On the theory of the second optical harmonic in nonlinear crystals with a regular domain structure

The theory of the second-harmonic generation in crystals with a regular domain structure (RDS) is developed in the approximation of the fixed intensity of the fundamental radiation. An analytical expression for the second-harmonic intensity is derived. The results are compared with the data obtained in the fixed-field approximation and the numerical solution. The maximum number of layers n _max at which the theory remains valid is determined. Numerical estimates of n _max are presented for the frequency doubling of an ultrashort light pulse in the LiNbO₃ RDS crystal.     

Spectroscopic analysis of Er transition in Mg/Er-codoped LiNbO3 crystal

We present a Judd-Ofelt spectroscopic analysis on the Mg/Er-codoped congruent lithium niobate (LiNbO₃) crystals. The Judd-Ofelt model is applied to the room temperature unpolarized absorption intensities of Er³⁺ ions on eleven transition bands to determine their intensity parameters: Ω₂=2.36×10⁻²⁰ cm², Ω₄=0.76×10⁻²⁰ cm², Ω₆=0.30×10⁻²⁰ cm² in Er:LiNbO₃ crystal heavily codoped with MgO. The radiative lifetime of ²H_9/2 becomes longer when MgO is added into Er:LiNbO₃ crystal. The experimental lifetimes are obtained using microsecond time-resolved spectra at 400 nm femtosecond pulse excitation to predict radiative quantum efficiency. Combining higher radiative quantum efficiency with longer radiative lifetime, we conclude that Mg/Er-codoped LiNbO₃ crystals are more suitable than Er: LiNbO₃ ones in laser materials.     

Photoconductivity and photovoltaic behaviour of LiNbO3 and LiNbO3 waveguides at high optical intensities

The photoconductivity and photovoltaic currents of pure LiNbO₃ and proton exchanged waveguides in LiNbO₃ have been measured as a function of the optical intensity up to about several kW/cm² by the use of surface electrodes. For pure LiNbO₃ the observed dependences are a simple extrapolation of the well known low intensity behaviour. The photoconductivity of proton exchanged waveguides is considerably increased compared with pure LiNbO₃ and the curves are strongly nonlinear in the high intensity region. These results can explain, at least qualitatively, the previously observed characteristic time and intensity dependence of light-induced refractive index changes in this type of waveguides. Both the time and temperature behaviour of the dark conductivity of all proton exchanged waveguides give strong evidence of ionic charge transport in the proton exchanged region.     

The dielectric behaviour of doped near-stoichiometric lithium niobate in the terahertz range

The dielectric properties of near-stoichiometric LiNbO₃:Fe and LiNbO₃:Ce single crystals have been investigated using terahertz time domain spectroscopy in a frequency range of 0.7-1.6 THz at room temperature. When coupled with an applied external optical field, obvious photorefractive effects were observed, resulting in a modulation of the complex dielectric constant for the crystals. The variation in refractive index, |Δn|, had a linear relationship with the applied light intensity, accompanied by a step-like decrease at high intensity. The findings were attributed to the internal space charge field of the photorefraction and the light-induced domain reversal in the crystals.     

铌酸锂晶体中多畴层带区倍频光的研究

本文研究用声光调Q-YAG激光器作为基波光源,测量了LiNbO₃及BNN晶体中倍频光强-温度曲线。在生长层区,倍频曲线上往往出现双峰,同时有一一对应的NPM双光环出现。LiNbO₃的双峰之一在极化处理后消失,本文对此峰的产生进行了分析。LiNbO₃多畴层带区倍频光强比单畴区大10倍左右,实验表明此乃层带区180°畴的形成导致应力松弛的结果。 关键词：     

Temperature characteristics of a Y-cut Z-propagation LiNbO3 light modulator for application to polarimeters

