Photorefractive Sensitivity Research Articles

Growth and photorefractive properties of Mg:Ce:Fe:LiNbO3 crystals

The congruent Ce(0.1mol%):Fe(0.03wt%):LiNbO3 crystals doped with different concentrations of 2.0mol%, 4.0mol% and 6.0mol% MgO were grown by the Czochralski method. Content of impurities was analyzed by means of Ultraviolet (UV)–Visible (Vis) spectroscopy. The photorefractive properties of the Mg:Ce:Fe:LiNbO3 crystals were experimentally studied by using the two-wave coupling method. The results of the two-wave coupling experiments showed that as the doping concentration of Mg increased, the diffraction efficiency decreased, but the holographic response time and photorefractive sensitivity increased. The photorefractive sensitivity and dynamic range improve obviously when used the 532nm laser as light source instead of 633nm laser.

Spectral sensitivity of nominally undoped photorefractive Sn2P2S6

Nominally undoped tin hypothiodiphosphate is shown to possess photorefractive sensitivity from its band edge in the visible region of the spectrum up to 1.3 μm in the near infrared. The gain factor decreases with the increasing wavelength while the characteristic rate of photorefractive build-up roughly follows the spectral dependence of photoconductivity.

The correlation between defects and photorefractive properties in Fe:In:LiNbO 3 crystals

A series of Fe:In:LiNbO 3 crystals with a varying content of In doping were grown by Czochraski technique. The absorption and infrared spectra measurement and two-wave coupling experiment are employed to study the effect of In doping on photorefractive properties of Fe:LiNbO 3 crystals. The results indicate that the photorefractive properties can be greatly enhanced by proper level of In doping in Fe:LiNbO 3. Among the four samples of Fe:In:LiNbO 3 crystals, the Fe:In (2 mol%):LiNbO 3 crystal exhibits the highest photorefractive sensitivity and the largest dynamic range (1.79 cm J −1 and 2.10, respectively) and can resist stronger laser intensity without distortion. In terms of ions’ site occupation model, the change of photorefractive properties due to different content of In ions existing in LiNbO 3 is discussed systematically.

Photochromic effect in LiNbO_3: Fe :Co

In this paper, Co(2)O(3) was codoped in LiNbO(3): Fe crystals. It was found that Co codoping can give rise to the strong photochromic effect in LiNbO(3): Fe. Based on the UV-VIS-NIR absorption spectra, photorefractive sensitivities, and EPR results for both virgin and sensitized states of the crystal, the photochromic mechanism in this material was suggested as follows. During sensitization, electrons transfer from O(2-) to Fe(3+) directly while holes are thermally excited from O- into the valence band and then partly trapped by Co(2+), which leads to the darkening of the crystal. In bleaching process, the electrons are excited from Fe(2+) to the conduction band by green light and recombine with the holes on the Co(2+) level. Sensitizing/bleaching experiments were also carried out in LiNbO(3): Fe: Co crystals. Fast sensitization found for this material is beneficial to two-color recording.

Growth and optical properties of Mg:Ce:Fe:LiNbO 3 crystals

The congruent Ce(0.1 mol%):Fe(0.03 wt.%):LiNbO 3 crystals doped with different concentrations of 2.0 mol%, 4.0 mol% and 6.0 mol% MgO were grown by the Czochralski method. The Curie temperature and absorption spectra were measured to analyze their structure. The photorefractive properties of the Mg:Ce:Fe:LiNbO 3 crystals were experimentally studied by using two-beam coupling. The results of the two-wave coupling experiments showed that as the doping concentration of Mg increased, the diffraction efficiency decreased, but the response time and photorefractive sensitivity increased. Among all samples, the dynamic range of the Mg(4 mol%):Ce:Fe:LiNbO 3 crystal is the largest (3.18).

The photorefractive sensitivity and the effective electron transport length in two-centre holographic recording

In this paper the photorefractive sensitivity defined for single-centre holographic recording is modified to adapt two-centre holographic recording. Based on the time analytic solution of Kukhtarev equations for doubly doped crystals, the analytical expression of photorefractive sensitivity is given. For comparison with single-centre holographic recording and summing the electron competition effects between the deeper and shallower traps, an effective electron transport length is proposed, which varies with the intensity ratios of recording light to sensitive light. According to analyses in this paper, the lower photorefractive sensitivity in two-centre holographic recording is mainly due to the lower concentration of unionized dopants in the shallower centre and the lower effective electron transport length.

Enhanced photorefractive properties of LiNbO3:Fe crystals by HfO2 codoping

Photorefractive properties of congruent lithium niobate crystals codoped with HfO2 and Fe2O3 were investigated and it was found that Fe ions are still located at Li sites as photorefractive centers when the doping concentration of HfO2 goes above the threshold value. As a result, their photorefractive response speed and sensitivity are significantly enhanced. Meanwhile, the high saturation diffraction efficiency is still maintained. Experimental results definitely show that Hf is now the most effective doping element for LiNbO3:Fe crystal to improve its photorefractive properties.

