Growth and characterization of near stoichiometric LiNbO3 single crystal

A near stoichiometric LiNbO₃ single crystal has been grown by the Czochralski method from a 58.5% Li melt hold in a large platinum crucible. High resolution X‐ray rocking curves of 30 0 and 0006 reflections indicated that the near stoichiometric LiNbO₃ crystal possesses the high structural quality. Compared with the congruent LiNbO₃, the near stoichiometric LiNbO₃ possesses shorter ultraviolet absorption edge, thus higher Li concentration. The OH^– infrared absorption band analyses showed that the Li concentration in the near stoichiometric LiNbO₃ crystal is higher than that in the congruent LiNbO₃ crystal. This result is in good agreement with that of the ultraviolet absorption edge. The electro‐optic (EO) coefficient γ₂₂ of the near stoichiometric LiNbO₃ crystal was measured to be 6.75 pm/V higher than that of congruent LiNbO₃ crystal. It also proves the near stoichiometric LiNbO₃ electro‐optic Q‐switched requires a low driving voltage and it is advantageous for the device performance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of stoichiometric LiNbO3 nanopowder through a wet chemical method

Stoichiometric lithium niobate powder which are used as feeding material in near stoichometric LiNbO₃ crystal growth have been successfully prepared from commercial niobium hydroxide [Nb(OH)₅] and nontoxic DL‐malic acid by a wet chemical method. The synthesis temperature was pre‐determined by the results from thermogravimetric and differential thermal analysis. The structure and morphology of the as‐prepared samples were observed by using the infrared spectroscopy and the scanning electron microscopy. The X‐ray diffraction experiment showed that lithium niobate powder had an ilmenite structure, and its unit cell parameters were calculated to be a = b = 0.5140 nm, c = 1.3738 nm, and V = 0.3144 nm³. The melting point of the synthesized powder is 1239 °C and Curie temperature T_c is 1122 °C. This synthesis method would be helpful to grow the near‐stoichiometric LiNbO₃ crystal with double crucible techniques. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fluorescence properties of Nd3+ doped stoichiometric LiNbO3 fiber single crystals by micro‐pulling down method

Using the micro‐pulling down (μ‐PD) method, 1 and 3 mol% Nd₂O₃ doped near stoichiometric lithium niobate (LiNbO₃) single crystal fibers were grown in 1 mm diameter and 35∼40 mm length. The grown crystal fibers were free of cracks and the homogeneous distribution of Nd³⁺ ion concentrations were confirmed by the electron probe micro analysis. The changes of fluorescence spectra were measured with respect to the Nd³⁺ ion doping concentration. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reduction of domain wall width in VTE treated near stoichiometric LiNbO3 crystals

LiNbO₃ is a ferroelectric crystal and grows with multi domains. Different domains are separated by boundaries which are known as domain boundaries. Domain walls for congruent and VTE (Vapor Transport Equilibration) treated near stoichiometric lithium niobate samples were visualised in different crystallographic directions using chemical etching technique. The sample is etched in the mixture of HF and HNO₃ (in 1:2 volume ratios) for 10 minutes at boiling temperature. Measured domain wall width was found approximately 15‐20 µm for congruent (CLN) and it reduces to 1‐3 µm for VTE treated near stoichiometric (SLN). Activation energies were also measured by two‐probe method and found to be increasing in stoichiometric sample. This activation energy is related to defect density in the crystals. Activation energy is higher for less defective crystals. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and optical properties of near stoichiometric Ce:LiNbO3 crystals

The near sotichiometric Ce:LiNbO₃ (Ce:SLN) crystals were grown by the top seeded solution growth (TSSG) method by adding K₂O flux to Li₂O‐Nb₂O₅ melt. Their UV‐vis absorption spectra and IR spectra were measured and discussed to investigate their defect structure. The results showed that the grown crystals were near stoichiometric and Ce ions in the crystals located the Li site. Photorefractive properties of Ce:SLN crystals were studied by two‐wave coupling experiment. The results of the two‐wave coupling experiments of the crystals showed that as the CeO₂ doping concentrations increased, the diffraction efficiency increased, photoconductivity decreased and the writing time and erasure time increased. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of Zn2+ ions concentration on the optical properties of Zn/Er:LiNbO3 crystals

