 mol% the light-induced scattering is suppressed significantly, in particular at a composition of 49.8 mol% the light-induced scattering disappears completely. It is also found that the light-induced scattering of the crystals with lower compositions can be suppressed completely by elevating the temperature to 150 ℃. However the temperature for the complete suppression of the light-induced scattering in the crystal with a composition of 49.3 mol% is only 80 ℃. Based on these results, it is suggested that a critical Li composition may exist in a range from 48.9 mol% to 49.3 mol% and beyond this composition the suppression and temperature dependence of the light-induced scattering will change significantly.

All-optical microfluidic chips for reconfigurable dielectrophoretic trapping through SLM light induced patterning

We explore a novel approach for fabricating polymeric microfluidic-channelled dielectrophoretic (DEP) chips by direct laser projection through a holographic Spatial-Light-Modulator (SLM) onto photorefractive crystal substrates. As the first step, an all-optical mould-free approach was used to fabricate the PDMS microfluidic channel, by exploiting the light induced space charge field in Fe-doped lithium niobate crystals, with the aim of integrating a microfluidic channel directly onto the functionalized substrate. Subsequently, as the second step, geometrical flexible DEP traps can be created onto the substrate by the same SLM holographic projection system. The experimental verification shows the trapping of flowing carbon nanotubes (CNTs) and the formation of chaining effects with graphite nanofibers. The main feature of the SLM is the ability to display an arbitrary light intensity pattern that is used here for fabricating the channels. Moreover, the reconfigurable trapping of CNTs is possible simply by the optical writing/erasing of various light intensity patterns projected by the SLM.

Large pyroelectric effect in Fe-doped lithium niobate induced by a high-power short-pulse laser

We demonstrate the pyroelectric effect induced by a short pulse laser with an intensity up to 6 MW/cm2 which is used to heat the Fe-doped Lithium niobate crystal. 0.2 °C temperature rise induced by a 10 ns laser pulse causes a very large current peak (up to 25 mA) since the rate of temperature rise is extremely large comparing with that induced by using an ordinary heater. On the other hand, the pyroelectric current during the cooling is much smaller because of the slow cooling rate by thermal conduction.

Site-selective investigation of site symmetry and site occupation of iron in Fe-doped lithium niobate crystals

Lithium niobate (LN) crystals, in particular after strong oxidation, are of great relevance for applications in photorefraction and nonlinear optics. Crystals doped with 2 or 4 wt % of iron are studied by extended x-ray absorption fine structure spectroscopy. The Fe atoms are observed to be sixfold coordinated by oxygen atoms and incorporated into the Li site. No evidence for formation of iron oxide clusters is found. In situ x-ray absorption spectroscopy measurements of the thermoelectric oxidation of a LN:Fe 2 wt % crystal and the analyses of the partially site-selective x-ray absorption near edge structure spectra of a LN:Fe 4 wt % crystal provide site-selective structural information. We found that the Fe2+ and Fe3+ sites have the same site symmetries and positions in the LN matrix, i.e., the Fe2+ and Fe3+ atoms are both octahedrally coordinated by six oxygen atoms and both occupy the Li site.

Fixed holograms in iron-doped lithium niobate: simultaneous self-stabilized recording and compensation

We analyze the mechanisms leading to a highly diffractive fixed hologram in photorefractive Fe-doped lithium niobate crystals by simultaneous self-stabilized holographic recording and compensation at moderately high temperatures. We show that a partially compensated running hologram is produced during recording under this condition and discuss the performance of the process in terms of the operating temperature, the degree of oxidation ([Fe(3+)]/[Fe(2+)] ratio) of the sample, and the effect of the absorption grating arising from the spatial modulation of the Fe(2+) concentration produced during photorefractive recording. We experimentally measure the evolution of the uncompensated remaining hologram during recording and the evolution of the diffraction efficiency of the fixed hologram during white-light development and show that the maximum fixed grating modulation to be achieved is roughly limited by Fe-dopant saturation. A reproducible eta approximately 66% efficiency fixed grating was obtained on a sample exhibiting an otherwise maximum fixed eta approximately 3% when using the classical three-step (recording at room temperature--compensating at high temperature--developing at room temperature) process.

