Structure and nonvolatile holographic recording of Mg:Ce:Cu:LiNbO3 crystals

A series of Mg:Ce:Cu:LiNbO₃ crystals has been grown by Czochralski method. Their infrared transmittance spectra and ultraviolet-visible absorption spectra were measured and discussed to investigate their defect structure. The nonvolatile holographic recording of Mg:Ce:Cu:LiNbO₃ crystals was characterized by the two-photon fixed method. We found that the recording time of Mg:Ce:Cu:LiNbO₃ crystals became shorter and nonvolatile diffraction efficiency decreases with the increase of Mg doping concentration, especially doping with Mg approaches and exceeds the so-called threshold. And the nonvolatility vanishes when the concentration of MgO exceeds 4 mol%. The intrinsic and extrinsic defects were discussed to explain the nonvolatile holographic properties in the Mg:Ce:Cu:LiNbO₃ crystals.     

Nonvolatile hologram storage properties of tri-doped Zn:Ce:Cu:LiNbO3 crystals

Congruent Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was grown by the Czochralski method in air. The occupation mechanism of the Zn²⁺ was discussed by an infrared transmittance spectrum. The nonvolatile holographic recording in Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was measured by two-photon fixed method. Zn(7 mol%):Ce:Cu:LiNbO₃ single crystals present the faster recording time and higher light-induced scattering resistance ability comparing with Ce:Cu:LiNbO₃ single crystals.     

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.     

Information capacity and resolution in three-dimensional imaging

In this paper we report experimental studies of nonvolatilephotorefractive holographic recording in doping In(3mol%), which exceed the so-called threshold, in Ce:Cu:LiNbO₃ crystals. The In:Ce:Cu:LiNbO₃ crystal was grown by the Czochralski method and oxidized. The OH⁻ absorption band of In:Ce:Cu:LiNbO₃ crystal is 3507cm⁻¹. The nonvolatile holographic recording in In:Ce:Cu:LiNbO₃crystal is realized by two-photon fixed method. From the experiment, we found that the recording time of In:Ce:Cu:LiNbO₃ crystal is lower than that of Ce:Cu:LiNbO₃ crystal and there are a little lose of nonvolatile diffraction efficiencies between In:Ce:Cu:LiNbO₃ and Ce:Cu:LiNbO₃ crystals. In this paper, the mechanism of OH⁻ absorption band shifting and two-photon fixed was discussed, too.     

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.     

Grating spacing dependence of nonvolatile holographic recording with arbitrary charge transport lengths

Grating spacing dependence of nonvolatile holographic recording in doubly doped lithium niobate crystals is theoretically investigated allowing arbitrary charge transport lengths. It is shown that the nonvolatile refractive index modulation initially increases with increasing grating spacing, then a saturation behavior arises because of the dominant bulk photovoltaic effect. Although different charge transport length results in different nonvolatile refractive index modulation, the grating spacing dependence of nonvolatile holographic recording obeys almost the same rules for arbitrary charge transport lengths. The experimental results obtained by recording nonvolatile holograms in LiNbO₃:Cu:Ce crystals with different grating spacing are consistent with the theoretical analyses.     

Investigation of nonvolatile holographic storage in LiNbO3:Mn:Ce:Fe crystal

We investigate the persistent holographic recording in triply doped LiNbO₃:Mn:Ce:Fe crystals at different oxidation/reduction states. The experimental results show that there is an optimum oxidation/reduction state, which results in the best dynamic range M/#. Compared with doubly doped LiNbO₃:Ce:Fe, we found that the nonvolatile diffraction efficiency and the best dynamic range M/# obtained in triply doped samples are larger than that obtained in doubly doped samples. The reason for the increase of the crystal about the nonvolatile diffraction efficiency and the dynamic range M/# was also explained.     

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.     

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.     

Growth and photorefractive properties of near-stoichiometric In:Fe:Cu:LiNbO3 crystals

The near-stoichiometric LiNbO₃ crystal co-doped with In₂O₃, Fe₂O₃, and CuO has been grown from a Li-rich melt (Li/Nb = 1.38, atomic ratio) by the Czochralski method in air atmosphere for the first time. The OH⁻ absorption spectra were characterized to investigate the structure defects of the crystals. The appearance of the 3506 cm⁻¹ absorption peak manifests that the composition of the grown crystal is close to the stoichiometric ratio. The photorefractive properties were also measured by the two-wave coupling experiments. The results show that the near-stoichiometric In:Fe:Cu:LiNbO₃ crystal has a larger refractive index change, higher recording sensitivity and larger two-wave coupling gain coefficient than those obtained in the congruent In:Fe:Cu:LiNbO₃ crystal under the same experimental conditions. The material of near-stoichiometric In:Fe:Cu:LiNbO₃ crystal is a promising candidate for blue photorefractive holographic recording.     

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.     

