Nonvolatile Holographic Storage Research Articles

Magnetism and optical properties of LiNbO3 doped with (Fe,Ni,Ga): First-principles calculations

Based on the first principles of the generalized gradient approximation method, the magnetic and optical properties of Fe-, Ni-doped and (Fe,Ni) co-doped LiNbO3 crystals are studied. The results show that the mono-doped LiNbO3 crystals contain magnetic atoms (Fe,Ni) leading to spin polarization, and (Fe,Ni) co-doped LiNbO3 crystals tend to the ferromagnetic state with △EFM of −143 meV. The total magnetic moment of the (Fe,Ni) co-doped LiNbO3 crystal is 5.97 μB, and the local magnetic moments of Fe and Ni are 2.99 μB and 0.93 μB, respectively. It is interesting that the introduction of the Ga atom makes the ferromagnetic state (△EFM of −285.4 meV) of the (Fe,Ni) co-doped LiNbO3 system more stable than the anti-ferromagnetic state. Although Ga atoms do not contribute to the magnetic moment, the (Fe,Ni,Ga) co-doped LiNbO3 is more conducive to the spin-polarized state. In addition, with the introduction of Fe, Ni, and Ga atoms, the visible region absorption changes significantly. Non-volatile holographic storage can be achieved by (Fe,Ni) co-doped LiNbO3, thus increasing the absorption of the visible light range. In summary, this research provides theoretical guidance for the application of LiNbO3 crystals in spintronic devices.

Nonvolatile photorefractive properties of triply doped stoichiometric lithium tantalate single crystals

Triply doped Mg:Fe:Mn:LiTaO3 crystals with near stoichiometry were grown by the top seeded solution growth technique. Defect structure was investigated by infrared absorption spectra and Curie temperature. Using blue laser as light source, excellent photorefractive properties were obtained. Nonvolatile holographic storage properties were investigated by dual wavelength technique. High fixed diffraction efficiency and sensitivity were acquired. The energy of blue light is high enough to excite holes of deep (Mn) and shallow (Fe) trap centers, which enhance dramatically blue photorefractive properties and nonvolatile holographic storage. Mg2+ ion is no longer damage resistant under blue laser, but enhances photorefractive characteristics. Using 476 nm laser to record grating, 633 nm laser as reading light, the fixed diffraction efficiency of 62.5% was obtained in 2 mol% Mg2+ doped crystal, the sensitivity of nonvolatile holographic storage was improved to 0.335 cm/J.

Nonvolatile photorefractive properties in triply doped stoichiometric Mg:Fe:Mn:LiTaO3 crystals

We have grown triply doped Mg:Fe:Mn:LiTaO3 crystals with near stoichiometry using the top seeded solution growth technique. The defect structure was investigated by infrared absorption spectra and Curie temperature. Using a blue laser as the source, excellent photorefractive properties were obtained. Nonvolatile holographic storage properties were investigated using the dual wavelength technique. We got a very high fixed diffraction efficiency and nonvolatile holographic storage sensitivity. The blue light has more than enough energy to excite holes of deep (Mn) and shallow (Fe) trap centers with the same phase, which enhance dramatically the blue photorefractive properties and the nonvolatile holographic storage. Mg2+ ion is no longer damage resistant at blue laser, but enhances photorefractive characteristics.

Enhancement of nonvolatile holographic storage properties in In:Ce:Mn:LiNbO3 crystal by [Li]/[Nb] ratios

In:Ce:Mn:LiNbO3 crystals were grown by Czochralski technique with various [Li]/[Nb] ratios of 0.946, 1.05, 1.20, 1.38 in the melt. The infrared (IR) spectrum and inductively coupled plasma–atomic emission spectroscopy (ICP–AES) were used to investigate defect structure of these crystals. The nonvolatile holographic storage properties of In:Ce:Mn:LiNbO3 crystals were studied with the two-wave mixing technique. The grating was recorded by a Kr+ laser (476nm) and read by a He–Ne laser (633nm). Experimental results indicated In:Ce:Mn:LiNbO3 crystals with the [Li]/[Nb] ratios of 1.38 has the shortest access times of 310s and the highest photorefractive sensitivity of 3.23mm/J.

