Study of near-infrared nonvolatile two-center holographic recording in LiNbO3:Fe:Rh crystal

The near-infrared nonvolatile holographic recording has been realized in a doubly doped LiNbO₃:Fe:Rh crystal by the traditional two-center holographic recording scheme, for the first time. The recording performance of this crystal has been investigated by recording with 633 nm red light, 752 nm red light and 799 nm near-infrared light and sensitizing with 405 nm purple light. The experimental results show that, co-doped with Fe and Rh, the near-infrared absorption and the photovoltaic coefficient of shallow trap Fe are enhanced in this LiNbO₃:Fe:Rh crystal, compared with other doubly doped LiNbO₃ crystals such as LiNbO₃:Fe:Mn. It is also found that the sensitizing light intensity affects the near-infrared recording sensitivity in a different way than two-center holographic recording with shorter wavelength, and the origin of experimental results is analyzed.     

Preparation and holographic storage properties of tri-doped Mg:Mn:Fe:LiNbO3 crystals

Doping MgO, MnO and Fe₂O₃ in LiNbO₃ crystals, tri-doped Mg:Mn:Fe:LiNbO₃ single crystals were prepared by the conventional Czochralski method. The UV-vis absorption spectra were measured and the shift mechanism of absorption edge was also investigated in this paper. In Mg:Mn:Fe:LiNbO₃ crystal, Mn and Fe locate at the deep level and the shallow level, respectively. The two-photon holographic storage is realized in Mg:Mn:Fe:LiNbO₃ crystals by using He-Ne laser as the light source and ultraviolet as the gating light. The results indicated that the recording time can be significantly reduced for introducing Mg²⁺ in the Mg:Mn:Fe:LiNbO₃ crystal.     

Reversible photochromic effects in doped single crystals of bismuth germanium (Bi12GeO20) and bismuth silicon oxide (Bi12SiO20)

Absorption of Bi₁₂GeO₂₀ and Bi₁₂SiO₂₀ single crystals doped with Mn and Cr were investigated before and after illumination with visible light. Pronounced photochromic effects were found. The effects are explained in terms of light induced charge transfer Mn⁴⁺→Mn⁵⁺ and Cr³⁺→Cr²⁺.     

The correlation of doping ions to holographic storage properties of Mg:Mn:Fe:LiNbO3 crystals

The congruent Mn(0.1 wt%):Fe(0.03 wt%):LiNbO₃ crystals doped with different concentration of MgO(0,1,3,6 mol%) have been grown by Czochralski method in air atmosphere. Some crystal samples were reduced in Li₂CO₃ powder. The defects and doping ions location in crystals were investigated by UV-Vis. absorption spectrum as well as infrared transmittance spectrum analysis. In two wave coupling experiments we determined the writing time, maximum diffraction efficiency and the erasure time of four crystal samples with He-Ne laser at 633 nm. The results indicated that Mg(3 mol%):Fe:Mn:LiNbO₃ was the most proper holographic recording media material among four crystals in the paper.     

Influence of post-growth treatment on the optical properties of In:Ce:Cu:LiNbO3 crystals

Congruent In (3 mol%):Ce:Cu:LiNbO₃ crystals have been grown by the Czochralski method in air. Some crystal samples were reduced in Li₂CO₃ power, and others were oxidized in Nb₂O₅ power. The structure of crystals was studied by an infrared transmittance spectrum. The resistance ability to optical damage and the photorefractive properties were measured by light-induced scattering experiments and two-beam coupling, respectively. It has been found that the reduction treatment increased the photoconductivity , which resulted in decreased erasure time and diffraction efficiency, but higher light-induced scattering resistance ability. The oxidation treatment caused the inverse affect. Finally, the nonvolatile holographic recording in In:Ce:Cu:LiNbO3 crystals is realized.     

Defect structure and nonvolatile hologram storage properties in Hf:Fe:Mn:LiNbO3 crystals

A series of Hf:Fe:Mn:LiNbO₃ crystals with various levels of HfO₂ doping were grown by Czochralski technique. The infrared spectra and ultraviolet spectra were measured and discussed to investigate their structure and defects. The optical damage resistance was characterized by the transmitted beam pattern distortion method. The nonvolatile two-color holographic recording experimental results showed that the recording speed was faster with the increase of HfO₂ doping concentration and at the same time little loss of nonvolatile diffraction efficiencies could be achieved.     

Comparison of two-color holography among different doped LiNbO3 crystals at low intensities

The effects of material and experimental parameters on the nonvolatile two-color holographic recording space charge field and sensitivity for different doped LiNbO₃:Fe crystals have been studied theoretically based on a two-center model. When the direct electron transfer between the deep-trap centers and the shallow-trap centers was considered, the near-stoichiometric LiNbO₃:Fe is confirmed theoretically to be of bigger space charge field and higher recording sensitivity than the LiNbO₃:Fe:Mn and LiNbO₃:Cu:Ce in the low intensity region. A further improvement of the recording sensitivity can be achieved by doping concentration, thermal reduction treatment of Fe, appropriate gating and recording wavelengths with large photo-excitation cross sections.     

