Stoichiometric LiNbO3 Crystals Research Articles

Regular domain structures fabricated by an electron beam in stoichiometric LiNbO3 crystals

Electron beam writing of regular domain structures in Z-cuts 0.75 mm thick of stoichiometric and close to stoichiometric LiNbO3 crystals has been carried out. Crystals have been grown by the Czochralski method from a melt with excess Li2O (58.6 mol %) and from a congruent-composition melt in the presence of 6 wt % K2O alkali solvent (flux). In both crystals, threshold charge doses required to form individual domains have been determined, and the optimal conditions of periodic structure patterning by sequential local irradiations have been found. Domain gratings of similar type (with periods of 6.5, 7, and 10 μm) are formed in both types of stoichiometric crystals.

Defect structure and light‐induced scattering of Zr‐doped near‐stoichiometric Mn:Fe:LiNbO3 crystals grown by TSSG method

AbstractNear‐stoichiometric Mn:Fe:LiNbO3 crystals doped with various concentration of ZrO2 were grown by top seed solution growth (TSSG) method in the air atmosphere. The Zr concentration in the crystal was determined by inductively coupled plasma optical emission spectrometer. The defect structures were analyzed by means of ultraviolet‐visible and infrared transmittance spectra. The appearance of vibration peak at 3466 cm‐1 in infrared spectra manifested that Li/Nb ratio in crystals approached to stoichiometric proportion. The fundamental absorption edge represented continuous red‐shift which was discrepancy with congruent doped LiNbO3 crystals showed that doping ions possessed different location mechanism. The light‐induced scattering of the doped stoichiometric LiNbO3crystals were quantitatively scaled via incident exposure energy. The results demonstrated that Zr(2 mol%):Mn:Fe:LiNbO3 crystal had the weakest light‐induced scattering and the mechanism related to their defect structures was discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Near field diffraction analysis of optical domain wall image in stoichiometric LiNbO3 crystal

Observing diffraction patterns instead of direct measurement of domain wall morphology makes it possible to obtain effective in situ optical visualization of anti-parallel domains. Domain polarities are easily distinguished by applying proper voltage along the crystal z-axis. The polarization-dependent electro-optic effect gives opposite phase difference on probe beam at the anti-parallel domains, and maximum optical contrast is obtained at the 180° phase shift. The diffraction pattern width is minimal on the sample surface and diffused with the distance to the monitoring screen. Calculations based on the near field diffraction of the wave front are in good agreement with the experimental observations.

Wideband and high-gain frequency stabilization of a 100-W injection-locked Nd:YAG laser for second-generation gravitational wave detectors

Second-generation gravitational wave detectors require a highly stable laser with an output power greater than 100 W to attain their target sensitivity. We have developed a frequency stabilization system for a 100-W injection-locked Nd:YAG (yttrium aluminum garnet) laser. By placing an external wideband electro-optic modulator used as a fast-frequency actuator in the optical path of the slave output, we can circumvent a phase delay in the frequency control loop originating from the pole of an injection-locked slave cavity. Thus, we have developed an electro-optic modulator made of a MgO-doped stoichiometric LiNbO(3) crystal. Using this modulator, we achieve a frequency control bandwidth of 800 kHz and a control gain of 180 dB at 1 kHz. These values satisfy the requirement for a laser frequency control loop in second-generation gravitational wave detectors.

A Fe[Nb]-Li center in stoichiometric LiNbO3 crystals: Mechanism of formation

A new iron center in stoichiometric lithium niobate crystals has been studied by the EPR method. The angular dependences of the EPR spectrum of the center have been used to derive the parameters of its spin Hamiltonian. The data amassed on the variation in the concentrations of two iron centers in lithium niobate crystals annealed in a Li2CO3 powder have provided an insight into the mechanism of formation of the new center, as well as corroborated its model proposed by us earlier. According to this model, the center represents a complex of two defects aligned with the polar axis in the crystal: the iron ion at the niobium site and an interstitial lithium ion filling the nearest structural vacancy (Fe3+[Nb]-Li+[V]). The structure of other Fe3+ centers revealed earlier in LiNbO3 crystals, in which the iron ion occupies the niobium site, has been discussed.

Structure and properties of optical waveguides in stoichiometric LiNbO3 crystals

The structures of proton-exchanged and implanted waveguides are studied by X diffraction analysis and vibration spectroscopy. On the basis of the absorption’s spectroscopy data in the visible region and the data on the shift of the fundamental absorption edge in the nearultraviolet region, a comparative analysis of technologically conditioned variations in the nonlinear optical properties of proton-exchanged and implanted waveguides in stoichiometric LiNbO3 crystals is conducted.