High photorefractive sensitivity and fast response time of near‐stoichiometric and MgO doped LiNbO3:Fe crystals

AbstractCongruent LiNbO3:Fe and LiNbO3:Mg,Fe crystals were grown by Czochralski method, and vapor transport equilibration technique was employed to improve the [Li]/[Nb] ratios of these crystals. The influence of stoichiometry and MgO dopant on the photorefractive sensitivity and response time of LiNbO3:Fe crystals was investigated. Both stoichiometry and MgO dopant can effectively reduce the amount of intrinsic defects, but MgO can also decrease the concentration of Fe2+ ions in Li‐sites. Near‐stoichiometric and MgO doped LiNbO3:Fe crystal has high photorefractive sensitivity and fast response time. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of post‐treatment conditions on photoreactive effect of Zn:Fe:LiNbO3 crystals

AbstractThree kinds of different Zn:Fe:LiNbO3 wafers are prepared by proper reduction or oxidation post‐treatment processes. The optical‐damage‐resistance abilities and photorefractive properties are studied using the optical compensator technique and two‐wave coupling measurement, respectively. The holographic storage properties, that is, diffraction efficiency, writing time, erasure time, photorefractive sensitivity, and dynamic range, are also measured or calculated. The analyses indicate that the sample disposed by reduction is the most proper media for holographic storage application. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 986–988, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21542

Nematic-like mesophase photoconductive polymer for photorefractive applications

A new nematic-like mesophase photoconductive polymer PPT–TPA consisting of wholly aromatic rigid backbone of poly( p-phenyleneterephthalate), PPT and pendent hole-transporting triphenylamine (TPA) groups attached to the ends of oxydecyl spacers has been synthesized. The photorefractive composite contains the photoconductor PPT–TPA, the chromophore diethylaminodicyanostyrene (DDCST), and the photosensitizer C 60. Although no plasticizer was added, the glass transition temperature T g of the composite is 15 °C, which characterizes it as a low- T g photorefractive material. We investigate the correlation between the mesophase structure and its optical/physical properties by X-ray diffraction, photoconductive and photorefractive experiments. The new composite and its properties are compared to PPT–CZ composites with only a different charge transporting agent (carbazole, CZ) but a much more ordered mesophase structure, which were studied previously and have shown very good photorefractive properties. Despite of a lower photoconductivity of the new photorefractive composite PPT–TPA ( n=10):DDCST:C 60 this material shows a higher photorefractive sensitivity S n2 of 2±0.2 cm 2/kJ at E=50 V/μm than the previously synthesized composite PPT–CZ ( n=10):DDCST:C 60.

Growth and photorefractive properties of In:Fe:LiNbO 3 crystals with various [Li]/[Nb] ratios

In:Fe:LiNbO 3 crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 48.6 and 58 mol% Li 2O. The crystal composition and defect structure were analyzed by the UV–Vis and IR spectroscopy. The photorefractive properties of the In:Fe:LiNbO 3 crystals were experimentally studied by using the two-wave coupling method. The results of the two-wave coupling experiments showed that as the [Li]/[Nb] ratio increased, the diffraction efficiency decreased, but the holographic response time and photorefractive sensitivity increased. Among all samples, the dynamic range of the In:Fe:LiNbO 3 crystal with [Li]/[Nb] ratio of 1.2 was the largest (up to 10.22).

Photorefractive performance of polymer composite sensitized by CdSe nanoparticles passivated by 1-hexadecylamine

The performance of a photorefractive polymer composite sensitized by 1-hexadecylamine capped CdSe nanoparticles is reported. The polymer composite also comprises the charge transporting matrix poly(N-vinylcarbazole) and the electro-optic chromophore 1-(2-ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo) benzene. At an applied field of 70 V μ m−1 two beam coupling gain of 13.2 cm−1 was observed, confirming the photorefractive nature of the induced grating. At the same field, a holographic contrast of 9.12×10−4±6×10−6, a photorefractive sensitivity of 5.1×10−4 ±0.2×10−4 cm3 J−1 and a space-charge field rise time of 13±1 s were obtained.

Growth and photorefractive properties of near-stoichiometric Zn:Fe:LiNbO 3 crystal

The near-stoichiometric lithium niobate (LN) crystal co-doped with Fe 2O 3 and ZnO has been grown by K-flux method and is compared with the same doping congruent Fe:Zn:LN on photorefractive properties for the first time. In comparison with congruent Fe:Zn:LN, the absorption edge of near-stoichiometric Fe:Zn:LN shifts to the violet, and its lattice constants get smaller. Stoichiometric Fe:Zn:LN exhibits higher photorefractive sensitivity than that of congruent one. The mechanism of photorefractive effect improvement for near-stoichiometric Fe:Zn:LN crystal is investigated combined with doping ions location analysis.

