Photorefractive Sensitivity Research Articles

Photoelectric, nonlinear optical, and photorefractive properties of polymer composites based on supramolecular ensembles of Ru(II) and Ga(III) complexes with tetra-15-crown-5-phthalocyanine

Techniques of the production of polymer nanocomposites based on tetrapyrrole compounds with photoelectric, nonlinear optical, and photorefractive properties are considered. Supramolecular ensembles of Ru(II) and Ga(III) complexes with tetra-15-crown-5-phthalocyanine are found to form upon the deposition of thin films from tetrachloroethane solutions that have undergone several cycles of heating followed by slow cooling. The surface formation of supramolecular ensembles is studied with the use of atomic force microscopy. The assemblage is found to give rise to photoelectric and photorefractive sensitivity in the near-infrared range (measurements were carried out at 1064 and 1550 nm). The quantum yield of thermalized electron-hole pairs at 1064 nm in composites of polyvinyl carbazole and assembled Ga(III) complexes is 20 times as high as that in composites based on Ru(II) complexes. The largest two-beam coupling gain coefficient at 1064 nm (Γ = 120 cm−1 at E0 = 120 V/μm) was also observed for the Ga(III)-complex-containing composite. The third-order dielectric molecular susceptibility measured in tetrachloroethane solutions of the complexes weakly depends on the nature of the central atom and its axial neighborhood and falls in a range of γ = 4−5.2 × 10−32 esu.

Photoelectric and photorefractive properties of composites based on poly(vinylcarbazole) and ruthenium(II) tetra-15-crown-5-phthalocyanine with axially coordinated pyrazine molecules

The photoelectric and photorefractive characteristics of composites based on poly(vinylcarbazole) containing crown-substituted ruthenium phthalocyanine with axially coordinated pyrazine molecules, (R4Pc)Ru(pyz)2 (where R4Pc−2 is [4,5,4′,5′,4″,5″,4″′,5″′-tetrakis-(1,4,7,10,13-pentaoxatridecamethylene)phthalocyanine ion], pyz is pyrazine), have been studied. Supramolecular ensembles of these complexes are responsible for optical absorption and photoelectric and photorefractive sensitivity in the near IR region. It has been found that under the action of a 1064-nm laser, the quantum efficiency of formation of mobile charge carriers, estimated from the photocurrent, corresponds to the Onsager equation when the quantum yield of formation of thermalized electron-hole pairs φ0 = 0.35 and the separation distance is 9.8 A, irrespective of the (R4Pc)Ru(pyz)2 content. The kinetic curves of amplification of the information laser beam are bellshaped. This suggests that the photorefractive characteristics are underestimated owing to lowering to zero of the field E0 inside the layer as a result of buildup of space charge in the near-electrode space upon passing the dark current. The two-beam gain coefficient at an (R4Pc)Ru(pyz)2 content of 7 wt % is Γ = 62 cm−1 as estimated from the maximum of the bell-shaped curve.

Photorefractive properties for holographic application of Ce:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

Ce:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 48.6 and 58 mol% Li2O. The Ce, Li and Nb concentrations in the grown Ce:Fe:LiNbO3 crystals were analyzed by the inductively coupled plasma atomic emission spectrometer (ICP-AES). It was found that as the [Li]/[Nb] ratio increases in the melt, the [Li]/[Nb] ratio in the crystal and the distribution coefficients of Ce ions increase also. The photorefractive properties of the Ce:Fe:LiNbO3 crystals were experimentally studied by the two-wave coupling method. The results show that as the [Li]/[Nb] ratio increases, the dynamic range decreases, but the photorefractive sensitivity and the signal-to-noise ratio improve. In a coherent volume 0.192 cm3 of a Ce:Fe:LiNbO3 crystal with [Li]/[Nb] ratio of 1.2, 3800 holograms with 800×600 pixels have been successfully multiplexed in a compact volume holographic data storage system.

Photoelectric, nonlinear optical, and photorefractive properties of composites based on poly(N-vinylcarbazole) and gallium phthalocyaninate

Poly(N-vinylcarbazole) layers containing tetra-5-crown-5-gallium phthalocyaninate (R4Pc)Ga(OH) are shown to possess photoelectric and photorefractive sensitivity at a wavelength of 1064 nm. This effect is associated with the formation of supramolecular ensembles of (R4Pc)Ga(OH) molecules with electronic optical absorption in the near-IR range and nonlinear optical properties. For the composite containing 5 wt % (R4Pc)Ga(OH) supramolecular ensembles, the dependence of the quantum efficiency of mobile-charge photogeneration on electric field E0 is well fit by the Onsager equation expanded to E03 at a quantum yield of electron-hole pairs of φ0 = 0.9 s with an initial separation radius of r0 = 9.8 A susceptibility χ(3) equal to 1.85 × 10−10 esu is measured via the well-known method of electric-field-induced second-harmonic generation. Two-beam-coupling gain coefficient Γ is found to be 80 cm−1 at E0 = 120 V/μm.