Nonvolatile holographic storage in triply doped LiNbO3: Hf, Fe, Mn crystals

It has been suggested to use LiNbO3:Fe,Mn crystal for solving the problem of information volatility during the read-out process with all-optical facilities,but the minute order response time is far from the requirements for the real-time information processing.We present the nonvolatile holographic storage properties of LiNbO3:Hf,Fe,Mn.The response time is shortened to 5.0 s,and the sensitivity S is enhanced to 0.22 cm/J in this triply doped crystal.The experimental results show that the HfO2 doping threshold is 5.0 mol.%.Thus it seems that we have found a useful tetravalent dopant for LiNbO3:Fe,Mn that can obviously improve the nonvolatile holographic recording sensitivity.     

Structure and optical damage resistance of Zn:Mn:Fe:LiNbO3 crystals

The new non-volatile holographic storage materials, Zn:Mn:Fe:LiNbO₃ crystals, were prepared by Czochralski technique. Their microstructure was measured and analyzed by infrared (IR) transmission spectra. The optical damage resistance of Zn:Mn:Fe:LiNbO₃ crystals was characterized by the transmitted beam pattern distortion method. It increases remarkably when the concentration of ZnO is over a threshold concentration. Its value in Zn(7.0 mol%):Mn:Fe:LiNbO₃ crystal is about three orders of magnitude higher that in Mn:Fe:LiNbO₃ crystal. The photoinduced birefringence change was measured by the Sénarmont's method. It decreased with ZnO concentration increasing. The dependence of the defects on the optical damage resistance was discussed.     

Photoconductivity and light-induced absorption in KNbO3:Fe

Measurements of photoconductivity and light-induced absorption in KNbO₃: Fe are performed at different light intensities and crystal temperatures. The results are interpreted in terms of a two-center charge transport model. Different model parameters may be evaluated from the experimental data. A complete set of parameters is suggested explaining the dependences of photoconductivity and light-induced absorption on light intensity and temperature for the KNbO₃: Fe crystal investigated.     

Formation dynamics of nonvolatile crossed-beam photorefractive gratings in doubly doped LiNbO3 crystals: Theoretical investigation

The formation dynamics of crossed-beam photorefractive gratings formed by the method of two-center holographic recording in doubly doped LiNbO₃ crystals is investigated in this paper based on the theoretical model combining the two-center band transport model with the two-dimensional coupled-wave theory. The numerical simulations are presented for two-center holographic recording crossed-beam photorefractive gratings in LiNbO₃:Fe:Mn crystals. The temporal and spatial evolutions of the refractive index modulation and the diffraction efficiency are shown. The spatial variation of the wave intensity is also presented.     

Nd0.5Pb0.5-xSrx (Mn, Fe)O3的局域结构畸变和磁输运性质的研究

通过X光衍射，穆斯堡尔谱和输运性质的测量研究了掺杂Fe的Nd0.5Pb0.5-xSrxMnO3样品的结构和磁输运性质。x光衍射和穆斯堡尔谱都表明了样品随Sr的增多结构畸变减小。Sr的掺杂同时也降低了室温时的电阻率并使磁电阻增大。由于在样品中Mn3+ 与 Mn4+的比率是一样的，也就意味着具有相同的双交换作用。所以Sr对样品性质的改变可以认为是由于晶格畸变所产生的。由于Jahn-Teller 效应而产生的Mn(Fe)O6八面体的局域结构畸变可以局域电子。由我们的实验可以看到在居里温度Tc以上，除了双交换作用之外，Jahn-Teller 效应对于输运性质也起了重要作用。     

Studies of absorption spectra and the photovoltaic effect in LiNbO3:Mg:Fe crystals

The absorption spectra, photoconductivities and photovoltaic currents of LiNbO₃:Fe crystals with different Mg doping levels and Li/Nb ratios in the oxidized state have been investigated at room temperature. The Fe²⁺ ions in LiNbO₃:Mg:Fe with Mg content above a critical value are more easily oxidized than in crystals with Mg content below a critical value. The photoconductivity of LiNbO₃:Mg:Fe crystals with Mg content above a critical value is one order of magnitude higher than those with Mg content below a critical value, however, the photovoltaic current of the former is one order of magnitude lower than the latter. The differences are postulated to be due to different sites of Fe in these two classes of crystals.     

Preparation and characterization of TiO2 photocatalysts co-doped with iron (III) and lanthanum for the degradation of organic pollutants

Titanium dioxide photocatalysts co-doped with iron (III) and lanthanum were prepared by a facile sol-gel method. The structure of catalysts was characterized by X-ray diffraction (XRD), Raman spectroscopy, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of the samples were evaluated by the degradation of methylene blue in aqueous solutions under visible light (λ > 420 nm) and UV light irradiation. Doping with Fe³⁺ results in a lower anatase to rutile (A-R) phase transformation temperature for TiO₂ particles, while doping with La³⁺ inhibits the A-R phase transformation, and co-doping samples indicate that Fe³⁺ partly counteracts the effect of La³⁺ on the A-R transformation property of TiO₂. Fe-TiO₂ has a long tail extending up the absorption edges to 600 nm, whereas La-TiO₂ results in a red shift of the absorption. However, Fe and La have synergistic effect in the absorption of TiO₂. Compared with Fe³⁺ and La³⁺ singly doped TiO₂, the co-doped simple exhibits excellent visible light and UV light activity and the synergistic effect of Fe³⁺ and La³⁺ is responsible for improving the photocatalytic activity.     

