OH⁻ absorption and holographic storage properties of Sc(0, 1, 2, 3):Ru:Fe:LiNbO₃ crystals

Abstract

A series of Sc:Ru:Fe:LiNbO₃ crystals with various levels of Sc₂O₃(0, 1, 2, and 3 mol%) doping were grown from congruent melts in air by using the Czochralski technique. The defect structures and photorefractive properties of the Sc:Ru:Fe:LiNbO₃ crystals were investigated by acquiring infrared spectra of the crystals and performing two-wavelength nonvolatile experiments, respectively. Our results showed the holographic storage properties of Ru:Fe:LiNbO₃ crystals to be enhanced by doping them with a high concentration of Sc₂O₃, and indicated Sc:Ru:Fe:LiNbO₃ crystals to constitute a promising medium for holographic storage.

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1. Introduction

With excellent nonlinear optical, electro-optical, acoustic-optical, ferroelectric and photorefractive properties, LiNbO₃(LN) crystals have become very promising materials.^1,2 In the past few decades, due to its high storage capacity, fast parallel processing and content addressability, the LiNbO₃ crystal has garnered great interest,³ and has been successfully applied in integrated electro-optical devices and holographic memory devices.⁴ However, the volatility of stored information is a major obstacle in the practical application of LiNbO₃ crystals. In order to solve this problem, photorefractive ions such as those of Fe,⁵ Ce,⁶ Cu,⁷ Ru,⁸ Mn, Ti,⁹ Er^10–14 are introduced into the crystal to enhance the photorefractive effect, and it was found that a nonvolatile readout can be realized in some doubly doped LiNbO₃.^15,16

Based on this development, a two-centered recording model was proposed in the doubly doped Fe:Mn:LiNbO₃ crystal by Buse et al. in 1998,¹⁷ and holographic recording and nondestructive readout were implemented in Fe:Mn:LiNbO₃. The key point of the technique is that the doped ions can provide both relatively shallow and deep centers in the crystals. Many new doubly doped crystals, such as Cu:Ce:LiNbO₃, Mn:Ce:LiNbO₃ and Fe:Cu:LiNbO₃ crystals, have since been reported to have such properties.^18,19 The Ru:Fe:LiNbO₃ crystal was recently found to be another excellent medium for holographic storage. Fe and Ru are both transition mental ions and are located at similar positions of the periodic table; and in the Ru:Fe:LiNbO₃ crystal, Ru is used as a deep center and Fe as a shallow center.²⁰ Generally speaking, the higher the concentration of the doping ion, the stronger the photorefractive effect; therefore, a lower response time, higher sensitivity, higher diffraction efficiency and other excellent parameters can be achieved by using a higher doping concentration. But it is not easy to grow large and high-quality Ru:Fe:LiNbO₃ crystals, because of the low solubility of Fe and Ru in the LiNbO₃ crystal. Comparatively speaking, doping ions resistant to optical damage is a more feasible method: it not only can eliminate the intrinsic defects caused by the Li composition deficiency but also can improve optical resistance ability of the crystal.

The choice of doping ions is critical for the characteristics and applications of LN crystals. In this work, the Sc³⁺ (ref. 21–24) ion was chosen as the doping ion. The ionic radius of Sc³⁺ is similar to that of Li⁺ but larger than that of Nb⁵⁺, and the Sc³⁺-doped LN has been used in integrated optics such as titanium-diffused optical LN waveguides doped with Sc³⁺. The LN crystal is susceptible to photorefractive damage under laser irradiation. To suppress photorefractive damage, the LN crystal must be doped with more than 4.6 mol% Mg,²⁵ more than 6.2 mol% Zn,²⁶ more than 3 mol% In,²⁷ or more than 2 mol% Sc.²⁸ We chose Sc³⁺ ion as the doping ion since it was effective at a concentration lower than were any of the other ions.

Based on the above considerations, a series of Sc:Ru:Fe:LiNbO₃ crystals with various concentrations of Sc₂O₃ were grown by using the Czochralski method. The defect structures of Sc:Ru:Fe:LiNbO₃ crystals were investigated by acquiring their infrared spectra, and the holographic storage properties of Sc:Ru:Fe:LiNbO₃ were investigated by taking two-wavelength nonvolatile measurements.