Influence of X-rays on optical properties of lithium tantalite crystals

The influence of X-ray radiation on the optical absorptance, photorefractive sensitivity, and the birefringence of pure and doped (with Cu and Rh impurities) LiTaO3 crystals has been investigated. The radiation-induced birefringence δΔnR of LiTaO3:Cu exceeds the same quantity of other previously investigated oxygen-octahedral ferroelectrics by more than an order of magnitude. The increased photorefractive sensitivities of LiTaO3 and LiTaO3:Cu observed after irradiation are related to metastable radiation-induced photovoltaic centers, which is assumed to arise from charge exchange between existing oxygen vacancies.

Influence of Li/Nb ratios on defect structure and photorefractive properties of Zn: In: Fe: LiNbO3 crystals

A series of Zn: In: Fe: LiNbO3 crystals are grown by the Czochralski technique with various ratios of Li/Nb=0.94, 1.05, 1.20 and 1.38 in the melt. The Zn, In, Fe, Nb and Li concentrations in the crystals are analyzed by inductively coupled plasma (ICP) spectrometry. The results indicate that with increasing the [Li]/[Nb] ratio in melt, [Li]/[Nb] ratio increases and goes up continuously in the crystal, the segregation coefficients of both Zn and In ions decrease. The absorption spectra measurement and two-wave coupling experiment are employed to study the effect of [Li]/[Nb] ratio on photorefractive properties of Zn: In: Fe: LiNbO3 crystals. It is found that the [Li]/[Nb] ratio increases, the write time is shortened and the photorefractive sensitivity is improved.

Two-photon luminescence of small polarons in reduced LiNbO3 crystals

The photoluminescence spectra of as-grown and chemically reduced nominally pure LiNbO3 crystals were measured. A powerful laser beam at 514.5 nm was used as pump to generate the small polarons, and a laser weak beam at 632.8 nm was used to probe the luminescence. The dependences of luminescence spectra on the pump beam power and crystal composition have been investigated. The luminescence intensity correlates with the so-called bipolaron absorption band in the chemically reduced crystal. We conclude that this gated luminescence at 800–950 nm is caused by the radiative transitions involving polaron states and it may be directly correlated with the two-color photorefractive sensitivity used for gated holographic recording.

Growth and nonvolatile holographic storage properties of Hf:Ce:Cu:LiNbO3 crystals

The congruent Ce(0.1%):Cu(0.2%):LN crystals doped with different concentrations of 0, 2.0 and 4.0mol% HfO2 were grown by the Czochralski method. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and the infrared (IR) spectrum were measured and are discussed in terms of the spectroscopic characterization. The nonvolatile holographic storage properties of Hf:Ce:Cu:LN crystals were experimentally studied using the two-color holographic recording method. The results indicated that the codoping with Hf eliminated undesirable intrinsic traps, which strongly enhanced the charge transition speed. As the doping concentration of Hf increased, the fixed diffraction efficiency decreased, but the holographic response time and photorefractive sensitivity improved.

Improved sensitivity of nonvolatile holographic storage in triply doped LiNbO_3:Zr,Cu,Ce

We have designed and grown triply doped LiNbO(3):Zr,Cu,Ce crystal and investigated its characteristics of nonvolatile holographic storage. It's observed that the photorefractive sensitivity of LiNbO(3):Zr,Cu,Ce has improved to 0.099 cm/J, which is about one order of magnitude larger than that of congruent LiNbO(3):Cu,Ce. And LiNbO(3):Zr,Cu,Ce also has high suppression to light-induced scattering. Our results indicated that triply doped LiNbO(3):Zr,Cu,Ce is an excellent candidate for nonvolatile holographic data storage.

