Near-stoichiometric LiNbO3 Research Articles

Characterization of near-stoichiometric LiNbO_3 crystals from Li-55 and 58mol% solution

Characterization of near-stoichiometric LiNbO_3 crystals from Li-55 and 58mol% solution

Study on optical properties of Er3+-doped near-stoichiometric LiNbO3 crystals

Near-stoichiometric Er∶LiNbO3 has many properties superior to the co ngruent one and has aroused extensive interest. It was obtained by the technique of vapor transport equilibration (VTE), its optical properties, including OH- absorption spectra, transmission spectra, emission spectra and the position of absorption edge were studied. The higher transmittance, the narrower of the peak width, the “blueshift" of the optical absorption edge, as well as the significant reduction of the OH- absorption band are indicative of the VTE treatment having brought the crystal closer to the stoichiometric composition. We also observed the increase of Er3+ luminescent intensity, and predicted that the VTE treatment must lead to the increase of fluorescence lifetime. The possible modification of materials in amplifier, self-frequency doubling laser and waveguide laser etc were expected.

The role of the Mg2+ ions in Cr3+ spectroscopy for near-stoichiometric LiNbO3crystals

The optical spectroscopy of Cr3+ ions doped into near-stoichiometricLiNbO3 crystals, pure and co-doped with MgO, has been investigated.In the near-stoichiometric LiNbO3:Cr(0.2 mol%):Mg(2 mol%) crystal,the optical spectra resemble those previously observed for congruentLiNbO3:Cr:MgO samples when the total MgO content exceeds the 4.6 mol%threshold. The coexistence of two types of Cr3+ centre ([Cr]Li and [Cr]Nb)characterized the optical and luminescence spectra of thissample. The concentration equilibrium between the two types ofcentre is strongly displaced towards the [Cr3+ ]Nbcentre, permitting us to obtain with accuracy the parameters ofthe broad bands. The R-line associated with the [Cr]Nbcentre is only observable in the low-temperature emission spectrum. The Fanoanti-resonance lines present have been observed to be more pronounced for thenear-stoichiometric samples than for congruent ones.

Reduced space-charge fields in near-stoichiometric LiTaO 3 for blue, violet, and near-ultraviolet light beams

The wavelength dependence of the space-charge field as induced in near-stoichiometric LiTaO3 crystals is investigated by measuring the bulk photogalvanic effect and the photoconductivity. LiTaO3 crystals of composition nearer to stoichiometry exhibit a pronounced reduction of the saturated space-charge field, which results from a larger increase in the photoconductivity relative to the photogalvanic effect. Especially, at near-UV wavelengths, the saturated space-charge field of the order of 0.1 kV/cm reaches a minimum that is two orders of magnitude lower than typical values observed in undoped near-stoichiometric LiNbO3.

Nonvolatile two-color holography in Mn-doped near-stoichiometric lithium niobate

Nonvolatile holograms are recorded in near-stoichiometric LiNbO3 doped with 8 ppm of Mn by use of a two-color holographic recording method. An IR laser with wavelength of 778 nm is used for writing and a UV laser with wavelength of 350 nm is used for gating. Dependences of two-color sensitivity and M/♯ on gating and writing intensities are presented. Significant improvements of sensitivity, M/♯, and dark decay are obtained compared with reduced near-stoichiometric LiNbO3. The obtained high two-color sensitivity of 0.21 cm/J with a gating intensity of 1.5 W/cm2 is attributed to large light-induced absorption of small polarons.

Ferroelectric domain inversion in near-stoichiometric lithium niobate for high efficiency blue light generation

LiNbO3 single crystals with a composition close to stoichiometry ([Li]/[Li+Nb]=0.496), 16 mm in diameter and 40 mm in length were grown by the Czochralski method using K2O flux. The domain reversal characteristics of near-stoichiometric LiNbO3 single crystals were investigated. The switching field required for 180° ferroelectric domain reversal in the near-stoichiometric crystal at room temperature was 7.5 KV/mm. This is about one third of the switching field required for conventional LiNbO3 crystals. Domain reversal (180°) in near-stoichiometric LiNbO3 samples of 1.0 mm thickness has been achieved. Samples have been evaluated by second harmonic generation and conversion efficiencies of up to 32% have been obtained.

Growth and property of stoichiometric LiNbO3 Crystal˚

Near-stoichiometric LiNbO3 and Fe:LiNbO3 were grown. The ultraviolet-visible absorption spectra, infrared absorption spectra, exponent gain coefficient, diffraction efficiency and response time of these crystals were measured. It is found that the response time of Nearstoichiometric LiNbO3 and Fe:LiNbO3 was one order of magnitude less than that of congruent LiNbO3

Characterization of two-color holography performance in reduced LiNbO_3:In

The two-color recording sensitivity S and the figure of merit M/# are measured in a reduced and congruent LiNbO(3):In crystal of 2.2-mm thickness. The results are compared with those before reduction treatment. It is found that S increases by an order of magnitude through reduction treatment but the M/# is unchanged. Measured values for S and M/# in the reduced crystal are found to be 3 x 10(-3) cm/J and 0.04, respectively, with a total writing intensity of 4.0 W/cm(2) at 780 nm and a gating intensity of 1.15 W/cm(2) at 488 nm. These values are close to those of near-stoichiometric LiNbO(3) crystals under similar recording conditions. We also show that photoconductivity plays an important role in the improvement of S in LiNbO(3):In.