Phase mapping of domain kinetics in lithium niobate by digital holographic interferometry

The phase mapping of domain kinetics under the uniform steady-state electric field is achieved and investigated in the LiNbO3 crystals by digital holographic interferometry. We obtained the sequences of reconstructed three-dimensional and two-dimensional wave-field phase distributions during the electric poling in the congruent and near stoichiometric LiNbO3 crystals. The phase mapping of individual domain nucleation and growth in the two crystals are obtained. It is found that both longitudinal and lateral domain growths are not linear during the electric poling. The phase mapping of domain wall motions in the two crystals is also obtained. Both the phase relaxation and the pinning-depinning mechanism are observed during the domain wall motion. The residual phase distribution is observed after the high-speed domain wall motion. The corresponding analyses and discussions are proposed to explain the phenomena.

Optical waveguides in LiNbO3 and stoichiometric LiNbO3 crystals by proton exchange

The formation of optical planar waveguides in LiNbO3 and stoichiometric LiNbO3 crystals by proton exchange was reported. The prism-coupling method was used to characterize the dark-line spectroscopy at the wavelength of 633 and 1539 nm, respectively. The mode optical near-field outputs from proton-exchanged LiNbO3 and SLN waveguides at 633 nm were presented. The mode field from stoichiometric LiNbO3 (SLN) waveguide is lighter and more uniform than that from LiNbO3 waveguide, which means the quality of the waveguide in SLN crystal is better than that of the LiNbO3 waveguide. For proton-exchanged LiNbO3 waveguides, the evolution of the refractive index profile with annealing was presented. The disorder profiles of Nb atoms in proton-exchanged LiNbO3 waveguides were obtained by Rutherford backscattering/channeling technique. It is shown that the longer the exchange time, the larger the displacement of Nb atoms.

Growth evolution and decay properties of the abnormally switched domains in LiNbO3 crystals

Direct domain writing is carried out in single-crystalline stoichiometric LiNbO3 crystals using a scanning probe microscope. The abnormally switched domains with polarization antiparallel to the poling field are observed, and their growth and decay processes are systematically studied. The radius of these domains is proportional to the pulse magnitude while it remains constant as the pulse width varies. These abnormal domains quickly decay, the lifetime of which is linear with the poling pulse magnitude while it stretched exponentially increases with the increasing pulse width. The experimental results are carefully discussed in relation to the reported charge injection model.

Investigation of new fluxes for the single-crystal growth of stoichiometric lithium niobate

Thermoanalytical and crystal growth investigations of the ternary system Cs2O-Li2O-Nb2O5 are presented in order to grow stoichiometric LiNbO3 (LN) crystals. Part of the phase diagram is determined and subsolidus phases are identified at room temperature by X-ray powder diffraction. Among the constituent phases, a new tetragonal cesium lithium niobate phase is assessed. From the Cs2O-Li2O-Nb2O5 system, good quality quasi-stoichiometric LN crystals can be grown.

Studies of photorefractive properties of different oxidation state of MnO doped near stoichiometric LiNbO3 crystals

In this paper, photorefractive properties of different oxidation state of undoped and MnO doped near stoichiometric LiNbO3 (SLN) were studied. The undoped and doped with different concentration of Mn ions SLN crystals have been grown by the top-seeded-solution-growth (TSSG) method by adding K2O flux to Li2O–Nb2O5 melt and been post-treated by oxidation and reduction respectively. The UV–Vis spectrum results showed, oxidation states and MnO doping concentration of the MnO doped SLN crystals have influences on their UV–Vis spectra. The photorefractive properties of different oxidation states of the crystals were experimentally studied by using two-beam coupling. The results of the two-wave coupling experiments showed that, oxidation and reduction treatment have opposite influences on the photorefractive properties of undoped and MnO doped SLN crystals.

Growth and characterization of near stoichiometric LiNbO3 single crystal

AbstractA 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$ \bar 3 $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)

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 1.7×108s−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 4μs when temperature grows up from 77 to 300K. 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.

Direct observation of pinning centers in ferroelectrics

We present a direct observation of nanoscale ferroelectric domain pinning centers in lithium niobate crystals. A high-voltage atomic force microscope has been used to tailor nanodomain structures in LiNbO3 crystal with high defect concentration. Domain pinning and depinning events have been captured following thermally induced domain decay process. The pinning centers’ influence on the domain wall dynamics has been analyzed by comparing domain growth in stoichiometric and congruent LiNbO3 crystals.

