Saturation Space-charge Field Research Articles

Highly reduced iron-doped lithium niobate for optoelectronic tweezers

We investigate the applicability of highly reduced lithium niobate samples doped with iron for the use as optoelectronic tweezers. Increasing the reduction degree of Fe-doped lithium niobate is well known to increase the photoconductivity and reduce the writing time of internal space-charge fields. Based on our measurements of the photorefractive properties, we determine the optimal conditions for dielectrophoretic trapping and present the application of Fe-doped lithium niobate as optoelectronic tweezers. For higher reduction degrees, an unexpected decrease in the photovoltaic current density and the saturation space-charge field is reported.

Fixed photorefractive holograms with maximum index-of-refraction modulation in LiNbO3:Fe

We report the recording of a fixed good quality transmission hologram in LiNbO3:Fe with maximum index-of-refraction modulation using the simultaneous recording/compensation process at 120 °C in a specially designed setup with λ=514.5 nm. This process was shown to be reproducible and in good agreement with an already reported theoretical model. The analysis of this recording process showed that material saturation was reached so that the maximum possible fixed index-of-refraction modulation was achieved. From the comparison of theoretical and experimental recording/compensation process data some material parameters (dielectric relaxation time τMe≈15 min, saturation space-charge field Eq=18.8 kV/cm, and photovoltaic-to-saturation field ratio Eph/Eq=0.80) were determined. The diffraction efficiency of this grating was measured using a λ=633 nm probe laser beam in an independent setup and its actual value computed, taking into account the angular divergence of the probe beam. The good grating performance as an optical Bragg filter was experimentally characterized by independently measuring its angular (approximately 1.2 mrad) and spectral (approximately 0.1 nm) selectivities, both at the probe beam wavelength of 633 nm.

Photovoltaic effect and photoconductivity in Sc-doped near-stoichiometric LiNbO 3 crystals

The photorefractive damage (optical damage) process in Sc-doped near-stoichiometric LiNbO 3 (Sc:SLN) crystals was investigated by measuring the photocurrents in these crystals for several Sc concentrations (up to approximately 1 mol%). The photovoltaic current density and photoconductivity were then estimated using the measured photocurrents. The saturated space-charge field, which is the ratio between the photovoltaic current density and photoconductivity and which causes the photorefractive damage, was then estimated. The photovoltaic current density decreased with an increasing Sc concentration while the photoconductivity increased with the concentration. This combination of decreased photovoltaic current density and increased photoconductivity means that the saturated space-charge field decreases with an increasing Sc concentration. This decrease in the saturated space-charge field apparently reduced the photorefractive damage in the Sc:SLN crystals. These behaviors are similar to those of Mg-doped LN (Mg:LN) crystals. The saturated space-charge field of the Sc:SLN crystals with no photorefractive damage was less than 100 V/cm (10 4 V/m), as it is for Mg:LN crystals. This means that the Sc concentration needed to obtain a less than 100-V/cm saturated space-charge field in SLN crystals is less than the corresponding value for Mg doping.

Optimizations for the uniformity of the non-volatile volume grating in doubly doped LiNbO3 crystals

The formation of the non-uniformity of the non-volatile volume grating in doubly doped LiNbO3 crystals is studied in detail. We find that the non-uniformity of the grating is mainly caused by strong ultraviolet light absorption, and the average saturation space-charge field is small and the diffraction efficiency is low as a result of the non-uniformity of the grating. In order to optimize the uniformity of the grating, we propose the recording scheme by using two sensitizing beams simultaneously from the two opposite sides of the crystals. Theoretical simulations and experimental verifications are performed. Results show that the well uniformed grating with high diffraction efficiency can be obtained by using this optimization scheme.

Reduced space-charge fields in near-stoichiometric LiTaO 3 for blue, violet, and near-ultraviolet light beams

The wavelength dependence of the space-charge field as induced in near-stoichiometric LiTaO3 crystals is investigated by measuring the bulk photogalvanic effect and the photoconductivity. LiTaO3 crystals of composition nearer to stoichiometry exhibit a pronounced reduction of the saturated space-charge field, which results from a larger increase in the photoconductivity relative to the photogalvanic effect. Especially, at near-UV wavelengths, the saturated space-charge field of the order of 0.1 kV/cm reaches a minimum that is two orders of magnitude lower than typical values observed in undoped near-stoichiometric LiNbO3.

Temporal evolution of beam fanning in LiNbO_3:Fe,In crystals

We investigated the temporal evolution of light-induced scattering in LiNbO(3):Fe, In crystals with different doping concentrations. A special behavior of the beam fanning was found when the intensity of the incident light was relatively weak. In this case the beam fanning became stronger at the beginning of the illumination and then was greatly reduced, which was observed only at strong incident light intensities. This phenomenon was analyzed on the basis of the saturation space-charge field. The intensity threshold effect and the concentration threshold effect were successfully explained.

Photorefraction in LiNbO3 as a function of [Li]/[Nb] and MgO concentrations

Photorefractive damage, photoconductivities, and photogalvanic currents of LiNbO3 single crystals have been investigated as a function of [Li]/[Nb] and MgO concentrations. Near stoichiometric LiNbO3 single crystal, having large [Li]/[Nb] ratios of 0.993, shows lower photorefractive damage resistance than congruent LiNbO3. However, crystals doped with a small amount of MgO (>0.78 mol %) exhibit no measurable photorefractive damage at 532 nm to intensities of as much as 2 MW/cm2. This remarkable decrease of photorefraction that was found can be attributed to the decrease of saturated space charge field results from a combination of the increased photoconductivity and the decreased photogalvanic current of the crystal.

Effect of the oxidation state of LiNbO_3:Fe on the diffraction efficiency of multiple holograms

We show that the oxidation state of Fe in LiNbO(3) has two competing effects on the diffraction efficiency of multiple holograms in 90 degrees -geometry holographic storage. For crystals with moderate absorption, the saturation space-charge field is larger after high-temperature reduction treatment. However, reduction also increases absorption, which reduces the overall diffraction efficiency. We develop a theoretical model that predicts achievable diffraction efficiency as a function of oxidation state, doping level, photovoltaic field, crystal length, and region of beam overlap. We compare this model with experimental results for achievable diffraction efficiency and erasure-time constant.

X-ray produced charge deposition and dose in dielectrics near interfaces including space-charge field and conductivity effects

When a dielectric material is exposed to x- or γ-radiation, charge buildup occurs in regions of the dielectric near interfaces with dissimilar media. The charge buildup results from the divergence of photo-Compton current near the interface. The charge density acts as a source for an electric field. If one assumes a transient dielectric conductivity equal to the bulk radiation-induced conductivity of the dielectric, one can predict a sufficiently large x-ray generated space-charge field at high exposures to either limit the photo-Compton current — hence perturb the depth-dose profile in the dielectric — or initiate dielectric breakdown. There also occurs at the interface, however, a gradient in dose which should produce a gradient in transient conductivity. This conductivity gradient, it is shown, profoundly modifies the charge buildup and space-charge field in the dielectric. Calculations of current density, charge density, and space-charge field are presented for a planar gold/polyethylene interface irradiated by 30, 100, 200, and 1000 keV x-rays. It is found that for the lower x-ray energies the saturation space-charge field is too low to significantly perturb the depth-dose profile or initiate breakdown, but for the 1000 keV x-rays the saturation space-charge field may be sufficiently high to significantly perturb the depth-dose profile. In all cases, the charge density profile is strongly perturbed by the conduction currents, even for relatively low exposures.