Mg-doped LiNbO3 Crystals Research Articles

Features of the Defect Structure of LiNbO3:Mg:B Crystals of Different Composition and Genesis.

We proposed and investigated a refinement of technology for obtaining Mg-doped LiNbO3 (LN) crystals by co-doping it with B. LN:Mg (5.0 mol%) is now the most widely used material based on bulk lithium niobate. It is suitable for light modulation and transformation. We found that non-metal boron decreases threshold concentrations of the target dopant in many ways. In addition, we earlier determined that the method of boron introduction into the LN charge strongly affects the LN:B crystal structure. So we investigated the point structural defects of two series of LN:Mg:B crystals obtained by different doping methods, in which the stage of dopant introduction was different. We investigated the features of boron cation localization in LN:Mg:B single crystals. We conducted the study using XRD (X-ray diffraction) analysis. We have confirmed that the homogeneous doping method introduces an additional defect (MgV) into the structure of LN:Mg:B single crystals. Vacancies in niobium positions (VNb) are formed as a compensator for the excess positive charge of point structural defects. According to model calculations, boron is localized in most cases in the tetrahedron face common with the vacant niobium octahedron from the first layer (VNbIO6). The energy of the Coulomb interaction is minimal in the LN:Mg:B crystal (2.57 mol% MgO and 0.42 × 10-4 wt% B in the crystal); it was obtained using the solid-phase doping technology. The solid-phase doping technology is better suited for obtaining boron-containing crystals with properties characteristic of double-doped crystals (LN:Mg:B).

Strong nonlinear-polarization in ZnMgO epitaxial thin-films with Li incorporation

The second-order nonlinear-polarization originated from the interaction between thin-film materials with second-order nonlinear susceptibility (χ (2)) and high-power laser is essential for integrated optics and photonics. In this work, strong second-order nonlinear-polarization was found in a-axis oriented Zn1-x Mg x O (ZnMgO) epitaxial thin-films with Li incorporation, which were deposited by radio-frequency magnetron sputtering. Mg incorporation (x > 0.3) causes a sharp fall in the matrix element χ 33 of χ (2) tensor, although it widens optical bandgap (E opt). In contrast, moderate Li incorporation significantly improves χ 33 and resistance to high-power laser pulses with a little influence on E opt. In particular, a Zn0.67Mg0.33O:Li [Li/(Zn + Mg + Li) = 0.07] thin-film shows a |χ 33| of 36.1 pm V−1 under a peak power density (E p) of 81.2 GW cm−2, a resistance to laser pulses with E p of up to 124.9 GW cm−2, and an E opt of 3.95 eV. Compared to that of ZnO, these parameters increase by 37.8%, 53.4%, and 18.6%, respectively. Specially, the Zn0.67Mg0.33O:Li shows higher radiation resistance than a Mg-doped LiNbO3 crystal with a comparable E opt. First-principle calculations reveal the Li occupation at octahedral interstitial sites of wurtzite ZnO enhances radiation resistance by improving structural stability. X-ray photoelectron spectroscopy characterizations suggest moderate Li incorporation increases χ 33 via enhancing electronic polarization. These findings uncover the close relationship between the octahedra interstitial defects in wurtzite ZnMgO and its nonlinear-polarization behavior under the optical frequency electric field of high-power 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.

Robust, efficient, and broadband SHG of ultrashort pulses in composite crystals.

We experimentally demonstrate efficient second-harmonic generation (SHG) of tunable ultrashort pulses of 100 femtoseconds, using a novel method based on composite segmented periodically poled (CSPP) design. The scheme was borrowed from the nuclear magnetic resonance (NMR) composite pulses (CP) of Shaka and Pines. Using CSPP, a broadband and efficient conversion over a bandwidth of 35nm in very short interaction length was achieved. In addition, CSPP showed robustness to temperature changes up to 90°C. Two CSPP schemes with 15 and 31 segments and periodically poled design were implemented on Mg-doped LiNbO3 crystal. The CSPP performance was shown to be superior in all aspects. The experimental results are compared to numerical simulation with excellent agreement.

Generation of monocycle efficient terahertz pulses by optical rectification in LiNBO3 at 800 nm

Generation of efficient terahertz (THz) pulses was experimentally made by tilted pump pulse front scheme with a Mg-doped LiNbO3 crystal. In this study, a spitfire laser (Ti:sapphire laser, 800 nm, 3 mJ, 1 kHz) was used as an optical source for the generation and detection of THz pulses. The electro-optic (EO) detection optics consisting of a ZnTe crystal (1 mm in thickness) and a balanced photodetector was used. To obtain optimum THz characteristics and pump to THz power conversion efficiency, the image of the grating was made coincides with the tilted pump pulse front. The maximum THz electric field of 8.5 kV/cm and the frequency bandwidth of 2.5 THz were achieved by using pump pulse energy of 2.4 mJ and pump pulse width of 100 fs. The THz energy of 4.15 μJ was obtained and pump-to-THz conversion efficiency was estimated to be approximately 1.73 x 10-3.

