Damage threshold of lithium niobate crystal under single and multiple femtosecond laser pulses: theoretical and experimental study

The damage threshold of lithium niobate crystal under single and multiple femtosecond laser pulses has been studied theoretically and experimentally. Firstly, the model for the damage threshold prediction of crystal materials based on the improved rate equation has been proposed. Then, the experimental measure method of the damage threshold of crystal materials has been given in detail. On the basis, the variation of the damage threshold of lithium niobate crystal with the pulse duration has also been analyzed quantitatively. Finally, the damage threshold of lithium niobate crystal under multiple laser pulses has been measured and compared to the theoretical results. The results show that the transmittance of lithium niobate crystal is almost a constant when the laser pulse fluence is relative low, whereas it decreases linearly with the increase in the laser pulse fluence below the damage threshold. The damage threshold of lithium niobate crystal increases with the increase in the duration of the femtosecond laser pulse. And the damage threshold of lithium niobate crystal under multiple laser pulses is obviously lower than that irradiated by a single laser pulse. The theoretical data fall in good agreement with the experimental results.     

圆碟中回音壁模式的耦合条件

基于波导的几何光学理论,并考虑谐振腔的品质因素或光子寿命,给出了毫米级圆碟回音壁模式谐振时的条件,应用光子隧道效应,分析了输入输出波导与圆碟的耦合特性,详细分析并给出了品质因素为5×105环形电极铌酸锂圆碟调制器中回音壁模式与波导的耦合与限制条件.提出一种利用铌酸锂Mach-Zehnder波导与同质材料圆碟构成的微波接收转换装置,该装置能够实现推挽操作,在具有较高频率分辨特性的基础上能进一步降低驱动电压,而且器件尺寸进一步减小.     

Diffraction grating fabrication in lithium niobate and KDP crystals with femtosecond laser pulses

We report studies of the modifications induced inside lithium niobate and KDP crystals by Yb:KGW ultrafast laser pulses, having 300-fs pulse duration and operating at 100-kHz repetition rate. By focusing the laser beam with a 0.42 numerical-aperture objective, we have recorded homogeneous volume Bragg gratings in the bulk of the niobate crystal that showed excellent diffraction efficiencies, reaching up to 87%, and remained permanent after thermal annealing for one hour at 150°C. The refractive index modification level was found to be 0.002 in lithium niobate. The results show that lithium niobate is a very promising crystal candidate for microphotonics applications. On the contrary, in the KDP crystal no smoothly modified refractive index zones were created.     

The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser

Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxide-doped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 μm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 ± 2.83% and demonstrated second-harmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved.     

非线性光学晶体铌酸钾锂的二次谐波产生

研究铌酸钾锂晶体的近经这过光谱：利用铌酸钾锂晶体对８９０￣９６０ｎｍ的Ｔｉ：ｓａｐｐｈｉｒｅ近红外激光进行倍频,获得蓝得蓝绿光输出;研究了晶体的倍频特性,探讨了提高晶体倍频效率的方法。     

Highly reduced iron-doped lithium niobate for optoelectronic tweezers

We investigate the applicability of highly reduced lithium niobate samples doped with iron for the use as optoelectronic tweezers. Increasing the reduction degree of Fe-doped lithium niobate is well known to increase the photoconductivity and reduce the writing time of internal space-charge fields. Based on our measurements of the photorefractive properties, we determine the optimal conditions for dielectrophoretic trapping and present the application of Fe-doped lithium niobate as optoelectronic tweezers. For higher reduction degrees, an unexpected decrease in the photovoltaic current density and the saturation space-charge field is reported.     

Temperature-dependent Sellmeier equation for the index of refraction, n(e), in congruent lithium niobate

A Sellmeier equation for the extraordinary index of congruent lithium niobate is derived. The source data for the fit contain previously published data [Opt.Commun.17, 322 (1996); erratum 20, 188 (1997); J.Appl.Phys. 45, 3688 (1974)] and new measured tuning data for an optical parametric oscillator using periodically poled lithium niobate. Phase-matching predictions are accurate for temperatures between room temperature and 250 degrees C and wavelengths ranging from 0.4 to 5 mum .     

