Near-stoichiometric LiNbO3 Research Articles

Growth of large size near-stoichiometric lithium niobate single crystals with low coercive field for manufacturing high quality periodically poled lithium niobate

Periodically poled lithium niobate is an important component in mid-infrared lasers due to its perfect nonlinear optical property. However, fabrication of high-power laser based on periodically poled lithium niobate is still a great challenge, because lithium niobate crystal wafer with large thickness for large clear aperture periodically poled lithium niobate is hard to be completely poled due to the high coercive field of commercial congruent lithium niobate (LiNbO3). In this work, we have successfully grown 3-inch near-stoichiometric LiNbO3 by efficiently regulation of temperature gradient with the aid of thermal field simulation. Characteristics on the as-grown large size near-stoichiometric LiNbO3 proved that the [Li]/[Nb] of crystal is around 1:1, and the crystal possesses a typical single domain with high optical homogeneity. Compared with congruent and Mg-doped LiNbO3, the near-stoichiometric LiNbO3 crystal has better piezoelectric performance. The forward and backward coercive fields of the near-stoichiometric LiNbO3 crystal were 2.90 kV/mm and 2.43 kV/mm, respectively, which were approximately 8 times lower than that for CLN (21 kV/mm). The periodically polarization experiments proved that only 1.36 kV can realize the complete polarization for a 1 mm thick near-stoichiometric LiNbO3 wafer. The mass-productive near-stoichiometric LiNbO3 crystals grown in this work will have great applications in high-power mid-infrared solid-state laser.

Defect structure of near-stoichiometric Mg-doped LiNbO3 crystals prepared by different method

Mg-doped (0.5 mol% and 1 mol%) near-stoichiometric lithium niobate (nSLN) crystals were prepared by Li-rich growth method and VTE technique, respectively. Their defect structures were characterized by high resolution X-ray diffraction (HRXRD), OH– vibrational spectra, Raman spectra and ultraviolet-absorption spectra. The crystalline perfection analysis by HRXRD measurements reveals that diffraction curves of all Mg-doped nSLN crystals contain a single diffraction peak, signifying the absence of internal sub-grain boundary. In comparison to SLN-R crystals (grew by Li-rich growth method), the crystalline quality of SLN-V crystals (prepared by VTE technique) is better. The results indicate that MgNb3+ defect complexes exist in SLN-R crystals, even if Mg content is under the threshold, and that most of Mg2+ occupy Li site, extra low Mg2+ and the unavoidable trivalence impurities occupy Nb site in SLN-V crystals. The cation sublattice disorder of SLN-R crystals increases relative to SLN-V crystals. A remarkably weak OH– absorption band indicates that H+ content is extremely low in SLN-V crystals.

Distinct temporal evolution of PILS distribution in congruent and near-stoichiometric LiNbO3:Fe crystals detected by in-situ dynamic angular spectra

Distinct temporal evolution of PILS distribution in congruent and near-stoichiometric LiNbO3:Fe crystals detected by in-situ dynamic angular spectra

Conditions of application of LiNbO3 based piezoelectric resonators at high temperatures

We have researched piezoelectric, dielectric properties and ion conductivity in near-stoichiometric LiNbO3 crystals (NSLN) in order to establish conditions for application of LiNbO3 based piezoelectric resonators in the high-temperature region (∼300–850 K) which implies high conductivity. NSLN crystals have been grown from a congruent melt (R=Li/Nb≈0.946) with 5.5 wt% K2O flux. We have revealed that piezoelectric resonance in NSLN crystals appears in a certain temperature range ΔT and resonance frequencies ΔωR provided that τV−1≪ωR. The temperature range ΔT corresponds to certain values of conductivity (Δσ) and relaxation time (ΔτV) range. We have concluded that application of high-temperature piezoelectric resonators based on crystal NSLN is possible at certain kinetic parameters of charge transport in a temperature range ΔT and at piezoelectric resonance frequencies corresponding to the temperature range.