The temperature characteristics of a Y-cut Z-propagation LiNbO₃ crystal light modulator, with manufacturing errors, in the absence and presence of an electric field have been investigated by analyses and experiments. According to our analyses, when the Z-axis of the LiNbO₃ crystal is at an angle of 0.22° with respect to the normal of the input surface of the crystal, we found the theoretical fluctuation of the normalized output-light intensity with temperature to be less than 7:75 × 10⁻⁶/°C. This magnitude is less than 1% of the theoretical intensity fluctuation of a conventional temperature-compensation LiNbO₃ light modulator. The measured temperature characteristics of a prototype of this modulator were 2 × 10⁻⁴/°C in the absence of an electric field (OFF state) and 2:8 × 10⁻⁴/°C in the presence of an external field (ON state). During a running test of longer than 8 hours at room temperature, the intensity fluctuation of this prototype was 0.01% in the OFF state, and 0.07% in the ON state.     

掺镁铌酸锂亚微米结构畴反转的研究

研究了掺镁铌酸锂(MgO：LiNbO₃)的极化特性及其畴壁运动的性质,通过调节多个脉冲外加电场来控制畴壁的运动,在背向反转效应作用下,反转畴发生劈裂,制备出均匀的掺镁铌酸锂亚微米周期畴结构,并分析探讨了掺镁铌酸锂亚微米结构的成因及其反转机理. 关键词： 亚微米畴结构 掺镁铌酸锂 背向反转     

Mössbauer relaxation spectra of LiNbO3: Fe(III)

Mössbauer spectra of LiNbO₃: Fe(III)-monocrystals were measured in zero external magnetic field (5mK to 673K). They all show typical lineshapes due to relaxation effects. Below 77K temperature independent and above 77K a temperature-dependent relaxation is observable. In spectra measured in external magnetic fields relaxation effects occur only above 77K. We present least square fits of spectra measured at 300K and external magnetic fields using symmetry invariant reservoir spectral densities. The applied numerical formalism takes the complete electronuclear manifold of⁵⁷Fe(III) into account. We discuss the problems concerning the interpretation of zero field spectra.     

The use of Raman spectroscopy as a probe for the intermolecular bonding in crystalline tetraphosphorus trisulphide

The temperature and pressure dependences of the internal and external first-order Raman-active phonons of crystalline α-P₄S₃ are shown to provide a sensitive probe of the intermolecular bonding. Comparison with a similar study of α-P₄Se₃ leads to the conclusion that while intra-intermolecular coupling is present, it is weaker in crystalline α-P₄S₃. Unlike α-P₄Se₃, no rigid layer mode is observed nor is there any softening of the A₁ mode at 440 cm⁻¹ analogous to the 361 cm⁻¹ soft mode of α-P₄Se₃. Weak intra-intermolecular coupling is also supported by an analysis of the implicit-explicit contributions to the observed temperature coefficients. A pressure dependent increase in the intensity of the A₁ mode at 440 cm⁻¹ is compared to similar pressure dependent changes in intensity for phonons in other inorganic molecular solids.     

Anomalous increase in the unipolarity of doped lithium niobate crystals in the temperature range 300–400 K

The lithium niobate single crystals doped with B, Zn, and Gd at a content of 0.002–0.44 wt % have been grown. Their domain structure, static and dynamic piezoelectric properties, dielectric properties, and conductivity are investigated over a wide range of frequencies. The dielectric dispersion associated with the Debye-type relaxation process and considerable anomalies in ?′₂₂(T) and conductivity are revealed in the temperature range ～300–400 K. At these temperatures, the piezoelectric modulus d ₃₃ of the initial polydomain crystals LiNbO₃: Gd jumpwise increases up to the values close to those for the undoped single-domain crystal. This increase is accompanied by a substantial change in the etch patterns due to the domain structure of the crystal. The nature of the anomalies observed in LiNbO₃ in the above temperature range is discussed.     

Two field nonlinear Faraday rotation in rubidium vapor in a Doppler-free geometry

Nonlinear Faraday rotation with counter-propagating light beams was investigated in a rubidium vapor cell. Observations across all hyperfine and crossover resonances of the D₂ line of ⁸⁷Rb atoms were made. Additionally, rotation spectra as a function of pump and probe intensity, pump polarization and external magnetic field strength were studied.The investigations were performed in a regime where the pump beam power was sufficiently low relative to the probe power to avoid polarization rotation due to pump-induced anisotropy. Our results are analyzed in the context of Bennett structures and coherent optical processes.     