Ultraviolet photorefraction at 325 nm in doped lithium niobate crystals

We studied the photorefractive effect of lithium niobate (LiNbO3) doped with Mg, Zn, In, Hf, or codoped with Mg and Fe at an ultraviolet (UV) wavelength down to 325 nm. It is found that the UV photorefraction of LiNbO3 doped with Mg, Zn, In, or Hf was enhanced significantly as compared to that of the nominally pure LiNbO3. Our results show that the property of resistance against photorefraction in highly Mg, Zn, In, or Hf doped LiNbO3 is true only in the visible and near-infrared wavelength range. By contrast, these crystals exhibit excellent photorefractive characteristics at UV wavelength of 325 nm, even better than those at 351 nm. For example, the photorefractive two-wave coupling gain coefficient Γ and the photorefractive recording sensitivity at 325 nm were measured to be ∼38 cm−1 and 37.7 cm/J, respectively, in a LiNbO3 crystal doped with 9 mol % Zn. The photorefractive response time of a Mg:LiNbO3 with a 9 mol % Mg was measured to be 73 ms with a total recording intensity of 614 mW/cm2 at 325 nm. In highly Mg, Zn, In, or Hf doped LiNbO3 crystals, diffusion dominates over photovoltaic effect and electrons are the dominant charge carriers in UV photorefraction at 325 nm. The results are also of interest to the study on the defect structure of LiNbO3 near to the absorption edge.

Photorefractive polymer composites based on ruthenium (II) tetra-15-crown-5-phthalocyanate axially coordinating ethylisonicotinate molecules photosensitive in telecommunication range

Polyvinylcarbazole layers containing ruthenium (II) tetra-15-crown-5-phthalocyanate axially coordinating ethylisonicotinate molecules are found to have photoelectric and photorefractive sensitivity at 1550 nm. The effect is determined by the formation of supramolecular ensembles with the electronic optical absorption and photoelectric sensitivity in the near IR range up to 1600 or 1700 nm and nonlinear optical properties. A photorefractive two-beam coupling gain coefficient Γ measured at 1550 nm increases with an increase in the Ru(II) complex content in the polymer layer and equals 25 cm−1 at 6 wt %. The difference between the coupling gain and absorption coefficients (the actual gain coefficient) is Γ − α = 19 cm−1. Additionally introducing 3 wt % C60 fullerene into the layer does not change the characteristics, but the subsequent preirradiation (irradiation of the whole layer with a 633-nm laser beam in a range of the optical absorption of C60 before recording photorefractive curves) results in an increase in the gain coefficient at 1550 nm up to Γ = 48.3 cm−1 and, hence, in Γ − α = 42.3 cm−1. A time constant of the diffraction grating formation is about τ = 0.8 s.

Defect structure and optical fixing holographic storage of Mg:Mn:Fe:LiNbO3 crystals

AbstractMg:Mn:Fe:LiNbO3 crystals were grown by the Czochralski method. The defect structure was analyzed by UV‐vis spectra and IR spectra. The holographic storage of Mg:Mn:Fe:LiNbO3 crystals was measured by the two color fixed method. The results show that with the increase of MgO doping concentration, the writing time becomes shorter, the dynamic range decreases, photorefractive sensitivity increases and fixing diffraction efficiency decreases. When the MgO doping concentration exceeds 4.5 mol%, the fixing diffraction efficiency approaches zero. The effect of doping Mg ions on the holographic storage properties of Mn:Fe:LiNbO3 crystals is discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photorefractive properties of paraelectric potassium lithium tantalate niobate crystal doped with iron

We report the successful growth of paraelectric potassium lithium tantalate niobate (KLTN) single crystal doped with iron. Detailed investigations have been made on the photorefractive properties of the as-grown crystal. The key parameters such as space-charge field, grating response time, photorefractive sensitivity and sign of the dominant charge carrier were obtained by two-wave mixing technique. 1.7mm thick sample exhibits a high diffraction efficiency of 78% at the external field of 3.3kV/cm and a sensitivity of 1.49×10−10E0cm2/J. The two-wave mixing gain coefficient increases linearly with external field, and reaches a large value of 19.4cm−1 at 4kV/cm. Based on experimental results, iron is an effective dopant to KLTN which shows high diffraction efficiency and two-wave mixing gain coefficient.

Spectral dependence of absorption photoinduced in a Bi12TiO20 crystal by 532-nm laser pulses

The spectral dependences of absorption photoinduced in a pure bismuth titanium oxide crystal by 532-nm laser pulses are studied. It is shown that optical absorption in the crystal in the range from 492 to 840 nm increases with increasing exposure. The photoinduced absorption relaxes in the dark for more than 60 hours. A model of photoinduced absorption is proposed which assumes the population of two trap centres with the normal energy distribution law for the concentrations of electrons photoexcited from donors to the conduction band. This model well describes the spectral dependences of photoinduced absorption by using the average ionisation energies of the traps E1 = 1.60 eV and E2 = 2.57 eV. The model is used to estimate the increase in the photorefractive sensitivity of a bismuth titanium oxide crystal in the near IR region, which was earlier observed after exposing the crystal to visible radiation. It is predicted that the speed of response of dynamic holography devices based on BTO crystals exposed to green light can be increased.