Light-induced acoustic effect in LiNbO3：Fe：Ce crystals

The phenomena of acoustic emission in LiNbO3：Fe：Ce crystals have been observed in the process of light-induced quasi-breakdown. It is found that the ultra-high frequency acoustic signal introduced into the crystal is modulated by the low frequency acoustic waves. Its frequency increases with the increase of the intensity of incident light and its jump period of breakdown is the same as that of the photovoltaic current Ic, the change of light-induced refractive index △n and the diffracted light intensity L. This phenomenon has been analysed in this paper, which is caused by the inverse piezoelectric strain effect of the jump of space charge field during the quasi-breakdown.     

First-principles study of color centers in PbMoO4 crystals

Electronic structures of PbMoO₄ crystals containing Mn ion impurities located at Pb²⁺ sites are studied within the framework of the fully relativistic self-consistent Direc-Slater theory, using a numerically discrete variational (DV-Xα) method. The calculated results show that Mn²⁺ ions have donor energy levers in the forbidden band, which may correspond to the yellowish color absorption band of PbMoO₄ as-grown in air. The new-formed Mn³⁺ ions have acceptor energy levers in the forbidden band, which may relate to the photochromic effect in PbMoO₄ crystal.     

Mn, Cr-co-doped MgAl2O4 phosphors for white LEDs

The Mn-, Cr-doped and Mn, Cr-co-doped MgAl₂O₄ powders have been synthesized via a gel-solid reaction method. Energy transfer from Mn²⁺ to Cr³⁺ has been observed for the first time in the co-doped MgAl₂O₄ phosphors. When excited with blue light with a wavelength of 450 nm at room temperature, both green emission from Mn²⁺ around 520 nm and red emission from Cr³⁺ around 675and 693 nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn²⁺ and Cr³⁺. Therefore, MgAl₂O₄: Mn²⁺, Cr³⁺ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn²⁺ to Cr³⁺ is determined by means of radiation and reabsorption.     

Enhanced photocatalytic activity and stability of nano-scaled TiO2 co-doped with N and Fe

Titanium dioxide (TiO₂) nanoparticles co-doped with N and Fe were prepared via modified sol-gel process. The products were characterized by transmission electron microscopy (TEM), N₂ adsorption, X-ray diffraction (XRD), Raman spectroscopy, UV-vis spectroscopy, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). It is shown that the prepared TiO₂ particles were less than 10 nm with narrow particle size distribution. The addition of MCM-41 caused the formation of Ti-O-Si bond which fixed the TiO₂ on MCM-41 surface, thus restricted the agglomeration and growth of TiO₂ particles. The photocatalytic performance in the degradation of methylene blue showed that N, Fe co-doped TiO₂ exhibited much higher photocatalytic activity than doped sample with nitrogen or Fe³⁺ alone under both UV and visible light. N, Fe co-doping decreased the loss of doping N during the degradation reaction, thus increased the photocatalytic stability. It was also found that the nitridation time had significant influence on the photocatalytic activity of prepared TiO₂ catalysts.     

Charge transport processes in LiNbO3:Fe at high intensity laser pulses

Light-induced refractive index changes in LiNbO₃:Fe crystals are investigated at high light intensities (>10⁹ Wm^–2). Holographic gratings are recorded and erased with frequency-doubled pulses of a Q-switched Nd:YAG laser. We find new intensity dependent contributions to the holographic sensitivity, to the photoconductivity, and to the saturation value of refractive index change. Light-induced absorption changes are also detected. These results indicate that the Fe²⁺/Fe³⁺ charge transport model, well established for low intensities, has to be modified for high intensities by assuming additional centers which trap and supply electrons.     

镱、钠共掺的氟化钙晶体在1050nm的可饱和吸收作用

通过在稳定连续波运转的Yb:YAG 激光器中插入不同掺杂浓度的新型钠、镱共掺的氟化钙晶体的对比性实验,证明了镱、钠共掺的氟化钙晶体在1050nm具有明显的可饱和吸收作用,从而解释了该晶体作为增益介质在该波段总是趋于自调Q运转的原因.Yb³⁺离子是该晶体可饱和吸收作用的主要因素,但是共掺入适当的Na离子可以明显改善晶体的调Q效果.优化共掺镱、钠离子的浓度和比例后的氟化钙晶体能够作为1050nm波段激光器的被动Q开关. 关键词： 镱、钠共掺氟化钙 可饱和吸收体 调Q