Doped Lithium Niobate Crystals Research Articles

EVIDENCE OF MULTIPARTICLE OPTICAL CENTERS IN LiNbO3:Er3+-Yb3+ CRYSTAL

A characteristic feature of doped lithium niobate crystals is the liability of its structure to form complex multiparticle optical centers, which consist of two or more closely spaced trivalent impurity ions, even at low concentrations of dopants. This paper presents the results of studies aimed at identifying the characteristics of such centers in the fluorescent signal of the crystal LiNbO 3: Er 3+- Yb 3+ by combination of pulsed excitation at a wavelength of 808 nm with continuous excitation at 980 nm.

Orientationally polarized dependence of image contrast in doped lithium niobate crystals

In lithium niobate crystals doped with photorefractive iron and copper admixtures when lightening objects with broadband incoherent radiation one can observe typical orientation and polarized dependences of image contrast. The record does not realize at orientation of light bands in the image parallel to the polar axis of the crystal.

Pump and probe measurements of optical amplification at 584 nm in dysprosium doped lithium niobate crystal

Usually the Dy 3+ ions have been used as laser sources associated with the 6H 13/2 → 6H 15/2 transition about 3 μm. However, these ions exhibit emissions in the blue (480 nm) and in the yellow (575 nm) that are promising for laser operations. In this work, positive optical gain about 1.9 cm −1 (∼8.2 dB/cm) has been observed at 584 nm in trivalent dysprosium doped lithium niobate crystal, Dy:LiNbO 3, using a pump and probe experimental setup. High power laser pulses at 475 nm have been used as the pump source in order to strongly populate the 4F 9/2 level of the Dy 3+ ions due to ground state absorption. Low signal beam cw radiation at 584 nm has been used as the probe beam to stimulate the emission associated with the 4F 9/2 → 6H 13/2 electronic transition of the Dy 3+ ions. The process has been modelled as a four level system and the level populations have been analysed and simulated in order to study the gain dynamics. Optical amplification of the probe signal has been observed during the first 300 μs, which represents a good agreement between the measured lifetime of the 4F 9/2 level and the proposed simulation.

Influence of dopant concentration on the structural and optical characteristics in Ti doped (2 and 5 mol%) LiNbO 3 nonlinear optical single crystal

Congruent lithium niobate single crystal with titanium ion dopant (2 and 5 mol%) has been successfully grown in an air by Czochralski technique with Automatic Diameter Control facility. The results of analysis of the influence of the Ti dopant concentration on X-ray diffraction, refractive index, optical absorption edge and FTIR in lithium niobate single crystals were presented. High Resolution X-ray Diffraction technique was used to study the crystalline quality through full width half maximum values with the presence of dopant incorporation. The increase of refractive index values observed for doped samples than pure sample by prism coupling technique with different laser sources. The position of the fundamental absorption edge in the near UV region was analyzed by UV–Vis spectra. FTIR spectra revealed that 5 mol% Ti doped lithium niobate crystals clearly suppresses the OH − defect sites than 2 mol% which indicates the limited influence of the humidity. The reason for OH − defect site suppression is discussed. Also the obtained results from all measurements are compared with pure congruent lithium niobate crystal.

Lithium niobate crystals doped with iron by thermal diffusion: Relation between lattice deformation and reduction degree

We report, to our knowledge for the first time, on the experimental observation that the maximum lattice deformation induced at the surface of iron doped lithium niobate crystal by thermal diffusion depends on both the Fe concentration and the reduction degree of the doped layer itself. By exploiting a simple linear model, we suggest a description of this experimental evidence and we point out a procedure that allows the characterization of the in-depth profile of the [Fe2+]/[Fe3+] ratio.

Luminescence-induced photorefractive spatial solitons

We report the observation of spatial confinement of a pump beam into a photorefractive solitonic channel induced by luminescence [luminescence induced spatial soliton (LISS)]. Trapped beams have been obtained in erbium doped lithium niobate crystals at concentrations as high as 0.7 mol % of erbium. By pumping at 980 nm, erbium ions emit photons at 550 nm by two-step absorption, wavelength which can be absorbed by lithium niobate and originates the photorefractive effect. The luminescence at 550 nm generates at the same time the solitonic channel and the background illumination reaching a steady-state soliton regime.

The threshold effect of incident exposure energy flux for photorefractive light-induced scattering in doped lithium niobate crystals

The threshold effect of incident exposure energy flux is proposed to measure the light-induced scattering in doped LiNbO 3 crystals. The dependences of the exposure energy flux threshold on the concentration of damage-resistant dopants as well as oxidation–reduction treatment are investigated experimentally in doped LiNbO 3 crystals. The results show that doping with damage-resistant dopants and oxidation–reduction treatment are simple and effective methods to control and optimize the photorefractive properties of doped LiNbO 3 crystals. The high signal-to-noise ratio reconstructions are also demonstrated experimentally according to the incident exposure energy flux threshold in triply-doped LiNbO 3 crystals. Comparing with the known incident intensity threshold method, the exposure energy flux threshold method is more effective for estimating the light-induced scattering of photorefractive recording materials.