Fabrication of nano-scale domain structures in bulk Mg-doped LiNbO3 crystals

We present a new method to fabricate nano-scale domain structures in bulk Mg-doped LiNbO3 crystals with the assistance of thermal fixing technique. Domain structures with hundred-nanometer width and millimeter length can be fabricated using this method. SEM image reveals a feature of smooth sidewall for the nano-scale domain structures.

THz pulse generation using a contact grating device composed of TiO2/SiO2 thin films on LiNbO3 crystal

We developed a new contact grating device for terahertz (THz) pulse generation by optical rectification. The device was made from polycrystalline rutile TiO2 thin film in the grating region and an amorphous SiO2 layer deposited on a Mg-doped LiNbO3 crystal. Our calculations indicated that the TiO2 grating on the SiO2 layer would yield an increase in diffraction efficiency of up to 0.69. The prepared TiO2 thin film had a sufficient laser induced damage threshold (140 GW/cm2) to enable effective THz pulse generation. Using a prototype device, we demonstrated THz pulse generation and investigated the phase-matching conditions experimentally.

Yb:YAG thin-disk chirped pulse amplification laser system for intense terahertz pulse generation.

We have developed a 1 kHz repetition picosecond laser system dedicated for intense terahertz (THz) pulse generation. The system comprises a chirped pulse amplification laser equipped with a Yb:YAG thin-disk amplifier. At room temperature, the Yb:YAG thin-disk regenerative amplifier provides pulses having energy of over 10 mJ and spectral bandwidth of 1.2 nm. The pulse duration achieved after passage through a diffraction grating pair compressor was 1.3 ps. By employing this picosecond laser as a pump source, THz pulses having a peak frequency of 0.3 THz and 4 µJ of energy were generated by means of optical rectification in an Mg-doped LiNbO3 crystal.

Generation of a phase contrast Talbot array illuminator with electric controlling

A hexagonal phase contrast Talbot array illuminator based on an electro-optically tunable phase grating made from periodically poled Mg-doped LiNbO3 crystal is presented theoretically and experimentally. Both the effects from external electric field and diffraction distance on the intensity distribution of diffraction field are demonstrated by numerical simulation. The simulated results show that higher light compression ratio (LCR) in a hexagonal sample area can be achieved at a suitable diffraction distance under lower electric field. The experimental Talbot diffraction imaging was successfully obtained by modulating phase difference and fractional Talbot distance, and the intensity distribution agrees with the simulated results well. For the hexagonal phase contrast array illuminator with duty cycle of 63%, the highest LCR of 4.2 units can be obtained under phase difference of 0.80 π and diffraction position of 0.38 times Talbot distance.

Multiphoton photoluminescence contrast in switched Mg:LiNbO3 and Mg:LiTaO3 single crystals

We observed a multiphoton luminescence contrast between virgin and single-switched domains in Mg-doped LiNbO3 (LNO) and LiTaO3 (LTO) single crystals with different doping levels of 0–7 mol. % and 0–8 mol. %, respectively. A luminescence contrast in the range of 3% was measured between as-grown and electrically inverted domain areas in Mg:LNO samples, while the contrast reaches values of up to 30% for the Mg:LTO case. Under annealing, an exponential decay of the domain contrast was observed. The activation energy of about 1 eV being determined for the decay allowed a comparison with reported activation energies of associated defects, clearly illustrating a strong connection between thermal contrast decay and the H+ and Li+-ion mobility. Finally, performing similar experiments on oxidized samples undoubtedly demonstrated that the origin of the reported luminescence contrast is strongly connected with lithium ions.

Investigation of the defect complexes in highly Mg-doped LiNbO3 crystals by 93Nb NMR method

Investigation of the defect complexes in highly Mg-doped LiNbO3 crystals by 93Nb NMR method

基于周期极化掺镁铌酸锂晶体的六角可调相位阵列光栅

基于周期极化掺镁铌酸锂晶体的六角可调相位阵列光栅

Large-aperture, axis-slant quasi-phase matching device using Mg-doped congruent LiNbO_3 [Invited

We fabricated large-aperture axis-slant quasi-phase-matching (AS-QPM) device with 8 mm x 11 mm acceptable aperture size in 2-mm-thick Mg-doped LiNbO3 crystal at 65° slant angle with 75-µm surface period. The AS-QPM has a possibility of wafer-scale-aperture device, suitable for handling high power/energy lasers.

Light-induced superlow electric field for domain reversal in near-stoichiometric magnesium-doped lithium niobate

Light-induced domain reversal of near-stoichiometric Mg-doped LiNbO3 crystal was investigated with a focused 532 nm continuous laser beam. The lowest electric field applied to accomplish domain nucleation is only 30 V/mm and 1/80 of the coercive field, which is safe and convenient for us to fabricate domain structures. Under this superlow applied field, the pinning effect of domain wall is so obvious that the inverted domain reveals a gear shape contrary to the hexagon in a higher applied field. Then two-dimensional domain patterns with the smallest domain size of 4 μm have been fabricated.