Temperature dependence of the absorption and refraction of Mg-doped congruent and stoichiometric LiNbO3 in the THz range

The absorption coefficient and the index of refraction of undoped and Mg-doped stoichiometric and congruent LiNbO3 crystals were determined for polarization parallel to the z axis in the far-infrared (FIR) 30−180-cm−1 frequency range for different Mg-doping levels and temperatures down to 10 K. For stoichiometric LiNbO3, smaller absorption and index of refraction were found, than for congruent samples. At a Mg content near the photorefractive threshold, the FIR absorption coefficient has the lowest value in the stoichiometric crystals, which are most suitable for efficient pulse generation in the terahertz range.

Studies of photorefractive fields of MnO and Fe2O3 co-doped near stoichiometric LiNbO3 crystals

Abstract In this paper, photorefractive field properties of MnO and Fe 2 O 3 co-doped near stoichiometric LiNbO 3 (nSLN) crystals were studied. The nSLN crystals co-doped with 0.005 wt% Fe 2 O 3 and MnO at the concentration of 0, 0.003 wt%, 0.005 wt%, and 0.010 wt%, respectively, have been grown by the top seeded solution growth (TSSG) method by adding K 2 O flux to Li 2 O–Nb 2 O 5 solution. Their IR spectra showed that the crystals were near stoichiometric. In the two-wave coupling experiments of the crystals, from the measured maximum diffraction efficiency and the incidence angles between the reference and signal beams, the maximum saturation field E q , photovoltaic field E ph , diffusion field E d and space charge field E sc of the crystals were calculated by iterative method for the first time.

Optical properties of stoichiometric LiNbO3 waveguides formed by low-dose oxygen ion implantation

The planar waveguides in z-cut stoichiometric LiNbO3 crystal have been fabricated by 3.0 MeV oxygen ion implantation with the dose of 6×1014ions∕cm2 at room temperature. The modes of the waveguides were measured by the prism-coupling method with the wavelength 633 and 1539 nm, respectively. The refractive index profiles of the waveguides were reconstructed using reflectivity calculation method. The Rutherford backscattering/channeling technique was used to investigate the damage produced by the ion implantation, the minimum yield of the spectra was 4.52%. The propagation loss of waveguide was 0.61 dB/cm obtained by fiber probe technique. It was found that positive changes of extraordinary refractive indices happened in the guiding regions, and such change increased after the annealing treatment at 260 °C for 60 min.

Effect of Zn and Fe Concentrations on Structure and Optical Damage Resistances of Near Stoichiometric Zn:Fe:LiNbO<sub>3</sub> Crystals

Zn,Fe co-doped near stoichiometric LiNbO3 crystals with variant Zn concentrations and with same Fe concentrations were grown by top-seeded-solution-growth (TSSG) method. The optical damage resistances of SLN crystals were studied by the method of transmittance facula distortion. The UV-Vis spectra showed that the grown undoping crystal had a composition very close to stoichiometric LiNbO3 crystal, and the absorption edges moved as the doping concentration changed. The optical damage experiments showed that, the crystal with 2mol% of Zn was with high resistance to optical damage, and the optical damage resistance was improved evidently as the Zn concentration increased further.

1.5 μ m emission characteristics of Er3+-doped stoichiometric LiNbO3

The 1.5μm emission characteristics of stoichiometric LiNbO3 crystals with different Er3+ doping levels of 0.2, 0.5, 1.0, and 2.0mol% were investigated. All of the emission decays could be fitted by a single exponential curve. The lifetimes of the 1.5μm transition (I13∕24→I15∕24) were measured to be 3.0–3.2ms, which are about 25% longer than those reported for conventional Er-doped congruent LiNbO3. Our analysis showed that the longer lifetime in stoichiometric LiNbO3 is likely to originate from the reduced density of intrinsic defects.

Photochromic Effect in Stoichiometric LiNbO3:Fe,Mn

Stoichiometric LiNbO3 (LN) crystals doubly doped with Fe and Mn exhibited a strong reversible photochromic effect at room temperature. Irradiation with UV light induced a new absorption band extending from approximately λ=650 nm to the absorption edge, which could also be bleached by subsequent visible illumination. The dark decay times of induced absorption at room temperature were estimated to be approximately 2 months and more than a year for as-grown and oxidized crystals, respectively. Electron paramagnetic resonance analysis confirmed that the photochromic process in LN:Fe,Mn occurs by an intervalence charge transfer between deep Mn2+/3+ and shallow Fe2+/3+ energy levels. It was also found that only a small fraction of the doping ions are involved in the photochromic process.