Growth of Nd3+ doped LiNbO3 crystals using Bridgman method and its spectral properties

The growth of Nd3+ doped lithium niobate crystals using Bridgman method has been reported in this paper. By means of the optimum conditions such as proper feed materials, sealed platinum crucibles, growth rate of 1–1·5 mm/h and temperature gradient of 30–35°C/cm across the solid-liquid interface under the furnace temperature of 1300°C, single crystals containing Nd3+ ion with 0·54 mol% concentration were obtained. X-ray diffraction and ICP-AES were used to characterize the crystals and its composition. The absorption, emission and fluorescence lifetime are also measured. Based on the Judd-Ofelt theory, we obtained the optical parameters of the crystal such as the luminescent quantum efficiency, the radioactive lifetimes, the branching ratios and the emission cross-section.

Light-induced domain reversal in doped lithium niobate crystals

Light-induced domain reversals of Mg-doped, Zr-doped, and nominally pure LiNbO3 crystals were investigated with focused 514.5 nm laser beams. It was found the light-reduced values of electric field for domain reversal are almost the same, about 3 kV/mm. Inverted domains always first nucleate on the −c surface within the illuminated region but appear at the edge of the spot on the +c surface. The light-induced inverted domains are hard to be reversed by application of a reverse electric field or by heating to temperatures as high as 600 °C. According to these experimental results, we present a qualitative model on the light-induced domain reversal process in LiNbO3.

Depth Resolved Study of the Composition and Polaron Luminescence of Fe:LiNbO3 Diffused Crystals

Iron-doped lithium niobate crystals attract great attention for practical applications such as holographic storage systems, all-optical photonic devices and reconfigurable narrow band holographic filters. In this work we present some preliminary experimental results on the investigation of compositional and photorefractive properties at the surface of iron doped lithium niobate crystals prepared by thermal diffusion in oxygen atmosphere of a thin film deposited by magnetron sputtering. In particular both the Secondary Ion Mass Spectrometry and confocal polaron microluminescence are exploited.

Thermal lens spectrometry in pyroelectric lithium niobate crystals

In this work, the light-induced lens effect due to thermal and/or photorefractive processes was studied in pyroelectric (undoped and Fe2+-doped) lithium niobate crystals (LiNbO3) using thermal lens spectrometry with a two-beam (pump–probe) mode-mismatched configuration. The measurements were carried out at two pump beam wavelengths (514.5 and 750 nm) to establish a full understanding of the present effects in this material (thermal and/or photorefractive). We present an easy-to-implement method to determine quantitative values of the pyroelectric coefficient (dP s/dT), its contribution to the thermal effect and other thermo-optical parameters like thermal diffusivity (D), thermal conductivity (K) and temperature coefficient of the optical path length change (ds/dT). These measurements were performed in LiNbO3 and LiNbO3:Fe (0.1 ppm Fe2+) crystals with c axis along the direction of laser propagation.

Sensitized holmium upconversion emission in LiNbO3 triply doped with Ho3+, Yb3+, and Nd3+

Observed is 15 times enhancement of Ho3+ green upconversion (UC) emission in Ho3+(0.75 mol %)/Nd3+(0.75 mol %) doped lithium niobate crystal after incorporating with 4 mol % of Yb3+ under 800 nm femtosecond laser excitation. Yb3+ acting as bridging ion increases the energy transfer efficiency from Nd3+ F43/2 level to Ho3+ in these triply doped systems. The nonradiative energy transfer efficiency from Nd3+ F43/2 to Yd3+ F25/2 level can reach to 56%. The UC emission mechanisms are proposed based on spectral, kinetic, and pump power dependence analyses. The green UC emission is accomplished through a multi-ion interaction involving ground state absorption by the Nd3+ (I49/2→H29/2,F45/2) followed by three successive energy transfer processes involving one Nd3+-Yb3+ (F43/2+F27/2→I49/2+F25/2) and two Yb3+-Ho3+ pairs (F25/2+I58→F27/2+I56 and F25/2+I56→F27/2+S52,F54); whereas the red is dominated by excited state absorption of the Ho3+ (I57→F55) following two successive energy transfer processes involving the Nd3+-Yb3+ (F43/2+F27/2→I49/2+F25/2) and Yb3+-Ho3+ pairs (F25/2+I58→F27/2+I56) followed by the Ho3+ nonradiative relaxation to I57.

