Electrooptic characterization and Czochralski growth technique of pure and doped lithium niobate crystals
Tesfaye GebreD. EdwardsManmohan D. AggarwalA. K. BatraMatthew EdwardsDarayas PatelLaQuieta HueyRavindra B. Lal
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Abstract:
Ferroelectric Lithium Niobate (LN) possesses a combination of unique electrooptic, piezoelectric, pyroelectric, and photorefractive properties. These features make it suitable for applications in optical devices-as modulators, switches, and filters in communication systems and holographic recording medium, etc. Here, the growth of lithium Niobate doped with iron and doubly doped with iron and manganese ions will be described. The growth technique will be through Automatic Diameter Control Czochralski Design. From these grown crystals, critical electrooptical coefficients using null detection polarimetry are provided. The results of growth, electrooptic measurements, and some physical properties are compared and presented. Also, the use of doped LN crystals in devices is discussed.Keywords:
Characterization
Time evolution of Raman spectra depending on temperature were carried out on iron-doped photorefractive lithium niobate crystals with different Li content in the melt. The frequency shift of Raman lines are observed with time giving the possibility to calculate the saturation values of the photorefractive space charge field showing a linear decrease with temperature. Space charge field values are much lower for near-stoichiometric compsition sample comparing to near-congruent ones.
Stoichiometry
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We report the measurement of pyroelectric field in LiNbO3 by introducing a new method, based on optical interferometry. The pyroelectric coefficient of the congruent LiNbO3 crystals has been determined by this method and the value obtained for it, ∼95 μC/(m2 · K), is consistent with previous results reported in literature, obtained by other methods.
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The photorefractive effect in Indium-doped litium niobate crystal was studied. We found that the enhanced photorefractive sensitivity could be realized in the so-called photorefractive resistance lithium niobate crystals, even higher than the well-known iron doped ones. We explained the unusual qualitatively. The experimental result enriches us the knowledge of the properties of doped lithium niobate crystals and provides some advice in growing specific crystal.
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We studied the UV photorefractive effect and other optical properties of Na+-doped lithium niobate crystals. The behavior of the photorefractive resistance in Na+-doped lithium niobate crystals was found to be quite different from that in lithium niobate crystals doped with other well known optical-damage resistant metal ions, such as Mg2+, Zn2+, In3+ etc. It is also found that not only the defect structures but also the ratio of [Li]/[Nb] in our samples was changed by doping with Na+ ions. We theoretically analysed our experimental results, based on the Li-vacancy model.
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A model for beam propagation through a nonlinear material that takes into account inhomogeneous induced refractive index changes due to the nonlinearity is first developed. The theory based on this model can be used to analyze the propagation of Gaussian beams through photorefractive lithium niobate. A focused Gaussian beam of circular cross-section incident on the sample emerges as an elliptic Gaussian after interaction in this material. We observe that the value of the nonlinearity coefficient derived from a z-scan of photorefractive lithium niobate compares favorably with that found by varying the power P of a Gaussian beam focused at a fixed longitudinal position within the sample and monitoring the far-field beam ellipticity. We use the value of the nonlinearity coefficient to determine the donor to acceptor ratio of dopants in photorefractive lithium niobate samples. We show how our theory can be extended to include two species of impurities in the photorefractive material.
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Organic photorefractive materials
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When multiple holograms are recorded in a photorefractive crystal, the recording of recent holograms erases early ones. We can equalize the strengths of all recorded holograms by following an appropriate exposure schedule. However, this leads to a composite hologram whose strength decreases as the number of exposures increases. In this paper, we describe a system in which the composite photorefractive hologram reaches a steady state with overall efficiency independent of the number of holograms (N). Furthermore, the phases of the recorded holograms remain locked during this process.
Diffraction efficiency
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Pyroelectric crystal
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In the field of optical signal processing and all-optical integrated devices, photorefractive crystals can be used because of their capability to keep memory of a spatially varying light pattern. Among them, lithium niobate is particularly interesting because
its photorefractive response can be improved or inhibited by adding selected dopants: this opens the possibility of producing an integrated device in a lithium niobate single crystal, where each stage has different properties and different functions according to the doping. In particular, a photorefractive stage can be created by doping with Fe, which is known to enhance photorefractive effect. In the context of integrated devices,
it is necessary to perform a local doping of lithium niobate with Fe, in order to obtain a suitable substrate for photorefractive recording. This thesis deals with the preparation and characterisation of the locally doped crystal, i.e. with an investigation of the preparation conditions and how they affect the crystal quality. Many characterisation techniques, customary in materials science, such as secondary ion mass spectrometry, spectrophotometry and others, have been used and refined specifically for Fe doped lithium niobate. Besides the practical aim to find the best preparation conditions, many basic properties and features of this material have been investigated, leading to an advance in the knowledge of this material, as well as an advance in the usage of characterisation tools.
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The holograms fixing process in very well known sillenite type photorefractive crystals Bi 12 (Ge, Si, Ti)O 20 has important implications for various applications. An effect of prolonged memory of holographic recording with no applied electric field in photorefractive Bi 12 TiO 20 (BTO) crystals at room temperature has been reported. 1,2 The effect is established for high frequency holographic grating with K = 2π/Λ ~ 3×10 5 cm -1 , where Λ is the grating period.
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