Discrete spatial solitons formed in periodically poled lithium niobate by electro-optical effect

We report the numerical observation of discrete spatial solitons in a periodically poled lithium niobate waveguide array by applying an electrical field through electro-optical effect. We show that discrete spatial soliton can be controlled by applied voltage in the periodically poled lithium niobate.     

Structural examination of lithium niobate ferroelectric crystals by combining scanning electron microscopy and atomic force microscopy

The structure of lithium niobate single crystals is studied by a complex technique that combines scanning electron microscopy and atomic force microscopy. By implementing the piezoresponse force method on an atomic force microscope, the domain structure of lithium niobate crystals, which was not revealed without electron beam irradiation, is visualized     

Experimental and simulated performance of lithium niobate 1–3 piezocomposites for 2 MHz non-destructive testing applications

Lithium niobate piezocomposites have been investigated as the active element in high temperature resistant ultrasonic transducers for non-destructive testing applications up to 400 °C. Compared to a single piece of lithium niobate crystal they demonstrate shorter pulse length by 3×, elimination of lateral modes, and resistance to cracking. In a 1–3 connectivity piezocomposite for high temperature use (200–400 °C), lithium niobate pillars are embedded in a matrix of flexible high temperature sealant or high temperature cement.In order to better understand the design principles and constraints for use of lithium niobate in piezocomposites experiments and modelling have been carried out. For this work the lithium niobate piezocomposites were investigated at room temperature so epoxy filler was used. 1–3 connectivity piezocomposite samples were prepared with z-cut lithium niobate, pillar width 0.3–0.6 mm, sample thickness 1–4 mm, pillar aspect ratio (pillar height/width) 3–6, volume fraction 30 and 45%. Operating frequency was 1–2 MHz.Experimental measurements of impedance magnitude and resonance frequency were compared with 3-D finite element modelling using PZFlex. Resonance frequencies were predicted within 0.05 MHz and impedance magnitude within 2–5% for samples with pillar aspect ratio ⩾3 for 45% volume fraction and pillar aspect ratio ⩾6 for 30% volume fraction. Laser vibrometry of pulse excitation of piezocomposite samples in air showed that the lithium niobate pillars and the epoxy filler moved in phase. Experiment and simulation showed that the thickness mode coupling coefficient k_t of the piezocomposite was maintained at the lithium niobate bulk value of approximately 0.2 down to a volume fraction of 30%, consistent with calculations using the (Smith and Auld, 1991 [1]) model for piezocomposites.     

Anomalies in the pyroelectric properties of LiNbO<Subscript>3</Subscript> crystals of the congruent composition

A number of anomalies are revealed in the temperature dependence of the primary pyroelectric coefficient for single-domain crystals of lithium niobate LiNbO₃ in the temperature range 310–330 K. It is shown that these anomalies are associated with the transformation of the domain structure of the lithium niobate crystals. The problem regarding the correct performance of the pyroelectric measurements and the possible influence of the observed effects on the optical properties of lithium niobate crystals and on the recording of optical phase holograms are discussed.     

Structure of elastic and electric fields induced near the boundary of a LiNbO3 crystal in the course of recording of photorefractive gratings through the photovoltaic mechanism

This paper reports on the results of the investigation into the structure of elastic and electric fields induced near the boundary of the X cut of a lithium niobate crystal during the formation of a photorefractive grating with a wave vector K parallel to the threefold symmetry axis due to the photovoltaic effect. The elastic and electric fields induced by the photorefractive grating and the changes in the components of the dielectric impermeability tensor of the lithium niobate crystal at the frequency of the light wave are calculated using numerical analysis. It is demonstrated that light waves with orthogonal polarizations can effectively interact at a photorefractive grating formed near the electrically short-circuited boundary of a lithium niobate crystal.     