Photorefraction of Pb-doped tin hypothiodiphosphate

Photorefractive properties of deliberately lead-doped tin hypothiodiphosphate (Sn2P2S6, SPS) are investigated. In contrast to nominally undoped crystals, the space-charge gratings are formed by charge species of only one type, holes, what ensures a high beam-coupling gain in the steady-state. No optical sensitization is needed for the recording with near infrared light, but a photorefractive sensitivity does not extend beyond a wavelength of 0.9μm. Like for some other SPS crystals, in SPS:Pb the effective trap density was found to be intensity dependent in the red spectral range.

Optical and holographic properties of Fe+Mn co-doped Bi4Ge3O12 crystals

The effect of iron and manganese co-doping on the light-induced, dark- and photoconductivity and holographic properties of Bi4Ge3O12 crystals in comparison with Fe- and Mn-single doping is studied. Fe+Mn co-doped crystals exhibited strong photochromic effect at room temperature, the photochromic effect being fully reversible by thermal annealing. The observed light-induced absorption changes and non-linear dependence of photoconductivity on light intensities indicate deep and shallow photoactive levels contribution to the charge transport mechanism of Fe+Mn co-doped BGO. Holographic gratings are successfully recorded at different wavelengths and a possibility to extend the photorefractive sensitivity into the red spectral range is demonstrated. It is found furthermore that Fe+Mn-doping combination leads to faster response time during holographic recording, especially after a preliminary exposure with ultraviolet light, in comparison with Fe- and Mn-single doping.

Influence of the Various Li/Nb Ratios on Defect Structure and Photorefractive Properties of In: Eu: Fe: LiNbO<sub>3</sub> Crystals

In: Eu: Fe: LiNbO3 crystals were firstly grown from a congruent melt according to the Czochralski method, with the various ratios of Li/Nb =0.885, 0.946, 1.051. The new crystal composition and defect structure were analyzed by the UV-Vis spectroscopy. In the two-wave coupling experiments, writing and erasure time, maximum diffraction efficiency, photorefractive sensitivity and dynamic range were measured and calculated. The dependence of the photorefractive properties on the defect structure of In: Eu: Fe: LiNbO3 Crystals were discussed.

HOLOGRAPHIC STORAGE PERFORMANCE OF NEAR-STOICHIOMETRIC In:Fe:LiNbO3 CRYSTALS

The near-stoichiometric In:Fe:LiNbO 3 crystals (In:Fe:SLN) were grown by the TSSG technology for the first time. The infrared spectra and the photo-scattering resistance ability were measured. The threshold concentration of In 3+ decreased to 2 mol% from 3 mol% of the congruent In:Fe:LiNbO 3 (In:Fe:CLN) crystal. The mechanism of OH - absorption peak shift was studied with the infrared spectra. The diffraction efficiency, writing time and erasure time were studied by using the two-wave coupling method, and the photorefractive sensitivity and dynamic range were calculated. The comprehensive indexes of the photorefractive properties of In:Fe:SLN crystals were superior to those of Fe:LN and In:Fe:CLN crystals. The In:Fe:SLN crystal was used as the storage material and the relative recognition of photorefractive volume holographic images was first realized with the material and good results were obtained.

Comparative Study of Composition Dependences of Photorefractive and Related Effects in LiNbO3 and LiTaO3 Crystals

The results of a comparative study of composition dependences of the light-induced refractive index change, photorefractive sensitivity, polaron luminescence, and intrinsic optical absorption in nominally pure LiNbO 3 and LiTaO 3 crystals are reported. Our investigation demonstrates that the particular light-induced defects traced by the photoluminescence are secondary photorefractive centers itself. The experimental results obtained are well described by light-induced dissociation of bipolarons into small polarons that are unstable at room temperature.

Defect structure and photorefractive properties of In:Eu:Fe:LiNbO 3 Crystals with various Li/Nb ratios

In: Eu: Fe: LiNbO 3 crystals were grown by the Czochralski technique with various ratios of Li/Nb =0.946, 1.051, 1.200, 1.382 in the melt. The crystal composition and defect structure were analyzed by the UV-visible and IR spectroscopy and X-ray diffration. The results showed that increasing the Li/Nb ratio can decrease the In 3+ doping threshold concentration of In: Eu: Fe: LiNbO 3 crystals. In two-wave coupling experiments, the writing and erasure time constants, maximum diffraction efficiency, photorefractive sensitivity, photoconduction, photovoltaic field and space-charge field of the crystals were measured and calculated. The dependence of the photorefractive properties on the defect structure of In: Eu: Fe: LiNbO 3 crystals were discussed.