2. Crystal growth

In our experiment, the Sc(0, 1, 2, and 3 mol%):Ru:Fe:LiNbO₃ crystals with 0.3 mol% Fe₂O₃ and 0.2 mol% RuO₂ were grown from congruent melts in air by using the Czochralski technique. The raw materials used in crystal growth were Nb₂O₅, LiCO₃, Sc₂O₃, RuO₂ and Fe₂O₃. The purity of raw material is critical for optical quality, so the purity levels of all raw materials were at least 99.99%. All raw materials including Nb₂O₅, LiCO₃, Sc₂O₃, RuO₂ and Fe₂O₃ were mixed for 24 hours in order to obtain uniform materials. Then the materials were placed into a platinum crucible heated up to 750 °C for 2 hours to remove CO₂ and then heated up further to 1150 °C for 2 hours to form a polycrystalline material as a result of a solid-state reaction. The growth condition was selected as follows: the temperature gradient was 2.5 °C mm⁻¹, the polling rate and rotation rate were controlled to be in the range 0.8 to 1.5 mm h⁻¹ and 17–26 rpm, respectively. After growth, the crystals were cooled to room temperature at a rate of 65 °C h⁻¹. In order to prevent spontaneous polarization of the crystals, all of the crystals needed to be polarized artificially in a medium frequency furnace for 8 h, in which the temperature was 1100 °C, the temperature gradient was almost equal to zero, and the current density was 5 mA cm⁻². Several 8 mm × 10 mm × 2 mm (x × y × z) wafers were obtained by cutting from the middle of the Sc:Ru:Fe:LiNbO₃ crystals along the y-axis. The samples with different Sc³⁺ ion concentrations were labeled as ScRuFe-0, 1, 2 and 3. A photograph of one of the ScRuFe-3 crystals is shown in Fig. 1.

Fig. 1 A sample ScRuFe-3 crystal.

3. Measurements

3.1 Infrared absorption spectra of Sc:Ru:Fe:LiNbO₃ crystals

The water in the raw materials and growth atmosphere were expected to cause H⁺ ions to enter the crystal lattice and form O–H–O during the crystal growth. The frequency and energy of infrared light can only make molecules vibrate and their rotation levels change. Since the vibration of O–H is very sensitive to its environment, including surrounding ions, we expected the defects and structure of our crystals to be amenable to analysis using infrared spectroscopy.²⁹

The infrared absorption spectra of the crystals were each acquired in the wavenumber range 3400 cm⁻¹ to 3540 cm⁻¹ at room temperature. As shown in Fig. 2, the OH⁻ absorption bands of samples ScRuFe-0, 1 and 2 were all located at about 3482 cm⁻¹, while the OH⁻ band of sample ScRuFe-3 was located at 3507 cm⁻¹. The shape of an OH⁻ absorption band is related to the crystal composition and ions surrounding the OH⁻. The OH⁻ absorption band of LiNbO₃ is also located at 3482 cm⁻¹.³⁰

Fig. 2 Infrared transmittance spectra of Sc:Ru:Fe:LiNbO₃ crystals.

The mechanism underlying the shift of the absorption peak can be described as follows. The [Li]/[Nb] ratio in congruent LiNbO₃ crystals has been previously indicated to be about 0.946, and to have caused the generation of intrinsic defects. During the growth of such crystals, a lattice position missing the Li⁺ ion would form a Li vacancy V_Li⁻, which would be filled by a Nb³⁻ ion to form an anti site Nb Nb_Li⁴⁺. The concentration threshold of anti-photorefraction can be defined as that when the doped ions enter the normal Nb position. In our experiment, the concentrations of Fe³⁺ and Ru⁴⁺ ions were fixed, so we just took the Sc³⁺ ion into account; as indicated above, the concentration threshold of Sc³⁺ ions has been reported to be 2 mol%. The concentrations of Sc₂O₃ added into the growth melts of samples ScRuFe-0, 1, 2 and 3 were measured using ICP-AES spectrometry to be 0 mol%, 0.78 mol%, 1.44 mol% and 2.07 mol% respectively. For the Sc:Ru:Fe:LiNbO₃ crystals with a dopant concentration below its threshold value, the doping Sc, Ru and Fe ions, according to the proposed mechanism, replaced Nb_Li⁴⁺ and V_Li⁻ to form the defects Sc_Li²⁺, Ru_Li³⁺ and Fe_Li²⁺, respectively. Due to the repulsion between these ions and H⁺, the H⁺ ions were still attracted by V_Li⁻, which caused the OH⁻ absorption bands of samples ScRuFe-0, 1, 2 to be quite similar. According to the proposed mechanism, for the concentration of Sc³⁺ ions exceeding the threshold, Sc³⁺ occupied Nb sites and formed the defect Sc_Nb²⁻. Sc_Nb²⁻ attracts H⁺ ions more strongly than does V_Li⁻, so the H⁺ ions gathered near Sc_Nb²⁻ according to our proposal. The shift of the absorption band of sample ScRuFe-3 to 3507 cm⁻¹ was attributed to the above mechanism. Note also that the OH⁻ absorption band of sample ScRuFe-3 was sharper than those of the others, and this observation was attributed to the formation of more Sc_Nb²⁻ defects resulting from the higher concentration of Sc³⁺ in this sample.