IMPROVEMENT OF NONVOLATILE BLUE PHOTOREFRACTIVE PROPERTIES IN In:Ce:Cu:LiNbO3 CRYSTALS

The In : Ce : Cu : LiNbO 3 crystals used in this work were grown by Czchralski method. The key parameters: the exponential gain coefficient Γ, the diffraction efficiency ηS, the response time τw, the dynamic range M/#, the sensitivity S and the maximal refractive index change Δn max were measured with conventional two beam coupling. The optimal values of Γ, ηS, M/#, S and Δn max achieved are 44.7, 68.6%, 6.02, 0.471 cm/J and 5.37 × 10-5, respectively. It was found that the blue light photorefractive properties of Γ, ηS, M/#, S and Δn max increased with increasing In 3+ ions concentration, while the red light photorefractive properties of Γ, ηS, Δn max decreased with increasing In 3+ ions concentration. The nonvolatile holographic storage properties were measured by both the dual wavelength and two color techniques. The maximal diffraction efficiency and fixing diffraction efficiency using the dual wavelength technique were doubled compared to those obtained in using the two color technique. The sensitivity and nonvolatile sensitivity were one order of magnitude higher than those by the two color technique. The photorefractive property enhancement by the dual wavelength technique was investigated.

OH− absorption and nonvolatile holographic storage properties in Mg:Ru:Fe:LiNbO3 crystal as a function of Mg concentration

Mg:Ru:Fe:LiNbO3 crystals with various concentrations of MgO (in mole) and fixed content of RuO2 and Fe2O3 (in mass) are grown with the Czochralski method from the congruent melt. Their infrared transmission spectra are measured and discussed to investigate the defect structure. With the increase of Mg2+ concentration the blue nonvolatile holographic storage capability is enhanced. The nonvolatile holographic storage properties of dual-wavelength recording of Mg(7 mol%):Ru:Fe:LiNbO3 nonvolatile diffraction efficiency, response time, and nonvolatile sensitivity reach 59.8%, 70 s, and 1.04 cm/J, respectively. Comparing Mg(7 mol%):Ru:Fe:LiNbO3 with Ru:Fe:LiNbO3 crystal, the response time is shortened apparently. The nonvolatile diffraction efficiency and sensitivity are raised largely. The mechanism in blue photorefractive nonvolatile holographic storage is discussed.

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.

Recent Advances in the Photorefraction of Doped Lithium Niobate Crystals

The recent advances in the photorefraction of doped lithium niobate crystals are reviewed. Materials have always been the main obstacle for commercial applications of photorefractive holographic storage. Though iron-doped LiNbO3 is the mainstay of holographic data storage efforts, several shortcomings, especially the low response speed, impede it from becoming a commercial recording medium. This paper reviews the photorefractive characteristics of different dopants, especially tetravalent ions, doped and co-doped LiNbO3 crystals, including Hf, Zr and Sn monodoped LiNbO3, Hf and Fe, Zr and Fe doubly doped LiNbO3, Zr, Fe and Mn, Zr, Cu and Ce triply doped LiNbO3, Ru doped LiNbO3, and V and Mo monodoped LiNbO3. Among them, Zr, Fe and Mn triply doped LiNbO3 shows excellent nonvolatile holographic storage properties, and V and Mo monodoped LiNbO3 has fast response and multi-wavelength storage characteristics.

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 (476nm) and He–Ne laser (633nm). 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 476nm 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.

Improved nonvolatile holographic storage sensitivity of near-stoichiometric LiNbO3:Fe:Mn crystals

We investigate the nonvolatile holographic storage characteristics of near-stoichiometric LiNbO3:Fe:Mn crystals with different Li2O contents. Experimental results indicate that the optimal value of Li2O content is about 49.6 mol%. Nonvolatile sensitivity S′ considerably improved to 0.15 cm/J because of the use of near-stoichiometric LiNbO3:Fe:Mn with 49.6 mol% Li2O.

Improved nonvolatile holographic storage properties in Zr:Ru:Fe:LiNbO 3 crystal by blue light recording

Lithium niobate crystal triply doped with zirconium, ruthenium and iron was grown by conventional Czochralski method and its nonvolatile storage characteristics were investigated by means of two-wavelength technology, where blue and red beam were used as recording and readout light, respectively. In oxidized Zr:Ru:Fe:liNbO 3 crystal, short response time of 9.1 s and high recording sensitivity of 0.92 cm/J were demonstrated, together with high fixed diffraction efficiency of 44.9% maintained. These excellent properties were attributed to the elimination of intrinsic defects by ZrO 2 doping and effective excitation of electrons by blue light. Our experimental results indicated this doped LiNbO 3 crystal was an outstanding medium for holographic storage applications.