Congruent Er³⁺(3 mol%):LiNbO₃ crystals codoped with ZnO (X mol %, X=0, 3, 6 and 7) were grown by the Czochralski technique. The Er contents in the crystals were measured by an inductively coupled plasma atomic emission spectrometer (ICP‐AES). Under 800 nm excitation, the upconversion emission spectra reveal an enhancement of the green emission with respect to the red emission when the Zn²⁺ ions are introduced into Er:LiNbO₃ crystal. The effect of Zn²⁺ ions concentration on the intensity ratio of the green to red emission has been investigated. Two cross‐relaxation processes (²H_11/2 + ⁴I_13/2 → ⁴I_11/2 + ⁴F_9/2 and ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2) are involved in populating the ⁴F_9/2 state, which bypass the green‐emitting states. The OH^‐ absorption spectra indicate that the Zn²⁺ codoping leads to a decreased concentration of Er³⁺ cluster sites contributing to the enhancement of the green emission. The studies on UV‐vis absorption spectra show that the heavily codoped with Zn²⁺ results in the reformation of the Er³⁺ cluster sites in Er:LiNbO₃. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of near‐stoichiometric LiNbO3 crystals and Li2O contents determination

Two kinds of near‐stoichiometric LiNbO₃ crystals (SLN11 and SLN19) were grown by a flux pulling method from stoichiometric melt with addition of 11mol%K₂O and 19mol%K₂O, respectively. Compared with the congruent melting LiNbO₃, the ultraviolet absorption edges of two crystals shift towards shorter wavelengths, and the locations of the OH^‐ infrared absorption band have obvious change and the bandwidths become greatly narrower. From these experimental results, the Li₂O contents are determined indirectly to be about 49.6mol% for SLN11 and 49.9mol% for SLN19, respectively. The Li₂O content in SLN19 is very close to the ideal value of 50mol%. The coercive fields of two crystals were measured by the poling method at room temperature. A linear relationship between the Li₂O content and the coercive field was fitted. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Inhomogeneity of composition in near‐stoichiometric LiNbO3 single crystal grown from Li rich melt

A near‐stoichiometric LiNbO₃ single crystal has been grown by the Czochralski technique from a melt of 58.5 mol% Li₂O. Its composition homogeneity was assessed by measuring the UV absorption edge. It was found that the maximum composition difference is about 0.03 mol% in the radial direction and 0.05 mol% in the axial direction. Differential scanning calorimetry (DSC) analysis was performed on the powder from the synthesized raw material and the frozen melt after crystal growth. The analytical results indicate that, during crystal growth, the magnitude of lithium volatilization from the melt surface is more than the degree of segregation from the crystal. The volatilized lithium diffuses into the crystal to compensate for the lithium segregation in the LiNbO₃ crystal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cr3+ luminescence quenching in stoichiometric lithium niobate crystals

Cr³⁺-doped stoichiometric LiNbO₃ crystals were grown and detailed spectroscopic investigations were performed. The samples are characterized by extremely low frequency factor of the luminescence thermal quenching. Its numerical value 1.7 × 10⁸ s⁻¹ is about 10⁴ times lower than in crystals of lithium-aluminum fluorides. Under such conditions, radiative transitions of Cr³⁺ ions compete successfully with non-radiative ones, resulting in a relatively high quantum yield of the broadband luminescence at room temperature. The quenching of luminescence is counteracted by the effect of the dynamic removal of the radiative transition exclusion, and as a result, Cr³⁺ radiative lifetime in LiNbO₃ decreases from 8 to 4 μs when temperature grows up from 77 to 300 K. Therefore quantum yield of the broadband luminescence at room temperature is not 5–10%, as reported previously, but about 30%. The results obtained in the present study show the stoichiometric lithium niobate doped with Cr³⁺ ions to be a potential active media for tunable lasers.     

Optical Absorption Edge of Mg + Zn: LiNbO3

The optical transmittance of LiNbO₃ single crystal double doped with MgO and ZnO was measured from the ultraviolet to the visible range. The wavelength dependence of the absorption coefficient α and its root α^1/2 (α versus hv and α^1/2 versus hv, respectively) were calculated and the characteristics of the absorption edge were discussed. The energy gaps E_g and E_g of the crystals which correspond to the direct transition and the indirect transition, respectively, and the energy of phonons taking part in the indirect transition were calculated. The effects of dopants Mg and Zn on the optical absorption properties are discussed. It was found that the energy E_g of our sample which was double doped with MgO and ZnO was smaller than that of congruent LN, causing the indirect transition absorption edge to move towards the infrared.     

铒镱双掺的钒酸钇和铌酸锂晶体中的铒离子的上转换发光

利用J-O理论,计算了在铒、镱双掺的钒酸钇和铌酸锂晶体中的铒离子在室温下的晶场唯象参数Ωλ(λ=2,4,6)及辐射跃迁几率、无辐射跃迁几率和共振跃迁几率.考虑到铒、镱间的能量转移,写出了在这些晶体中的铒离子的速率方程.速率方程的解表明,在铒、镱双掺的钒酸钇晶体中的铒离子的550 nm的上转换发光,比它在铒、镱双掺的铌酸锂晶体中更为有效.这一理论结果与我们的实验观察结果一致.     