3.2 Two-wavelength nonvolatile measurements

Two-wavelength nonvolatile measurements were taken to study the holographic storage properties of the Sc:Ru:Fe:LiNbO₃ crystals. The experimental setup is shown in Fig. 3. A Kr⁺ laser with λ = 476 nm and an He–Ne laser with λ = 633 nm were used as the recording and readout beams, respectively. By using a continuously adjustable beam splitter, the recording beam was split into two beams, I_S and I_R, of equal 120 mW cm⁻² intensity. The two beams were then polarized in the incidence plane, and then directed to the crystal at the corresponding Bragg angle of 16°. The two beams intersected symmetrically inside the crystal and made the grating vector along the c-axis. During the recording process, the two beams were first directed onto the crystal at the same time, and then the beam I_S was blocked from time to time by a shutter, and the diffraction efficiency of another beam I_R was detected for a short duration of 10 s in order to eliminate the impact of erasure. After the grating store was saturated, I_S and I_R were both blocked only by the He–Ne beam directed onto the sample, and the intensities of transmitted I_t and diffracted I_d were determined. This process was the nonvolatile readout. By carrying out these experiments, the holographic storage properties of the samples were determined.

Fig. 3 Two-wavelength nonvolatile experiment setup.

Next we discuss the holographic storage properties of the Sc:Ru:Fe:LiNbO₃ crystals.

The diffraction efficiency η was defined as, the diffraction efficiency η was defined as

η = I_d/I_t × 100% (1)

where I_t is the transmitted light intensity and I_d the diffracted light intensity of the readout beam.

The recording and erasure time constants were described by using eqn (2) and (3).

In these equations, τ_w and τ_e are the recording and erasure time constants respectively. Also, η_sat is the saturation diffraction efficiency during recording, and it was fixed by the function

where η_max is the maximum value of diffraction efficiency, d is the thickness of the samples, λ is the wavelength of the recording beam, and θ_cry is the refraction angle of incidence light within the crystal.

Sensitivity (S) and its dynamic range (M/#) were calculated using eqn (5) and (6).^31,32

In these equations, I is the total optical intensity, and L is the crystal plate thickness.

The dual-wavelength nonvolatile holographic recording-readout curves are shown in Fig. 4.

Fig. 4 Time dependence of diffraction efficiency during dual-wavelength nonvolatile holographic recording of the samples.

The holographic storage parameters of the Sc:Ru:Fe:LiNbO₃ crystals measured by performing the dual-wavelength experiment are listed in Table 1. The results showed that the holographic storage parameters improved as the Sc₂O₃ doping concentration was increased. Compared to the values of ScRuFe-0, τ_w of ScRuFe-3 decreased by a factor of 3.0, η_s increased by a factor of 1.6, S increased by a factor of 4, and M/# increased by a factor of 7.9. The η_s, S, and M/# values of sample ScRuFe-3 were in fact higher than the corresponding values of the other tested samples.