Highly efficient photorefractive composites based on layered photoconductive polymers

High-performance photorefractive materials based on the layered photoconductive polymers PPT-CZ [rigid backbone of poly(p-phenyleneterephthalate) with pendent carbazole groups] are studied. The composites are composed of PPT-CZ and are doped with the sensitizer C60 and nonlinear optical chromophores. Despite the absence of a plasticizer and the lower concentration of the carbazole photoconductive moieties as compared with poly(N-vinyl carbazole), these materials show high photorefractive sensitivity, i.e., Sn2 of 70± 7 cm2/kJ at E0=100 V/μm for PPT-CZ(n=12):diethylaminodicyanostyrene:C60. The low glass transition temperature (<0°C) leads to a high rotational mobility of the chromophores that results in large refractive-index changes. For all the composites that were investigated, the two-wave mixing gain Γ exceeds 100 cm-1 at an applied field of 50 V/μm.

The role of Zn ions in holographic storage in Zn:Fe:LiNbO 3

The iron-doped lithium niobate (Fe:LiNbO 3) and zinc-iron-doubly-doped lithium niobate (Zn:Fe:LiNbO 3) crystals have been grown by Czochralski method in air atmosphere. Based on results of X-ray diffraction analysis, it is indicated that doubly doped Zn:Fe:LiNbO 3 changes lattice more slightly than singly doped Fe:LiNbO 3. The holographic storage properties of two crystals have been measured by two-wave coupling with Ar + laser at 488 nm. The experimental results show that Zn:Fe:LiNbO 3 exhibits a large improvement in photorefractive sensitivity and writing time, especially in noise suppressing due to Zn ions existence in LiNbO 3 crystal.

Improvements of sensitivity and refractive-index changes in photorefractive iron-doped lithium niobate crystals by application of extremely large external electric fields

Applying external electric fields of as much as 65 kV/mm reveals significant enhancement of refractive-index changes Δns and of photorefractive sensitivity S. Samples with different iron concentrations and oxidation–reduction states were investigated. Refractive-index changes of up to Δns=11.5×10-4 and sensitivities as high as S=40 cm/J were measured (ordinary light polarization; light wavelength, λ=488 nm). The refractive-index changes are space-charge limited, but the photorefractive sensitivity S exhibits for large externally applied electric fields a linear increase up to the highest field values used. No nonlinear enhancement of the charge transport was observed. However, the excellent high-field material performance and the record values for Δns and S obtained improve the applicability of LiNbO3 crystals.

An increase of photorefractive sensitivity in In:LiNbO3 crystal

The high sensitivity of photorefractive effect in indium doped lithium niobate was observed based on the two-wave mixing experiment. The sensitivity S was measured up to 2 cm 3 /J, even larger than the iron doped samples under the same configuration. The slightly indium doping seems to be an effective method to improve the photorefractive sensitivity of lithium niobate. 2002 Elsevier Science B.V. All rights reserved.

Magnetic-field-induced changes in the photorefractive sensitivity of lithium niobate

The effect of a magnetic field on the average photorefractive sensitivity of an undoped LiNbO3 crystal is studied by phase-mismatched second-harmonic generation. The experimental data obtained show the photorefractive sensitivity to reverse sign as the external magnetic field exceeds B1=−0.38±0.04 T. The magnetic field is oriented perpendicular to the crystal optical axis and to the plane of laser radiation polarization. The variation of the photorefractive sensitivity is associated with paramagnetic iron centers, whose photoionization probability depends on the direction of their magnetic moment relative to the optical axis.

Broadband absorption changes and sensitization of near-infrared photorefractivity induced by ultraviolet light in LiNbO3:Mg

We observe ultraviolet-light-induced absorption changes in spectral regions from the near-ultraviolet to the near-infrared in as-grown LiNbO3:Mg. Such a broad light-induced absorption band is unstable at room temperature and decays nonmonoexponentially in the dark with a time constant on the order of tens of seconds. We attribute the appearance of the broadband light-induced absorption to the creation of the intermediate state O− in which holes excited from deep defect centers are trapped at O2− sites near cation vacancies. It is also found that near-infrared photorefractive sensitivities increase by several orders of magnitude by pre-exposing the crystal to incoherent ultraviolet light. These properties may be useful for an application of LiNbO3:Mg to nonvolatile two-color holography.

Temperature Dependence of Two Beam Coupling, Light-induced Erasure Decay and Dark Decay in As-Grown and Reduced BaTiO3:Rh

Two beam coupling, light-induced erasure decay and dark decay are investigated in as-grown and reduced BaTiO3:Rh as a function of temperature. The photorefractive sensitivity increases with temperature, mainly due to the increase of the speed. As deduced from the dark decay measurement, the shallow trap effect decreases while the dark conductivity increases with increasing temperature. These two factors determine how the two beam coupling gain depends on intensity and the intensity dependence power coefficient with response time varies as temperature increases. For the as-grown sample, the photorefractive properties are controlled by the competition between deep and shallow levels when the temperature is below 60°C. Above 60°C, large dark conductivity plays a dominant role. For the sample reduced at an atmosphere of 10-10 atm oxygen partial pressure, the photorefractive properties are controlled by the competition between deep and shallow traps from room temperature to 100°C. For the sample reduced at an atmosphere of 10-14 atm oxygen partial pressure, the photorefractive properties are mainly dependent on the dark conductivity though it has a high photoconductivity.