Injection pulse-seeded terahertz-wave parametric generator with gain enhancement in wide frequency range.

An injection pulse-seeded terahertz-wave parametric generator (ips-TPG) has been demonstrated with gain enhancement in wide tuning range. Theoretical analysis denotes that the compensation of initial Stokes energy is favorable to the THz gain enhancement in wide frequency range, which is attributed to the improvement on interaction of stimulated polariton scattering (SPS) and difference frequency generation (DFG) processes. In the experiment, the THz frequency tuning range from 1.04 THz to 5.15 THz was achieved based on near-stoichiometric LiNbO3 (SLN) crystal. Compared with the traditional terahertz parametric oscillator (TPO) under the same experimental conditions, a significant enhancement of THz output energy was occurred in high frequency range. As the THz frequency increased from 1.9 THz to 3.6 THz, the enhancement ratios from 1.6 times to 34.7 times were obtained. Besides, the 3dB bandwidth of ips-TPG was measured to be 2.1 THz, which was about 2.6 times that of SLN-TPO. This THz parametric source with a relative flat gain in wide frequency range is suitable to a variety of practical applications.

Mechanism of the UV band-edge photorefractivity enhancement in near-stoichiometric LiNbO3

The UV photorefractive properties of near-stoichiometric LiNbO3 single crystal are found to be significantly enhanced compared with the congruent one at 325 nm. The temperature dependence of the band edge of near-stoichiometric LiNbO3 crystal is investigated. Significant thermal-induced spectral shift in band gap which obeys the Bose-Einstein expression is observed, and the fundamental band gap at zero absolute temperature is found to be much larger than the congruent one. New absorption bands near the UV band edge which are much stronger in the near-stoichiometric LiNbO3 than those in the congruent LiNbO3 crystal show up at temperatures lower than ∼400 K. Note that the UV photorefractivity is enhanced in SLN, which has exactly the same tendency as the absorption strength.

Light-induced absorption and optical damage in Sc-, Mg-, and Zn-doped near-stoichiometric LiNbO3

By measuring the ultraviolet-light-induced absorption in Sc-, Mg- and Zn-doped near-stoichiometric lithium niobate (LiNbO[Formula: see text], we find that the steady-state ultraviolet-light-induced absorption coefficient changes with respect to the doping concentration. There is a strong ultraviolet-light-induced absorption when doping concentration is below its photorefractive threshold and a really weak absorption when the crystal is highly doped. We also use OH[Formula: see text] infrared absorption spectra and the transmitted light spot distortion method to verify the result. Thus, we can determine if the doping level in these doped near-stoichiometric LiNbO3 crystals is above or below their photorefractive threshold by measuring the ultraviolet-light-induced absorption.

Impact of crystal defect on upconverted white-light emission in lanthanide-doped LiNbO3 single crystal

Efficient and stable upconversion white-light is critical for optical applications. Herein, LiNbO3:Ho/Yb/Tm single crystals with different [Li]/[Nb] ratios were grown by Czochralski method. It is found that the crystal structures are not changed with [Li]/[Nb] ratios which varied from 0.946 to 0.991 in the single crystals. The RE ion concentrations are reduced faintly and crystal structures approach perfect with increasing the [Li]/[Nb] ratios. More ideal upconversion white-light is obtained when the [Li]/[Nb] ratio is close to 1. The integrated effects of intrinsic defect concentrations and RE ion concentrations affect the upconversion emission intensity of the single crystal. The lower crystal defect in the bulk single crystal has a positive impact on the enhancement of upconversion emission intensity. In addition, all the CIE coordinates are hardly changed with [Li]/[Nb] ratios and located in the white-light region. The upconversion mechanism analysis indicates the longer lifetimes of intermediate energy levels and the shorter lifetimes of luminous energy levels are beneficial to the upconversion process, and the larger intensity variation of upconversion emission arises from its greater lifetime variations of energy levels. The near-stoichiometric LiNbO3:Ho/Yb/Tm single crystal demonstrates potential applications in stable white-light devices and photoelectric instruments.