Infrared dispersion of dielectric function in Mg‐doped LiNbO3 crystals with polaronic‐type conductivity

AbstractWe have measured the refractive indices of Mg:LiNbO3 crystals before and after chemical reduction at different annealing temperatures by the spontaneous parametric down‐conversion technique and near‐forward Raman scattering by polaritons in the range 2–11 µm. The polaron‐induced contribution to the dispersion of the real part of the dielectric function was revealed near the phonon absorption edge. It was found that this contribution increases considerably with increase in the polaron concentration. Copyright © 2007 John Wiley & Sons, Ltd.

Ultraviolet photorefractive effect in Mg-doped near-stoichiometric LiNbO3

The ultraviolet photorefractive effect of Mg-doped near-stoichiometric LiNbO3 crystals prepared by vapor transport equilibration (VTE) technique was studied at 351nm. It was found in the near-stoichiometric LiNbO3 crystals that the ultraviolet photorefractive effect could be enhanced greatly with the increase of Mg concentration. Based on the activation energy of dark decay of the photorefractive grating, possible centers responsible for the ultraviolet photorefractive effect were also discussed.

Reduction-induced polarons and optical response of Mg-doped LiNbO3 crystals

We studied the visible and IR dispersion of absorption coefficient and refractive index for congruent LiNbO3 and Mg:LiNbO3 crystals before and after chemical reduction at different annealing temperatures. The concentration of Mg in Mg:LiNbO3 samples was just below or above the photorefractive threshold. The reduction-induced changes in the absorption coefficient reveal the formation of polarons typical for doped LiNbO3 crystals. It was shown that the polaron concentration is maximal when the Mg concentration is just below the photorefractive threshold and the annealing temperature is near 500 °C. This temperature is optimal for the most efficient polaron formation at all considered concentrations of Mg. The fitting of the experimental absorption dispersion curves indicates that intermediate polarons are formed in LiNbO3:Mg crystals preferably. The spectral dependence of transmission for samples of lithium niobate of various thicknesses was studied. The results indicate that there are spatial regions with much greater absorption than that of bulk crystals. We assume that, in general, polarons are localized in thin near-surface regions. The spectral dependence of the refractive index in the vicinity of the phonon absorption edge indicates some essential changes of the phonon subsystem taking place after reduction. The infrared contribution into the dispersion of the dielectric function real part increases considerably after reduction.

Study on laser-micro-Raman spectra in near-stoichiometric LiNbO3 crystals

The laser-micro-Raman spectra of near-stoichiometric LiNbO3 crystals were measured and analyzed. The crystals were grown by flux pulling method. Compared with the congruent LiNbO3 crystal, the number and frequency of Raman spectral lines of A1(TO) and E(TO) modes have no change. It was validated that the LiNbO3 crystal is substitutional solid solution. The relative intensity of 738cm-1 Raman peak(A1(LO)) exhibited obvious change when the Li2O contents increase in pure LiNbO3 crystal, the 738cm-1 Raman peak disappeared in the Mg-doped and Zn-doped LiNbO3 crystals. These Raman peaks were discussed and explained from the occupied site and structure according to the Li vacancy model. The full width at half maximum (FWHM) of the 152 and 872cm-1 Raman peaks narrows with increasing the Li/Nb in the crystals. Their relationships were fitted linearlly, using the fitting equation, and the Li/Nb ratio can be calculated according to the FWHM of Raman peaks.

Ultraviolet-light-induced near-infrared photorefractivity and two-color holography in highly Mg-doped LiNbO3

We investigate the ultraviolet (UV)-light-induced transient photorefractivity in a highly Mg-doped LiNbO3 crystal. The thermal activation energy of the UV-light-induced defect centers responsible for the induced transient photorefractivity was measured to be 0.65 eV and holes were found to be the major charge carriers. These results provide a solid basis for the model that intermediate states O− are the UV-light-induced defect centers in the crystal. We also demonstrate quasinondestructive two-color holography using UV-light-induced transient photorefractivity. The two-color recording sensitivity was measured to be as high as 0.05 cm/J with a recording intensity of 5 W/cm2 at 780 nm and a gating intensity of 0.47 W/cm2 at 365 nm.

Green-induced infrared absorption in MgO doped LiNbO3

Green-induced infrared absorption (GRIIRA) was investigated by a photothermal technique for undoped and Mg-doped LiNbO3 crystals that have different Li/Nb ratios. Threshold effect on GRIIRA was found, threshold MgO concentrations being the same for GRIIRA and photorefraction. We suggest that GRIIRA is associated with the formation of the small polaron that is located on Nb antisite defect. The remarkable decrease of GRIIRA in Mg:LiNbO3 can then be attributed to the elimination of this intrinsic defect, Nb in Li, following the incorporation of Mg on Li sites. For nonlinear optical applications, LiNbO3 doped with MgO at concentrations over threshold has a combined advantage of having almost no GRIIRA and photorefraction.