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO_3:Fe

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO(3):Fe crystal is investigated. Holographic gratings are recorded using three typical recording wavelengths including 488, 514, and 633 nm. Optimal switching from recording to thermal fixing is taken into consideration. The fixed holograms are developed by an original recording setup. Diffraction efficiencies of recording and thermal fixing are measured by a two-wave coupling technique. The theoretical and experimental results are analyzed and compared. With a blue beam, the nonvolatile hologram with maximum fixing efficiency is achieved. This work can obtain high persistent diffraction of the nonvolatile holographic storage in LiNbO(3):Fe crystals.

Investigations on growth and two-wavelength holographic storage properties varied with RuO2 codoping in Fe:LiNbO3 crystals

A series of Ru:Fe:LiNbO3 crystals with fixed concentration of Fe2O3(0.1wt%) and varied RuO2 were grown by the Czochralski technique via the optimum growth condition. After growth, the crystals were cut to wafers and then oxidized to Nb2O5 powder. The X-ray diffraction and UV–vis spectra were measured to analyze the defect structure of the crystals. The blue photorefractive properties of Ru:Fe:LiNbO3 crystals were investigated in a two-wave mixing experiment at the wavelength of 476nm. The results showed that Ru:Fe:LiNbO3 crystals with high concentration of RuO2 are promising candidates for holographic storage. The two-wavelength nonvolatile holographic storage experiment was carried out with sample 6.

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.

Fast responsive nonvolatile holographic storage in LiNbO_3 triply doped with Zr, Fe, and Mn

Iron and manganese doubly doped LiNbO(3) (LN:Fe,Mn) has been suggested for nonvolatile photorefractive recording; however, its response time is still of the order of minutes. Here we present results on LiNbO(3) triply doped with zirconium, iron, and manganese (LN:Zr,Fe,Mn). The codoping with Zr eliminates undesirable intrinsic traps, which strongly enhances the charge transition speed. The response time of LN:Zr,Fe,Mn for nonvolatile holographic storage shortens to only 0.95 s (wavelength of 532 nm and intensity of 400 mW/cm(2)), and the sensitivity reaches 1.31 cm/J. Thus it seems that we have found an excellent recording medium for practical holographic storage devices.

Two-step holographic recording in photorefractive lithium niobate crystals using ultrashort laser pulses

We investigate non-volatile holographic data storage in photorefractive lithium niobate crystals. Infrared picosecond laser pulses are used to write holograms after sensitizing the crystal with blue light from a cw-laser. The dependence of the dynamic range and the photoconductivity on the pulse intensities and the recording wavelength is investigated in detail. The results can be explained by a two-center model if the mean intensity of the laser pulses is considered. We demonstrate that several fixed holograms can be multiplexed by employing the wavelength multiplexing technique.

Influence of dopants on nonvolatile holographic storage in lithium niobate

By studying the ultraviolet-red two-color holographic storage performances of Mg-, Zn- and In-doped lithium niobate crystals, we found that the response time of the two-color recording could be shortened by as much as 3 orders of magnitude compared to that of one-color recording, and the two-color recording sensitivity was improved significantly, which was measured to be 11 cm/J in the crystal doped with Mg of 5 mol.%. Nonvolatile holographic storage was achieved in the crystals with doping concentrations above the damage-resistant threshold value. However, gratings on the deep centers and the shallow centers were out of phase in Mg- or Zn-doped lithium niobate, while those in In-doped lithium niobate were in phase. We consider that different defects induced by different dopants are responsible for the observed results.

Nonvolatile holographic storage of near-stoichiometric LiNbO_3:Cu:Ce with green light

The nonvolatile holographic storage of a near-stoichiometric LiNbO(3):Cu:Ce crystal with green light was investigated. With an increase in composition, improved nonvolatile holographic performance was obtained. The sensitivity S? of the near-stoichiometric LN(49.57):Cu:Ce crystal is 1 order of magnitude larger than the congruent crystal. And according to our research, Ce ions should be the deep centers of the LiNbO(3):Cu:Ce crystal.

Vectorial solution of Kukhtarev equations for doubly doped crystals and optimal choice of recording directions in nonvolatile holographic storage

Vectorial Kukhtarev equations modified for the nonvolatile holographic recording in doubly doped crystals are analyzed, in which the bulk photovoltaic effect and the external electrical field are both considered. On the basis of small modulation approximation, both the analytic solution to the space-charge field with time in the recording phase and in the readout phase are deduced. The analytic solutions can be easily simplified to adapt the one-center model, and they have the same analytic expressions given those when the grating vector is along the optical axis. Based on the vectorial analyses of the band transport model an optimal recording direction is given to maximize the refractive index change in doubly doped LiNbO3:Fe:Mn crystals.