Table 1 Experimentally determined holographic storage properties of the Sc:Ru:Fe:LiNbO₃ crystals

Sample	τw (s)	τe (s)	ηs (%)	S (cm J^−1)	M/#	Δns (× 10^−5)	σph (× 10^−12 cm Ω^−1 W^−1)
ScRuFe-0	389.2	937.2	41.2	0.04	1.55	4.17	0.64
ScRuFe-1	275.1	1105.7	44.5	0.06	2.68	4.33	0.90
ScRuFe-2	210.5	1461.5	53.4	0.09	5.07	4.75	1.18
ScRuFe-3	128.9	1937.8	65.6	0.16	12.18	5.26	1.92

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The holographic storage properties of the Sc:Ru:Fe:LiNbO₃ crystal are known to depend on the photorefractive sensitive center. In this crystal, there are two photorefractive ions, Ru⁴⁺ and Fe³⁺, which can form deep and shallow trap centers, respectively. The Sc³⁺ ion has a stable single valence state, so it cannot form a photorefractive center. First we discuss the carrier transport model in Sc:Ru:Fe:LiNbO₃ crystals. Here, Ru and Fe both have two different valences, which can form a donor level and acceptor level. Ru³⁺ and Fe²⁺ ions can bind many donor level electrons, while Ru⁴⁺ and Fe³⁺ can bind almost no accepter level electrons. When the incidence light was directed onto the crystal, the electrons in the donor level (Ru³⁺ and Fe²⁺) were excited to the conduction band, and after drift and diffusion, they were absorbed by accepter levels (Ru⁴⁺ and Fe³⁺). The process can be described as follows:³³

The Sc³⁺ ion enter into the lattice of Ru:Fe:LiNbO₃ crystal has no direct influence on the formation of the grating. The photorefractive ions Fe²⁺/Fe³⁺ and Ru³⁺/Ru⁴⁺ play a dominate role in photo-excited carrier transport processes. Doped Sc³⁺ ions influenced the ion arrangement and defects in the Sc:Ru:Fe:LiNbO₃ crystals, and a detailed mechanism for this influence was derived. According to this proposed mechanism, Sc³⁺ ions at doping concentrations below its threshold concentration replaced the Nb_Li⁴⁺ defects, while Sc³⁺ ions at concentrations exceeding its threshold concentration replaced the normal Li position. Due to the polarization ability of the Sc³⁺ ion being stronger than that of the Li⁺ ion, the ability of Sc³⁺ ions to capture electrons was also better than that for Li⁺ ions. In the process, the trap density of the electron acceptor increased and the saturation diffraction efficiency η_sat was also improved. The equation mentioned above indicated the dynamic range M/# to be approximately proportional to the saturation diffraction efficiency η_sat, so the dynamic range M/# increased with increasing concentration of the doped Sc³⁺ ions. For the LiNbO₃ crystal, the photoconductivity σ_ph has been shown to be related to the electron traps and to be inversely proportional to the Nb_Li⁴⁺ concentration. Increasing the concentration of the doped Sc³⁺ ions led to a decrease in the concentration of Nb_Li⁴⁺, which in turn led to the increase in the photoconductivity σ_ph and decrease in the response time. Sensitivity S is the comprehensive measure of saturation diffraction efficiency η_sat and photoconductivity σ_ph. As the results showed, increasing the concentration of Sc³⁺ doped into the Ru:Fe:LiNbO₃ crystals coincided with a decrease in writing time τ_w and increases in the dynamic range M/#, saturation diffraction efficiency η_sat, photoconductivity σ_ph and sensitivity S.

4. Conclusions

Sc:Ru:Fe:LiNbO₃ crystals with various concentrations of Sc³⁺ were grown by using the Czochralski method. The OH⁻ absorption experiment results showed the absorption bands of samples ScRuFe-0, 1, 2 to all be located at a similar wavenumber, of about 3484 cm⁻¹, when the doping Sc³⁺ ion concentration was below its threshold concentration. Once the Sc³⁺ concentration exceeded its threshold value, i.e., for ScRuFe-3, the absorption band shifted significantly, to 3507 cm⁻¹. The two-wavelength nonvolatile experiment results demonstrated that the holographic storage properties improved with increasing Sc³⁺ concentration. Compared to the other samples, ScRuFe-3, i.e., that with the highest Sc³⁺ doping concentration, showed the shortest response time, and the highest dynamic range M/#, saturation diffraction efficiency η_sat, and sensitivity levels, which are key parameters of volume holographic date storage. These results indicated the Sc:Ru:Fe:LiNbO₃ crystals to be promising materials for nonvolatile holographic storage.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Youth Science Fund of Heilongjiang Province of China (No. QC2015061).

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