Growth and mechanical properties of near-stoichiometric LiNbO3 crystal

The near-stoichiometric LiNbO3 crystal (SLN) with dimension of Φ25 × 45 mm3 has been successfully grown from Li-rich solutions by the Czochralski method. The results of X-ray rocking curve measurement indicate that the as-grown SLN crystal has a relatively high crystallinity. The mechanical properties of SLN crystal were determined by nanoindentation method. The hardness and elastic modulus values obtained revealed higher values for (300) plane of SLN crystal (10.02 and 225.6 GPa) compared to the (600) plane of SLN crystal (7.09 and 202.8 GPa). Different hardness and elastic modulus values for different planes suggest anisotropic character of mechanical properties in SLN crystals. The distribution characteristic of residual stress in SLN crystal along the optical axis is analyzed by means of photoelasticity effect.

First-principle calculation of electronic structures and absorption spectra of lithium niobate crystals doped with Co and Zn ions

In this paper, the electronic structures and absorption spectra of Co doped and Co, Zn co-doped LiNbO3 crystals are studied by the first-principle using the density functional theory, to explore the characteristics of charge transfer in Co, Zn co-doped LiNbO3 crystals, and to build the relationship between these characteristics and the holographic storage quality. The basic model is built as a supercell structure of 211 of near-stoichiometric pure LiNbO3 crystal with 60 atoms, including 12 Li atoms, 12 Nb atoms and 36 O atoms. Four models are established as the near-stoichiometric pure LiNbO3 crystal (LiNbO3), the cobalt doped LiNbO3 crystal (Co:LiNbO3), the zinc and cobalt co-doped LiNbO3 crystal [Co:Zn(L):LiNbO3] with doping ions at Li sites, and the other zinc and cobalt co-doped LiNbO3 crystal [Co:Zn (E):LiNbO3)] with zinc ions at Li sites and Nb sites. The last two models would represent the concentration of Zn ions below the threshold (6 mol%) and near the threshold, respectively. The charge compensation forms are taken as CoLi+-VLi-, CoLi+-ZnLi+-2VLi- and CoLi+-ZnNb3--2ZnLi+ respectively in doped models. The results show that the conduction band and valence band of pure LiNbO3 crystal are mainly composed of O 2p orbit and Nb 4d orbit respectively, and energy gap is 3.48 eV. The band gap of the doped LiNbO3 crystal is narrower than that of pure LiNbO3 crystal, due to the Co 3d and Zn 3d orbit energy levels superposed with that of O 2p orbit energy levels, and thus forming the upside of covalent bond. The band gap of Co:LiNbO3 crystal is 3.32 eV, and that of Co:Zn:LiNbO3 crystals are 2.87 eV and 2.75 eV respectively for Co:Zn(L):LiNbO3 and Co:Zn(E):LiNbO3 model. The Co 3d orbit is split into eg orbit and t2g orbit with different energies. The absorption peak at 2.40 eV appears in the band gap of Co:LiNbO3 crystal, which is attributed to the transfer of the Co 3d splitting orbital t2g electrons to conduction band. The absorption peaks of 1.58 eV and 1.10 eV could be taken as the result of eg electron transfers of both Co2+ and Co3+ in crystal, especially the latter ion. These two absorption peaks are obviously enhanced in Co:Zn (E):LiNbO3 crystal compared with in other samples in this paper. Based on that, it could be proposed that a charge transfer between Zn2+ and Co2+ as Co2++Zn2+Co3++Zn+ exist in the crystal, which results in the decrease of eg orbital electron number, but hardly affect the t2g orbital electron. The Co ion in crystal could act as the deep-level center (2.40 eV) or the shallow-level center (1.58 eV) with the different accompanying doped photorefractive ions in the two-light holographic storage applications. In both cases, the choice of Zn ion concentration near threshold could be helpful for the photo damage resistance and recording light absorption in storage applications.