Near‐stoichiometric LiNbO3:ZrO2 single crystal growth by micro pulling down method

Crack‐free, rod‐shaped single crystals of undoped and 0.5, 0.7 and 1.0 mol% ZrO₂‐doped LiNbO₃ with a near‐stoichiometric composition were grown by the micro‐pulling down (μ‐PD) method. The structural properties of the grown crystals were examined by powder X‐ray diffraction (XRD). Electron probe micro analysis (EPMA) of the near‐stoichiometric LiNbO₃ single crystals revealed the homogeneous incorporation of Zr ions. The change in the refractive index and IR transmission spectra of the grown crystals were examined as a function of the Zr concentration. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Improved blue photorefractive properties of near‐stoichiometric LiNbO3:Mn:Fe:Zr crystal

Near‐stoichiometric LiNbO₃ single crystal tri‐doped with ZrO₂, MnO and Fe₂O₃ was grown from Li‐riched melt by Czochralski method. The defect structures and composition of these crystals were analyzed by means of ultraviolet‐visible and infrared transmittance spectra. The appearance of 3466 cm^‐1 peak in infrared spectra showed that the crystal grown from Li‐riched melt was near stoichiometric. The photorefractive properties at the wavelength of 488 nm and 633 nm were investigated with two‐beam coupling experiment, respectively. The experimental results showed that the response speed and sensitivity were enhanced significantly and the high diffraction efficiency was obtained at 488 nm wavelength. This manifested that near‐stoichiometric LiNbO₃:Mn:Fe:Zr crystal was an excellent candidate for holographic storage. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

近化学计量组分掺铒铌酸锂的制备及性能测试

通过气相输运平衡技术,严格控制生长条件和气氛,制备了化学组分均匀、光学质量优异的近化学计量组分Er:LiNbO3晶体.通过对吸收边、喇曼谱及红外OH-吸收谱的测量,发现吸收边"蓝移";OH-吸收带显著减弱;Er:LiNbO3晶体中的E模(152cm-1)和A1模(632cm-1)的线宽也明显变窄.这些特征都证实了得到的Er:LiNbO3晶体已接近于化学计量组分,经过计算,其Li+含量达到49.7mol;.     

Cr3+‐doped LiNbO3 crystals grown by the Bridgman method

The growth of LiNbO₃ crystals doped with Cr³⁺ ions in 0.1, 0.2, and 0.5 mol % concentrations by Bridgman method were reported. The Cr³⁺ ion concentrations in crystals were measured by inductively coupled plasma spectrometry. Electron paramagnetic resonance had been used to investigate the sites occupied by the Cr³⁺ ions. Two Cr³⁺ ion centers located at Li⁺ and Nb⁵⁺ sites (Cr_Li³⁺ and Cr_Nb³⁺ centers, respectively) were observed. Optical absorption and temperature‐dependence emission spectra of the Cr³⁺ ions were reported. The crystal‐field parameters and Racah parameters of the Cr³⁺ ion defect sites were reported and compared with those grown by Czochralski technique. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and Characterization of LiNbO3: Mg + Ti Monocrystalline Fibers

LiNbO₃ : Mg + Ti monocrystalline fibers with a diameter of 200 – 1000 μm have been successfully grown by the laser-heated pedestal growth method. The lattice parameters, the dopant Mg- and Ti concentrations, and the ferroelectric domain structure of the fibers were determined. The results are compared with those in the large crystals grown by the Czochralski method.     

High photorefractive sensitivity and fast response time of near‐stoichiometric and MgO doped LiNbO3:Fe crystals

Congruent LiNbO₃:Fe and LiNbO₃:Mg,Fe crystals were grown by Czochralski method, and vapor transport equilibration technique was employed to improve the [Li]/[Nb] ratios of these crystals. The influence of stoichiometry and MgO dopant on the photorefractive sensitivity and response time of LiNbO₃:Fe crystals was investigated. Both stoichiometry and MgO dopant can effectively reduce the amount of intrinsic defects, but MgO can also decrease the concentration of Fe²⁺ ions in Li‐sites. Near‐stoichiometric and MgO doped LiNbO₃:Fe crystal has high photorefractive sensitivity and fast response time. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectroscopic properties of Yb3+ ions in halogeno-sulfide glasses

In this work, we report the optical properties of Yb³⁺ ions in halogeno-sulfide glasses of composition (75 − x)GeS₂–25Ga₂S₃–xCsCl (x = 5%, 10%, 15%, 20%, and 25% CsCl). This study includes an analysis of the influence of halide concentration on the absorption and emission cross-sections and lifetimes of Yb³⁺ ions. A blue shift of the absorption and emission bands and a decrease of the absorption and emission cross-sections and transition probability are observed as the halide concentration increases in the glass.