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.     

Squeezing by second-harmonic generation in a monolithic resonator

We have measured the intensity fluctuations of the second-harmonic mode generated in a MgO:LiNbO₃ external monolithic cavity pumped by a Nd:YAG laser. The cavity has mirror coatings for both the fundamental and the second-harmonic mode. We scan the cavity using the electro-optic effect of the crystal and observe that the second-harmonic beam of 2 mW exhibits a quantum noise reduction of 40(±5)%. In addition, we report on the active frequency stabilization of the monolithic device used in our squeezing experiments. Several fast tuning parameters such as the electro-optic effect, the photo-elastic effect, and the laser frequency have been investigated. With these tuning parameters the monolithic resonator can be locked on double-resonance at the phase-matching temperature, which is a prerequisit for observing squeezing in a cw-regime.     

The study of damage and optical properties in LiNbO3 implanted by MeV Er and He ions

Abstract

Two LiNbO₃ (X and Y cut) crystals from different companies were implanted by 3.0 MeV Er ions to a dose of 7.5 × 10¹⁴ ions/cm² and 3.5 × 10¹⁴ ions/cm² with different beam current densities, respectively. After annealing at 1060°C in air for 2 hours, one LiNbO₃ sample was implanted by 1.5 MeV He ions to a dose of 1.5 × 10¹⁶ ions/cm². The Rutherford backscattering/channeling and prism coupling method have been used to study the damage and optical properties in implanted LiNbO₃. The results show: (1) the damage in LiNbO₃ created by 3.0 MeV Er ions depends strongly on the beam current density; (2) after annealing at 1060°C in air for 2 hours, a good Er doped LiNbO₃ crystal was obtained; (3) there is waveguide formation possible in this Er-doped annealed LiNbO₃ after 1.5 MeV He ion implantation. It is suggested that annealing is needed to remove the damage created by MeV Er ions before the MeV He ion implantation takes place, to realize the waveguide laser for Er doped LiNbO₃.     

Domain kinetics in the formation of a periodic domain structure in lithium niobate

The evolution of the domain structure in LiNbO₃ with polarization switching in an electric field is investigated experimentally. Special attention is given to the formation processes of a regular domain applicable to nonlinear optical devices. A new method based on the spontaneous backswitching effect is proposed for creating a regular structure with a period of 2.6 μm in LiNbO₃ with a thickness of 0.5 mm. Fiz. Tverd. Tela (St. Petersburg) 41, 1831–1837 (October 1999)     

Light-induced damage mechanisms in α-phase proton-exchanged LiNbO3 waveguides

The overall power and far-field pattern of the beam out-coupled from a single-mode planar proton-exchanged LiNbO₃ waveguide in the α-phase have been studied for in-coupled intensities within the range 20–700 W/cm². The steady-state output versus input power response shows three definite stages designated as I, II, and III in order of increasing input intensity. In stage I the output varies linearly with input and the far-field pattern does not show appreciable changes. In stage II, the pattern is considerably broadened and displays a number of steady peaks and dips indicative of a filamentary structure of the beam. As in bulk LiNbO₃, these damage features are explained in terms of parametric processes involving the amplification of scattered (noise) light. An additional broadening is observed in stage III together with the occurrence of a fluctuating profile (chaotic response) attributed to random fluctuations in the coupling parameters. The threshold input intensity separating stages I and II is related to the intensity-dependence of the photovoltaic field. Received: 18 November 1998 / Revised version: 13 January 1999 / Published online: 12 April 1999     