Nonvolatile and quasi-nonvolatile holographic recording in near-stoichiometric lithium niobate doubly doped with Tb and Fe

We have observed that newly developed near-stoichiometric LiNbO 3 crystals doubly doped with Tb and Fe have three different types of energy levels: ultraviolet (UV) absorption centers just above the valence band, metastable shallow electron traps slightly below the conduction band, and deep traps located about 1.9 eV below the conduction band. Irradiation with UV light induced a stable absorption band extending from λ ≈650 nm to the absorption edge, which is caused by the photoinduced charge transfer from UV-sensitive absorption centers to deep traps (Fe 3+ ) via the conduction band. The electron lifetimes at shallow and deep traps could be controlled by doping concentrations. Based on these favorable energy states, nonvolatile two-color holographic recording has been demonstrated by use of 852 nm recording beams and UV gating light. Quasi-nonvolatile one-color recording at 532 nm has also been demonstrated in these codoped crystals. Hologram recording from the UV-exposed, colored state revealed a much improved sensitivity in comparison to that from the uncolored state. The obtained M/# was 1.73 for a 3.3 mm-thick crystal.

Photorefractive properties of near-stoichiometric LiNbO3 grown from congruent melts containing K2O

The photorefractive properties of nominally pure near-stoichiometric LiNbO3 crystals grown from congruent melts containing 11 mol % K2O are studied, and some photorefractive properties which are very different from those in congruent LiNbO3 crystals are observed. Large stationary energy transfer (Γmax=25 cm−1) exists in near-stoichiometric crystals though the photovoltaic field is dominant. The light-induced scattering fanning is markedly asymmetric in the crystals, which is very different from the symmetric fanning observed in congruent ones. Our analysis shows that these differences are caused by the large photovoltaic field and the relatively small density of filled traps in as-grown near-stoichiometric LiNbO3.

A synchrotron X-ray diffraction analysis of near-stoichiometric LiNbO3

Abstract Single-crystal diffraction data was collected at 120 and 294 K for an approximately spherical LiNbO3 crystal, 16 μm in diameter, using focused λ=0.7516 (1) Å wavelength synchrotron X-radiation and a fast avalanche photodiode (APD) detector. Extinction was significantly less than that recorded for previous single-crystal X-ray diffraction experiments. Anharmonic atomic displacements of Nb and Li were refined using a Gram-Charlier expansion to the fourth order. There was no significant change in the topography of the difference electron density (Δρ) map after anharmonic refinement. An accumulation of electrons present at the vacant site in the 294 K Δρ map was not evident in the 120 K Δρ map, indicating a migration of Li atoms to the vacant site as the temperature increased. The orientation of the electron density around Nb flipped by 90° going from 120 to 294 K.

Fast photorefractive response in strongly reduced near-stoichiometric LiNbO_3 crystals

The photorefractive response time of LiNbO(3) crystal is of the order of minutes, and such a long response time limits the crystal's practical applications. We report the photorefractive properties of nominally pure near-stoichiometric LiNbO(3) crystal that is strongly reduced in vacuum. A short photorefractive response time of the order of 100 ms is measured at a wavelength of 514.5 nm, with incident light intensity of 1.6 W/cm (2) , and possible corresponding mechanisms are discussed. To our knowledge this is the first experimental evidence of a subsecond photorefractive response in pure LiNbO(3) crystals. The diffraction efficiency of a holographic grating written in this reduced crystal is low but can be enhanced by an externally applied electric field.

Electron paramagnetic resonance investigation of the photochromic effect in near-stoichiometric LiNbO3 with applications to holographic storage

Near-stoichiometric LiNbO3 (LiNbO3, LiNbO3:Tb, and LiNbO3:Tb,Fe) crystals have exhibited a reversible photochromic effect at room temperature. Irradiation by UV light induced a visible absorption band extending from λ≈650 nm to the absorption edge, which could be bleached by subsequent illumination with visible light. Electron paramagnetic resonance (EPR) analysis at 15 K revealed an EPR signal of g≈2.0 appearing after UV irradiation with a simultaneous decrease in the Fe3+ signal intensity. This implies that the Fe3+ ions (natural impurities or dopants) act as electron traps and the induced absorption band is caused by the charge transfer from UV-sensitive absorption centers to Fe3+ ions via the conduction band. Using the induced absorption band, a high-sensitive holographic recording has been achieved with a continuous-wave green laser at λ≈532 nm. In addition, the fast response to UV light made it possible to quickly erase the stored information by UV illumination.