Enhanced nonvolatile holographic properties in Zn,Ru and Fe co-doped LiNbO3 crystals

A series of LiNbO₃ crystals doped with various concentrations of ZnO and fixed concentrations of RuO₂ and Fe₂O₃ have been grown by the Czochralski method from the congruent melts. The type of charge carriers was determined by Kr⁺ laser (476 nm) and He–Ne laser (633 nm). The results revealed that the holes were the dominant charge carriers at blue light irradiation. Dual-wavelength and two-color techniques were employed to investigate the nonvolatile holographic storage properties of Ru:Fe:LiNbO₃ and Zn doped Ru:Fe:LiNbO₃ crystals. The essential parameters of blue nonvolatile holographic storage in Zn:Ru:Fe:LiNbO₃ crystals were enhanced greatly with the increase of Zn concentration. This indicates that the damage resistant dopants Zn²⁺ ions enhance the photorefractive properties at 476 nm wavelength instead of suppressing the photorefraction. The different mechanisms of blue photorefractive and nonvolatile holographic storage properties by dual wavelength recording in Zn:Ru:Fe:LiNbO₃ crystals were discussed.     

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.     

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.     

Defect structure and optical damage resistance of Hf:Fe:LiNbO3 crystals

A series of Hf:Fe:LiNbO₃ crystals were grown by the Czochralski technique with various doping concentrations of HfO₂. Their defect structures were analyzed by the UV-visible absorption spectra and infrared absorption spectra. The optical damage resistance of Hf:Fe:LiNbO₃ crystals was measured by the photo-induced birefringence change and the transmitted light spot distortion method. The results show that the optical damage resistance ability of Hf:Fe:LiNbO₃ crystals enhances remarkably with the HfO₂ concentration increasing when the HfO₂ concentration is lower than its threshold concentration (4 mol%). However, when the HfO₂ concentration exceeds its threshold concentration, the optical damage resistance ability of the crystals returns to decrease. This unusual behavior is explained by using the photovoltaic field produced in the crystals.     

Enhancement of nonvolatile blue photorefractive properties in LiNbO<Subscript>3</Subscript>:In:Fe:Cu crystals

The nonvolatile photorefractive characteristics of LiNbO₃:Fe:Cu and In-doped LiNbO₃:Fe:Cu crystals are investigated. The stronger nonvolatile blue photorefraction observed can be ascribed to its remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers, which is one or two orders of magnitude higher than those obtained in conventional two-color recordings under the same recording conditions. Furthermore, it is interesting that, compared with LiNbO₃:Fe:Cu, the recording properties, such as the saturation refractive index change, nonvolatile sensitivity and response time at 488 nm wavelength are enhanced in LiNbO₃:In:Fe:Cu crystals under the same recording conditions. The so-called damage-resistant dopants such as In³⁺ ions in red photorefraction are not damage resistant at 488 nm wavelength but they enhance the blue photorefraction. PACS  42.40.Ht; 42.40.Lx; 42.70.Ln     

Effect of wavelength of sensitizing light on holographic storage properties in LiNbO3:Fe:Ni crystal

By sensitizing with 514 nm green light, 488 nm blue light and 390 nm ultraviolet light, respectively, recording with 633 nm red light, effect of wavelength of sensitizing light on holographic storage properties in LiNbO₃:Fe:Ni crystal is investigated in detail. It is shown that by shortening the wavelength of sensitizing light gradually, nonvolatile holographic recording properties of oxidized LiNbO₃:Fe:Ni crystal is optimized gradually, 390 nm ultraviolet light is the best as the sensitizing light. Considering the absorption of sensitizing light, to obtain the best performance in two-center holographic recording we must choose a sensitizing wavelength that is long enough to prevent unwanted absorptions (band-to-band, etc.) and short enough to result in efficient sensitization from the deep traps. So in practice a trade-off is always needed. Explanation is presented theoretically.     

Growth and optical properties of Hf,Ce co-doped lithium niobate crystals

In this paper, a series of Hf,Ce co-doped lithium niobate crystals with various HfO₂ concentrations were grown by Czochralski method. The ultraviolet-visible absorption spectra and the infrared transmittance spectra were measured to study defect structure of the crystals. The optical damage resistance was measured by the transmitted facula distortion method. The results showed that the optical damage resistance of Hf:Ce:LiNbO₃ was greatly improved when the Hf-doping concentration was above 4 mol%.     

Effect of [Li]/[Nb] ratios on the photorefraction and scattering properties in In: Fe: Cu: LiNbO3 crystals at 488 nm wavelength

The high sensitivity, fast response and the high quality reconstructions are observed in various [Li]/[Nb] ratios In:Fe:Cu:LiNbO₃ crystals at 488 nm wavelength based on the two-beam coupling experiment. The strong blue photorefraction is contributed by the two-center effect and the remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers. The blue photorefraction is enhanced significantly with the increasing of [Li]/[Nb] ratios under the same experimental conditions. The sensitivity S" is reduced to 0.46 J/cm, simultaneously the response time is as fast as 4.4 s and the erase phenomenon is not obvious in In:Fe:Cu:LiNbO₃ crystals which [Li]/[Nb] ratio is 0.986 in crystal. Increasing [Li]/[Nb] ratios improve the damage-resistant ability of the crystals, but lead to a more serious beam fanning. Experimental results definitely show that the near-stoichiometric In: Fe: Cu: LiNbO₃ crystal becomes a promising candidate for blue photorefractive holographic recording.