ESR study of rare-earth ions with the effective spin of 1/2 in LiNbO3 crystals

Rare-earth ions of Nd³⁺ and Er³⁺ in nearly stoichiometric and MgO-doped LiNbO₃ crystals, respectively, have been investigated by employing an X-band electron spin resonance (ESR) spectrometer. The grown crystal was heated in Li-rich powder at 1100°C in order to make it nearly stoichiometric by the vapor transport equilibrium technique. Due to the fact that the ESR linewidth is much narrower in the stoichiometric crystal than in the congruent LiNbO₃, we were able to determine the hyperfine constants of¹⁴³Nd and¹⁴⁵Nd at 4 K. By codoping MgO into LiNbO₃, a new Er³⁺ center has been observed with a differentg-tensor. We propose that the new Er³⁺ center in Mg-doped LiNbO₃ occupies the niobium site due to the local excessive Mg²⁺ ion at the lithium site, whereas Nd³⁺ and Er³⁺ in congruent crystals reside at the lithium site. The proposal is consistent with theg-value anisotropy.     

Domain inversion in LiNbO3 and Zn-doped LiNbO3 crystals by the electron-beam irradiation of the nonpolar Y-surface

Individual domains and domain gratings were fabricated on nonpolar Y-cuts of LiNbO₃ and LiNbO₃-Zn crystals by electron beam irradiation. The domains which nucleated in the irradiation points are frontally growing along the direction +Z within a thin (of about several microns) surface layer. The regularities of this motion are discussed in the framework of the approach to formation of space-charge fields under e-beam charging of insulators. The obtained dependency of the domain length on the exposure time permits us to propose the viscous-friction mechanism for the observed frontal domain growth. The velocity of the frontal growth in LiNbO₃-Zn is higher than in LiNbO₃ obviously due to a decreased number of pinning centers at the Nb-antisites. In LiNbO₃-4 %Zn crystals planar domain gratings were fabricated by means of point-to-point irradiations along the X- and Z-directions with specified distances between the irradiation points. It is shown that the domain gratings are generated by a total field of point charges $ \vec{E} = \sum\nolimits_{i = 1}^{n} {\vec{E}_{i} } $ , where E _i is the space-charge field induced in any irradiation point, and n is the number of points. Some preliminary estimates indicate that the frontal growth of domains under e-beam irradiation occurs at fields E < E _c.     

Emission characteristics, crystalline phase and composition of vapor-transport-equilibrated Er:LiNbO3 crystal codoped with 6 mol% MgO

Polarized downconversion, 980-nm-upconversion and near-infrared emission characteristics of vapor-transport-equilibrated (VTEed) bulk Er (0.4 mol%)/MgO (6 mol%)-codoped LiNbO₃ crystals were investigated. The downconversion and upconversion visible emissions display similar VTE effects including the drop of emission intensity and the weakening of polarization dependence. At 0.98 and 1.5 μm regions, the VTE has a weak effect on the emission intensity, but a strong effect on the spectral shape. The crystalline phases in these bulk Er/Mg-codoped VTE crystals are determined by comparing their infrared emission characteristics with those of pure ErNbO₄ powder and locally Er-doped MgO (4.5 mol%):LiNbO₃ crystal. The results show that the Er³⁺ ions present in these bulk Er/Mg-codoped VTE crystals as a mixture of Er:LiNbO₃ and ErNbO₄ phases. The percentages of the ErNbO₄ phase contained in these VTE crystals were evaluated from the 1531 and 1536 nm characteristic absorption areas. The contents of constituent elements were determined by chemical analysis.     

周期极化掺镁不同组分LiNbO3晶体的研究

利用气相平衡扩散法研制出掺镁不同组分的LiNbO₃晶体，并对其极化特性进行了研究.研究表明晶体的开关电场和自发极化不仅与晶体组分［Li］/［Nb］比有关而且与掺镁量有关，［Li］/［Nb］比为0.973掺入2mol% MgO的近化学比LiNbO₃晶体的开关电场仅为1.8kV/mm，是同成分晶体的1/12，且其极化结构的质量要远好于同成分LiNbO₃晶体和近化学比LiNbO₃晶体. 关键词： 气相平衡扩散 3晶体')" href="#">掺镁LiNbO₃晶体 周期极化