Growth and photorefractive properties of Zn, Fe double-doped LiTaO3 crystal
Introduction
Lithium tantalate (LiTaO3, LT) single crystal shows lots of properties similar to those of lithium niobate (LiNbO3, LN) single crystal, such as the same crystalline structure (trigonal system, 3 m point cluster), ferroelectric in room temperature and Li deficiency, but its melting point (1650 °C) was higher that of LN (1240 °C). As one of the most excellent and useful photorefractive material, LT crystal can be applied in many areas, such as, surface acoustic wave (SAW) devices [1], waveguides devices [2], and holographic storage field [3]. Kim et al. [4] believed that the photorefractive effect of the crystals was attributed to the transition metal (TM) impurities and stacking faults caused by the nonstoichiometry and deficiency of oxygen in LT crystals, so it was an effective method to improve the photorefractive effect by doping the transition metal (TM) ions like Fe, Cu and Mn. Similar to that of LN, in TM-doped LT crystals, the photo-induced charge transport process can be described bywhere TMn+ (Fe2+, Cu+ and Mn2+) ions acted as electron donors and TM(n+1)+ (Fe3+, Cu2+ and Mn3+) ions acted as electron traps. Charge carriers excited from TMn+ move until they are trapped by TM(n+1)+, thus space charge fields build up and the refractive index is modulated because of the electro-optic effect. The bulk photogalvanic effect, as the main driving force of the space charges movement, has been identified. In Fe:LN and Fe:LT, the one-center scheme was experimentally verified, in which the bulk photogalvanic current density is proportional to light intensity and Fe2+ concentration [5]. Despite of excellent photorefractive properties, there were serious disadvantages in TM-doped LT, e.g. long response time and low optical damage resistance. The so-called “optical damage” was also a photorefractive effect, which occurred when the crystals were irradiated with high-power laser beams of visible wavelengths. These effects limit its application in nonlinear optical fields.
It was well known that the optical damage resistance of LN crystals can be significantly improved by doping the optical damage resistance impurities such as Mg2+, Zn2+, In3+ and Sc3+ ions [6], [7]. We also found that the optical damage resistance could be increased considerably when doping Zn2+ ions in Fe:LT crystals. In addition, the photorefractive response time of Zn:Fe:LT was reduced to a considerable degree in comparison with Fe:LT crystal.
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Crystal growth
Due to the high melting temperature of LT crystal, an intermediate frequency (IF) furnace was used to grow the LT crystals. For comparison with LT crystals, the LN crystals were also grown from the IF furnace by the Czochralski method. The starting materials used to grow the crystals were Li2CO3, Ta2O5, Nb2O5, Fe2O3 and ZnO, which were of all 4 N purity. Compositions of the raw materials were shown in Table 1. The crystals were grown at rotating rate 10–20 rpm and pulling rate 1–3 mm/h, and the
Exponential gain coefficient
The photorefractive properties of the crystals were measured by the two-beam coupling experiments in the transmission geometry. Fig. 2 shows the typical experimental setup of the two-beam coupling. A weak probe wave (signal beam IS) and a pump wave (reference beam IR), which both originated from Ar+ laser (wavelength λ = 5145 Å), irradiate on a sample with polarization direction parallel to the c-axis of the sample, then intensive beams coupling occurred and caused the signal beam intensity
Optical damage resistance of Zn:Fe:LiTaO3 crystal
The optical damage resistance of Zn:Fe:LiTaO3 crystal was evaluated by the transmitted facula distortion measurement. The experimental setup was shown in Fig. 6. An Ar+ laser beam (wavelength λ = 488 nm), whose intensity can be controlled by an adjustable light attenuator and its polarizing direction was parallel to c-axis, irradiated on the samples after convergence through the convex lens. The crystal was placed on the focal plane of the lens.
The transmitted beam would not be distorted and the
Results and discussion
The intensive light crawling effect that exists in thin Zn:Fe:LT samples may be responsible for the higher exponential gain coefficient in a large angle range, similar to that case in LN [11]. It can be found from Table 2 that the response speed of Zn:Fe:LT crystal was about four times higher than that of Fe:LT crystal and also higher than that of Zn:Fe:LN crystal. The optical damage resistance of all the samples were also given in Table 2, which indicated that the optical damage resistance of
Conclusion
In conclusion, Zn:Fe:LiTaO3 crystals were prepared by the Czochralski method and their photorefracive properties were measured. It was found that the photorefractive response speed can be greatly improved by doping ZnO in Fe:LiTaO3 crystals, moreover, the optical damage resistance of Zn:Fe:LiTaO3 was two orders of magnitude higher than that of Fe:LiTaO3. Our analysis indicated that the increased photoconductivity was responsible for both fast photorefractive response and high optical damage
Acknowledgments
This work was supported by the National Natural Science Foundation of China (50232030, 10172030), The National Science Foundation of Heilongjiang Province, The Ministry of Science and Technology of China through the High-Tech Program (2001AA31304), and the National Committee of Defense Science and Technology.
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