Energy scaling and extended tunability of terahertz wave parametric oscillator with MgO-doped near-stoichiometric LiNbO3 crystal.

A widely tunable, high-energy terahertz wave parametric oscillator based on 1 mol. % MgO-doped near-stoichiometric LiNbO3 crystal has been demonstrated with 1064 nm nanosecond pulsed laser pumping. The tunable range of 1.16 to 4.64 THz was achieved. The maximum THz wave output energy of 17.49 μJ was obtained at 1.88 THz under the pump energy of 165 mJ/pulse, corresponding to the THz wave conversion efficiency of 1.06 × 10-4 and the photon conversion efficiency of 1.59%, respectively. Moreover, under the same experimental conditions, the THz output energy of TPO with MgO:SLN crystal was about 2.75 times larger than that obtained from the MgO:CLN TPO at 1.60 THz. Based on the theoretical analysis, the THz energy enhancement mechanism in the MgO:SLN TPO was clarified to originate from its larger Raman scattering cross section and smaller absorption coefficient.

First-principles study on the electronic structures and the absorption spectra of In: Mn: LiNbO3 crystals

The electronic structures and the absorption spectra of the indium and manganese codoped LiNbO3 crystals and their comparative groups are investigated by first-principles based on the density functional theory. The supercell crystal structures are established with 60 atoms, including four models:the near-stoichiometric pure LiNbO3 crystal (LN), the manganese doped LiNbO3 crystal (Mn:LN, charge compensation model as MnLi+-VLi+), the indium and manganese codoped LiNbO3 crystal (In:Mn:LN, charge compensation model as InLi2+-MnLi+-3VLi+), and the other indium and manganese codoped LiNbO3 crystal (In(E):Mn:LN, charge compensation model as InLi2+-InNb2--MnLi+-VLi+). The results show that the extrinsic defect levels within the forbidden band of Mn:LN crystal are mainly contributed by Mn 3d orbital electrons, which also affect the top of the valence band. The band gap of Mn:LN about 3.18 eV is narrower than that of LN; the band gaps of In:Mn:LN and In(E):Mn:LN sample are 2.82 and 2.93 eV respectively. The electron density of state (DOS) of manganese codoped LiNbO3 crystal shows that the orbits of Mn 3d, Nb 4d and O 2p superpose each other, i.e., forming covalent bonds, which result in conduction and valence bands shifting toward low energy. The indium ion does not contribute the extrinsic energy level within forbidden band, it affects the band gap through changing O2- electron cloud shape. The band gap narrows down if the indium ions occupy lithium ion positions, and becomes broad if the indium ions occupy niobium ion positions. It is found that the Mn:LN, In:Mn:LN and In(E):Mn:LN samples display the absorption peaks at 3.25, 3.11, 2.97, 2.85, 2.13 and 1.66 eV. The last absorption peak is contributed by the electron transferring from the Mn2+ energy level to conduction band, and the doping of indium ions leads to attenuation of this peak. The peak at 2.13 eV relates to the Mn3+, it is enhanced by the doped indium ions. The indium ions in crystal would influence the absorption, which relates to manganese ions, by transforming manganese ion valence via the formula as m Mn2++In3+→Mn3++In2+, that is, with the doping of the indium ions, the photorefractive center Mn2+ concentration decreases, which is responsible for the absorption peak at 1.66 eV. It must be mentioned that the Mn2+ possesses not only the shallow levels as mentioned previously, but also the deep ones which are responsible for the absorptions at 2.85 eV and other high energies. For the indium and manganese codoped LiNbO3 crystals, if the recording light is chosen at near 1.66 eV (748 nm), the relatively low concentration of indium ions is proposed to be chosen to achieve the high recording sensitivity.