Simple optical method for determination of crystal orientation in photorefractive crystals

A simple optical method for determination of crystallographic orientations +/-x, +/-y, and +/-z in photorefractive crystals based on light amplification via backward two-wave mixing (TWM) is presented. We consider a photorefractive Fe-doped lithium niobate crystal (LiNbO(3):Fe) with a standard x-y-z cut as an example to illustrate the principle, manipulation, and experimental results of this method in detail. Also we see that the direction of the light amplification, namely, the direction of the nonreciprocal steady-state energy transfer, strongly depends on the direction of the grating vector with respect to the corresponding crystallographic axes mainly under the diffusion mechanism. Finally, a way of determining the sign of the charge carriers in photorefractive materials is proposed.

Self-stabilized holographic recording in reduced and oxidized lithium niobate crystals

We investigate the behavior of non-stabilized and self-stabilized holographic recording kinetics in reduced and oxidized Fe-doped lithium niobate crystals. We show that self-stabilization modifies the recording process, always leads to 100% diffraction efficiency gratings for widely varying experimental conditions and any degree of oxidation and produces, in general, running holograms. The main features of self-stabilized recording are explained using the conventional dynamic coupled wave theory. The present conclusions are applicable to other photorefractive materials either photovoltaic or not.

Growth and optical properties of near-stoichiometric Fe-doped lithium niobate crystal

The Fe-doped near-stoichiometric lithium niobate (LiNbO 3) crystal has been grown by top seeded solution growth (TSSG) method from congruent melt doped with 0.01 wt.% Fe 2O 3 and 10.9 mol% K 2O. The intrinsic defects and Fe ions location in crystal are investigated on O–H vibration absorption peak position in infrared region and intrinsic absorption edge in UV–VIS region. The photorefractive properties of near-stoichiometric Fe/LiNbO 3 are measured by two-wave coupling method. Compared with congruent Fe/LiNbO 3 with the same Fe doping concentration, near-stoichiometric Fe/LiNbO 3 expresses a greatly improved response speed at the cost of a little decrease of diffraction efficiency.

Holographic phase-shift measurement during development of a fixed grating in lithium niobate crystals.

We report what is believed to be the first direct measurement of the grating phase-shift evolution during white-light illumination for the development of a fixed grating in an Fe-doped lithium niobate crystal. Stabilized holographic recording is shown to be essential for such measurements. Experimental data are in good agreement with theory and allow computation of the relevant material parameters for the sample under analysis. The results are of the utmost relevance for understanding the advantageous behavior of oxidized samples in hologram fixing.

鉄を添加したニオブ酸リチウム結晶の光誘起複屈折変化

For Fe-doped lithium niobate crystal (LN : Fe 0.1 wt%) grown by Czochralski technique, the effects of oxidation and reduction state were investigated on the photoinduced birefringence change by Ar-laser irradiation (488.0 nm). (1) A high accuracy apparatus for the measurement of the birefringence has been developed (Senarmont method). (2) In LN (undoped) the birefringence change is negligibly small (<10-6) regardless of the state of oxidation or reduction. (3) In LN : Fe the change is between 1.5 X 10-3 and 10-4. (4) It is possible to prepare a sample, 'oxidized LN : Fe', in which the change is still kept for long time even after stopping the irradiation.

Photoinduced Birefringence Change of Fe-doped Lithium Niobate Crystal

For Fe-doped lithium niobate crystal (LN:Fe 0.1 wt%) grown by Czochralski technique, the effects of oxidation and reduction state were investigated on the photoinduced birefringence change by Ar-laser irradiation (488.0 nm). (1) A high accuracy apparatus for the measurement of the birefringence change has been developed (Sénarmont method). (2) In LN (undoped) the birefringence change is negligibly small (<10-6) regardless of the state of oxidation or reduction. (3) In LN:Fe the change is between 1.5×10-3 and 10-4. (4) It is possible to prepare a sample in which the change is still kept for long time even after stopping the irradiation, which is `oxidized LN:Fe.'

Study of the equivalent electron drift field characteristics in LiNbO3 by phase holography

An analysis of the diffraction efficiency of photorefractive holograms in ferroelectric crystals is shown to provide a novel technique for determining the nature and magnitude of the equivalent electron drift field. For a Fe-doped lithium niobate crystal (0.05% per mole) we find that the total field consists of an intensity-independent internal field of 8.5 kV/cm and a photogenerated field given by the conversion factor 1.4 ×106 V cm/W.