Domain reversion process in near-stoichiometric LiNbO 3 crystals

LiNbO 3 single crystals with a composition close to stoichiometry ([Li]/[Li+Nb]=0.496) were grown by Czochralski method with K 2O flux for the domain reversal characteristics investigation. The switching field for the domain reversal in the near-stoichiometric crystal was 8.0±0.5 kV/mm, which is about one-third of the switching field required for the congruent LiNbO 3 crystals. The resulted long switching time enabled us to observe, for the first time, clear domain growth process in near-stoichiometric LiNbO 3 crystals. Slow growth is also found of both the needle-shaped nuclei into larger domains on the existing domain wall and along the electrode fingers.

Realization of fast photorefractive response speed in near-stoichiometric LiNbO3 crystals

Nominally pure near-stoichiometric LiNbO3 crystals are grown by Czochralski method from the congruent melt with addition of 11 mol% K2O. The photorefractive properties of strongly reduced near-stoichiometric LiNbO3 crystals are studied, and fast photorefractive buildup time (191 ms) and erasure time (78 ms) are observed at an intensity of 1.3 W/cm2. As we known, so fast response speed is firstly observed in nominally pure LiNbO3 crystals.

Nonvolatile two-color holography in indium-doped lithium niobate

We demonstrate nonvolatile two-color holography in an as-grown congruent LiNbO3:In crystal of 2.2 mm thickness. We show that the nonvolatile two-color holographic performance of the crystal is significantly improved as compared with that of other congruent LiNbO3 crystals reported so far. The figure of merit M/# and the two-color recording sensitivity S are measured to be ∼0.04 and ∼3.5×10−4 cm/J, respectively, with a writing intensity of 4 W/cm2 at 780 nm and a gating intensity of 1.15 W/cm2 at 488 nm. These values are comparable to those of unreduced near-stoichiometric LiNbO3 crystals under similar recording conditions.

Photochromic effect in near-stoichiometric LiNbO3 and two-color holographic recording

We have observed that newly developed near-stoichiometric LiNbO3 crystals have three different types of energy levels: ultraviolet (UV) absorption centers just above the valence band, metastable shallow electron traps slightly below the conduction band, and deep traps located about 1.9 eV below the conduction band. Irradiation with UV light induced a stable absorption band extending from λ ≈650 nm to the absorption edge, which is caused by the photoinduced charge transfer from UV-sensitive absorption centers to deep traps via the conduction band. The electron lifetimes at shallow and deep traps could be controlled by doping elements and concentrations. Based on these favorable energy states, nonvolatile two-color holographic recording has been carried out by use of 852 nm recording beams and UV gating light. Nonvolatile readout by a single 852 nm beam was demonstrated for several hours. It was found that two-color recording from the pre-exposed, colored state can also be an attractive way of achieving nonvolatile readout. In this article, we investigate the photochromic effect observed in near-stoichiometric LiNbO3 and its use in two-color holographic recording.

Angle-multiplexed hologram storage in LiNbO_3:Tb, Fe

We have performed hologram multiplexing with a cw laser at lambda=532 nm in a near-stoichiometric LiNbO(3) :Tb, Fe crystal that exhibits a visible absorption band induced by UV light. When 50 plane-wave holograms were angle multiplexed from the UV preexposed state, the recording sensitivity gradually decreased, owing to bleaching of the induced absorption. However, a conventional recording schedule of multiplexing many holograms of equal diffraction efficiency can be used in this photochromic material. The gradual decrease in recording sensitivity throughout multiplexing can be compensated for by the increased erasure-time constants of later-written holograms. An M/# of 1.73 was obtained in a crystal of 3.3-mm thickness.

Domain Reversal Characteristics of Near-Stoichiometric LiNbO3 Crystals

The domain reversal characteristics of near-stoichiometric LiNbO3 ([Li]/[Li+Nb] = 0.496) single crystals, which are grown by the Czochralski method with K2O flux, are investigated. The switch field for 180° ferroelectric domain reversal in the near-stoichiometric LiNbO3 crystal is 8.0 ± 0.5 kV/mm, which is only one third of the switching field required for the congruent LiNbO3 crystals. We have successfully achieved 180° domain reversal in near-stoichiometric LiNbO3 samples of 1.0 mm thickness.

Role of stoichiometric point defect in electric-field-poling lithium niobate

Non-congruent LiNbO3 crystals has been used to determine the influence of the intrinsic defect density in the ferroelectric domain inversion mechanism. The poling processes have been carried out either at high temperature or at RT throughout the electric-field-poling technique. It is shown that the coercive field of LiNbO3 crystals is strongly influenced by the intrinsic defect density in the crystals. Moreover it is shown that it is possible to realign the ferroelectric domain structure of near-stoichiometric LiNbO3 crystals to prepare periodic poled structures which allows second harmonic generation at shorter wavelengths than with congruent crystals at a fixed wavelength.