Study on the partial self-recovery process in Fe-doped lithium niobate crystals

We study the temperature and pump-intensity dependence of the temporal evolution of photoinduced light scattering in congruent, hafnium-codoped and near-stoichiometric LiNbO3:Fe crystals. A partial self-recovery process is found in the evolution trend of photoinduced light scattering in all crystals at room temperature while it disappears at −190 °C or above 120 °C. Moreover, a “scattering acceleration effect” is found for both Hf-doped and near-stoichiometric crystals. Basing the saturation refractive-index modulation the partial self-recovery process is explained by the slow recombination of light-induced space charges of the noise gratings with high wave vectors. The “scattering acceleration effect” is connected with the activation of the protons located around Fe ions and their impact on the photoconductivity.

Near-stoichiometric Ti-diffused LiNbO(3) strip waveguide doped with Zr(4+).

We report a near-stoichiometric Ti:Zr:LiNbO(3) strip waveguide fabricated from a congruent substrate with a technological process in the following sequence: Zr4+-diffusion-doping, diffusion of 8-μm-wide, 100-nm-thick Ti strips, and post-Li-rich vapor transport equilibration. We show that Zr(4+)-doping has little effect on the LiNbO(3) refractive index, and the waveguide is in a near-stoichiometric environment. The waveguide well supports both the TE and TM modes, shows weak polarization dependence, is in single mode at the 1.5 μm wavelength, and has a loss of ≤0.6/0.8 dB/cm for the TE/TM modes. A secondary ion mass spectrometry analysis shows that the Zr(4+)-profile part with a concentration above the threshold of photorefractive damage entirely covers the waveguide, implying that the waveguide would be optical-damage resistant.

Investigation of optical pumping on the dielectric properties of near-stoichiometric LiNbO3: Mg in terahertz range

The dielectric properties of near-stoichiometric LiNbO3:Mg crystal were measured by terahertz time-domain spectroscopy in a frequency range of 0.2THz to 0.9THz at room temperature. Experimental results show that the variation of refractive index |Δn| has a linear relationship on scale with the applied light intensity accompanied with a steplike decrease. These results were attributed to the internal space charge field of photorefraction and the light-induced domain reversal in the crystals.

Efficient 1.54μm laser property in near- stoichiometric Er:LiNbO3 crystal

Abstract Near-stoichiometric LiNbO3 crystal heavily doped with Er3+ ions (Er:NSLN) was grown by Czochralski technique. An enhancement of 1.54 μm emission and a lengthening lifetime of 4I13/2→4I15/2 transition observed in Er:NSLN crystal would meet the requirement of the greatly shortening optical diffusion route for Er-doped waveguide amplifiers (EDWAs). Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the concentration of Er3+ ion in the crystal. Based on Judd–Ofelt theory, the spectroscopic properties of Er:NSLN crystal were discussed, and the obtained intensity parameters Ωt implied that the increased [Li]/[Nb] ratio had a large influence on the value of Ω2. The emission cross-sections of 1.54 μm emission were calculated by Fuchtbauer–Ladenburg and reciprocity methods. Studies on the gain cross-section, estimated as a function of the population inversion ratio, suggested that Er:NSLN crystal could be considered as a potential material in the application of the tunable lasers around 1.54 μm wavelength.

Relationship between refractive index increase and Ti4+ concentration in Ti:LiNbO3 waveguide fabricated by Ti4+ diffusion in near-stoichiometric LiNbO3 substrate

Multi-mode Ti:LiNbO3 planar waveguide was fabricated by in-diffusion of 100-nm-thick Ti-metal film coated onto a near-stoichiometric LiNbO3 substrate prepared by vapor transport equilibration. The crystalline phase and composition in the waveguide were characterized. The guided modes in the planar waveguide were characterized by prism coupling technique. The refractive index profile is constructed from the measured mode indices using the inverse Wentzel–Krames–Brillouin method, and correlated with the Ti4+- concentration profile, which was obtained from secondary-ion-mass-spectroscopy analysis. The results show that the waveguide still retains the LiNbO3 phase, its composition is near-stoichiometric. The index change and Ti4+-concentration follow an exponential relationship with a power index 0.4 for an ordinary ray and 1.07 for an extraordinary ray. A comparison of various congruent and near-stoichiometric bulk materials or waveguides allows to conclude that the relationship is different from congruent to near-stoichiometric composition, from one fabrication method to another, and from bulk material to waveguide configuration. The difference is explained qualitatively.