Writing of crystal lines and its optical properties of rare-earth ion (Er 3+ and Sm 3+) doped lithium niobate crystal on glass surface formed by laser irradiation

A glass of Li 2O–Nb 2O 5–SiO 2–B 2O 3 with an addition of CuO or Sm 2O 3 crystallizing in nonlinear optical lithium niobate LiNbO 3 is developed. Crystalline lines of Lithium Niobate have been fabricated by continuous wave laser irradiation of the glass surface. The crystalline laser written lines have been found, by means of micro-Raman and Second Harmonic experiments, to be well oriented with its c-axis pointing along the laser scanning direction. The analysis of the confocal micro luminescence spectra obtained at the crystalline line have confirmed the incorporation of rare-earth ions in the crystalline network.

Two-photon-excited luminescence in a Tb^3+-doped lithium niobate crystal pumped by a near-infrared femtosecond laser

Blue (487.6 nm), green (544.1 nm), yellow (582.1 nm), and red (623.6 nm) upconversion (UC) luminescences are achieved in a Tb(3+)-doped lithium niobate crystal when an 800 nm femtosecond laser is loosely focused onto the sample at room temperature. The relationship between UC luminescence intensity and the pump energy indicates that a two-photon excitation process is dominant in this UC luminescence phenomenon. The Tb(3+) sensitive temperature dependence of the luminescence intensity is demonstrated via an obvious reduction of luminescence intensity with durative laser irradiation.

Domain reversal properties and refractive index changes of magnesium doped lithium niobate upon ion exposure

Irradiation of optical damage resistant, magnesium doped lithium niobate crystals with fast, high-energy He2+3 ions changes important material properties. In the interaction region, where the ions transmit through the material, the ferroelectric coercive field EC is diminished from 6.0kVmm−1 down to 5.0–5.4kVmm−1 after transmission of 41MeV He2+3 particles. This enables easier domain reversal in irradiated crystals compared to untreated material. Besides, large changes of the refractive index of the crystals on the order of 6×10−3 are induced by the treatment. Moderate annealing treatments do not diminish Δn, but refresh the coercive field.

Ni2+掺杂近化学计量比铌酸锂晶体的生长及光谱特性

Ni²⁺掺杂近化学计量比铌酸锂晶体的生长及光谱特性

Investigation on photorefractive properties of doped lithium niobate

AbstractCu:LiNbO3 crystal and Fe:Cu:LiNbO3 crystals were grown by the Czochralski method from congruent melt. The OH‐ absorption spectrum of doped lithium niobate crystals was measured. The photorefractive properties of doped crystals were studied by the two‐wave coupling method. The results of the two‐wave coupling experiments showed that as the concentration of doping ions increased, the diffraction efficiency and the dynamic range enhanced, the holographic response time shortened. The recording time of Fe(0.10wt%): Cu(0.10wt%): LiNbO3 crystal is only a tenth of that of Cu(0.05wt%): LiNbO3 crystal. Among all samples, the dynamic range of the Fe(0.10wt%): Cu(0.10wt%): LiNbO3 crystal was the most largest (up to 40.78). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Bond-Energy Study of Photorefractive Properties of Doped Lithium Niobate Crystals

A bond-energy method is proposed to study the photorefractive behaviors of doped lithium niobate (LN) crystals on the basis of the bond valence model. The dopant occupancy in the LN matrix can be determined by comparing the deviation of its bond energy in different lattice locations at both Li+ and Nb5+ sites. Furthermore, the optical damage resistant (ODR) ions can be easily distinguished from those dopants by the increased bond energy when they are introduced into the Nb5+ sites, which gives us a bond-energy criterion when searching for new ODR ions. The suppression of the photorefractive effect of doped ODR ions more than their threshold concentration can be attributed to the increase of photoconductivity; it has been shown that such a phenomenon results from the decreased concentration and mobility of the charge carriers with increasing bond energy. According to our bond-energy viewpoint, the threshold concentration of ODR ions in both congruent and near-stoichiometric LN crystals can be determined quantitatively via the assumption that all doped LN crystals should endure a same variation of bond energy.

Photoconductive and Electro-Optical Properties of Hf Doped Lithium Niobate Crystals

The optical and electrical properties of a set of Hf doped Lithium Niobate crystals with doping level from 2 to 8 mol% are reported. The photoconductivity is found to increase remarkably for Hf concentration equal and above 4 mol%. In contrast, the electro-optical coefficients keep almost constant with the crystal structural properties preserved. The conclusion is drawn that the decrease of the photorefractive response in Hf doped Lithium Niobate recently observed by other authors is mainly related to the increased photoconductivity.

Strongly sublinear growth of the photorefractive effect for increasing pump intensities in doped lithium-niobate crystals

The pump intensity dependence of the birefringence change due to the photorefractive effect in Hf-doped and Mg-doped congruent lithium niobate (LN) crystals is reported. The birefringence change is found to be very weakly dependent on the light intensity. This is in strong contrast with the behavior of undoped congruent LN, and much more similar to what happens in stoichiometric LN. The data also indicate that the minimum HfO2 concentration necessary to strongly reduce photorefractivity is lower than the value of 4 mol % indicated by previous experiments.