Study of structural defects and crystalline perfection of near stoichiometric LiNbO3 crystals grown from flux and prepared by VTE technique

Near-stoichiometric LiNbO3 (SLN) single crystals were grown/prepared by top seeded solution growth/vapor transport equilibration (VTE) technique, and investigated for stoichiometry, disorder and structural defects. The optical absorption and Raman line-width studies revealed higher stoichiometry (i.e., higher Li/Nb) for SLN prepared by vapor transport equilibration (SLN_V) technique in comparison to SLN grown from K2O flux (SLN_K) and Li-rich melt (SLN_L). The nearly symmetric single diffraction curve (DC), though broad, as observed for SLN_L specimen in high resolution X-rays diffraction (HRXRD) analysis depicted lesser low angle grain boundaries. On the other hand, relatively sharp DC with lowest full-width at half-maximum (FWHM ∼45 arc-sec) in HRXRD and lesser Urbach energy (∼80meV) in the absorption spectra for SLN_V crystal revealed less structural defects with respect to other SLN crystals. The higher FWHM of DCs in HRXRD for SLN_L and SLN_K is attributed to growth related imperfections usually observed in solution growth. Though, VTE process results in SLN crystals with better stoichiometry and lesser structural defects but the limitation being that samples up to ∼1mm thickness can be prepared with this technique. For bulk SLN, growth from K2O flux resulted in better stoichiometry whereas Li-rich flux resulted in better structural quality. The absorption spectra of the grown SLN crystals depicted oxygen vacancy induced electronic defects (Nb4+, polarons), which was further authenticated by X-ray absorption near-edge structure (XANES) analysis at Nb K edge revealing lesser Nb4+ defects in SLN with respect to congruent lithium niobate (CLN) crystal.

Comparison of two-color holography among different doped LiNbO3 crystals at low intensities

The effects of material and experimental parameters on the nonvolatile two-color holographic recording space charge field and sensitivity for different doped LiNbO3:Fe crystals have been studied theoretically based on a two-center model. When the direct electron transfer between the deep-trap centers and the shallow-trap centers was considered, the near-stoichiometric LiNbO3:Fe is confirmed theoretically to be of bigger space charge field and higher recording sensitivity than the LiNbO3:Fe:Mn and LiNbO3:Cu:Ce in the low intensity region. A further improvement of the recording sensitivity can be achieved by doping concentration, thermal reduction treatment of Fe, appropriate gating and recording wavelengths with large photo-excitation cross sections.

Upconversion for enlarging solar spectrum response in near-stoichiometric and congruent Er:LiNbO3 crystals

To increase the photovoltaic efficiency of solar cell, the visible luminescence produced at the excitation of the near infrared laser light in near-stoichiometric and congruent LiNbO3 crystals heavily doped with Er3+ ions (Er:NSLN and Er:CLN, respectively) were investigated. An enhancement of the green upconversion emission observed in Er:NSLN crystal was attributed to the inefficient cross relaxation processes of 2H11/2+4I13/2→4I11/2+4F9/2 and 4F7/2+4I11/2→4F9/2+4F9/2. The Er content in the crystal was measured by an inductively coupled plasma mass spectrometry (ICP-MS). The OH− absorption and UV–vis-near infrared absorption spectra indicated that the Er3+ cluster sites (ErLi2+–ErNb2-) were dissociated in Er:NSLN crystal. The strong green upconversion emission produced by Er:NSLN crystal will be beneficial for improving the practical performance of solar cells.