Structuring of material parameters in lithium niobate crystals with low-mass, high-energy ion radiation

Ferroelectric lithium niobate crystals offer a great potential for applications in modern optics. To provide powerful optical components, tailoring of key material parameters, especially of the refractive index n and the ferroelectric domain landscape, is required. Irradiation of lithium niobate crystals with accelerated ions causes strong structured modifications in the material. The effects induced by low-mass, high-energy ions (such as ³He with 41?MeV, which are not implanted, but transmit through the entire crystal volume) are reviewed. Irradiation yields large changes of the refractive index ??n, improved domain engineering capability within the material along the ion track, and waveguiding structures. The periodic modification of ??n as well as the formation of periodically poled lithium niobate (PPLN) (supported by radiation damage) is described. Two-step knock-on displacement processes, ³He??Nb and ³He??O causing thermal spikes, are identified as origin for the material modifications.     

LiNbO3:Fe晶体二波耦合弱光放大的物理机理

对LiNbO₃: Fe晶体中二波耦合过程进行了动态观测.进一步探讨了LiNbO ₃:Fe晶 体中弱光放大的物理机理.发现LiNbO₃ : Fe晶体中二波耦合过程的弱光放大 对c轴指 向有明显的依赖关系，虽然光生伏打效应对光生载流子的迁移有主要贡献，但扩散机理的贡 献仍不可忽略.弱光最终得到放大是瞬态能量转移与扩散机理引起的能量转移的共同贡献.弱 光放大达到准稳态之后的下降过程为瞬态能量转移的时间指数衰减过程与光散射引起的能量 耗散的共同贡献. 关键词： 光折变效应 光放大 掺杂铌酸锂     

Surface ablation of congruent and Mg-doped lithium niobate by femtosecond laser

Lithium niobate is praised as silicon in nonlinear optics for its wide usage both in fundamental science and applications in optics. But its photorefractive effect hinders it from more extensive application in optics. The deadlock has been broken by doping some metallic elements such as magnesium into the congruent lithium niobate crystal for its high damage resistance. The single-shot and multi-shot surface ablation experiments by femotosecond laser with the wavelength of 800 nm and the duration of 80 fs were conducted in the same condition and the ablation threshold fluence was gained for congruent lithium niobate crystal and 6 mol % Mg-doped lithium niobate crystal. The band gap widening after doping is responsible for the discrepancy of the ablation thresholds between the two samples. The laser threshold fluence dependence of lasershot number demonstrates the difference of the accumulation effect. The large reduction of trapping cross section for electrons after heavy doping of magnesium is focused more to interpret this distinction.     

Effect of broadband incoherent light on the photoresponse of lithium niobate crystals

The effect of broadband incoherent light on the local photoresponse of lithium niobate crystals has been experimentally investigated. It is shown that a previously induced electric field affects the dependence of the photovoltage on time and impurity type and concentration. The pyroelectric effect significantly influences the photoprocesses in lithium niobate crystals, especially in the initial stage of illumination.     

A Ferroelectric Frequency-Doubling Material-Potassium Lithium Niobate

The potassium lithium niobate crystals have been grown up. The shapes of solid-melt interfaces which maintaining the steady growth of the potassium lithium niobate crystals have been described. The optical transmission spectrum of the crystal has been surveyed. The perfected crystals showed good Second Harmonic Generation properties.     

Efficient generation of far-infrared radiation in the vicinity of polariton resonance of lithium niobate

We efficiently generated far-infrared radiation at the wavelengths centered at 20.8?μm in the vicinity of one of the polariton resonances of lithium niobate. Such an efficient nonlinear conversion is made possible by exploiting phase matching for difference-frequency generation in lithium niobate. The highest peak power reached 233?W.     

Solitons and critical breakup fields in lithium niobate type uniaxial ferroelectrics

Ferroelectric materials, such as lithium niobate, show interesting non-linear hysteresis behavior that can be explained by a dynamical system analysis. By using variational principle, a non-linear Klein-Gordon (K-G) equation is derived for lithium niobate type of uniaxial ferroelectrics involving various types of energy, which was not considered previously to construct the Hamiltonian. This leads to soliton solutions under different conditions of soliton velocity. The critical value of the (dimensional) effective electric field has been estimated to be 54–58 kV/cm for lithium niobate depending on the impurity content in these type inhomogeneous ferroelectrics. Beyond this critical field, there is no existence of solitons. This critical field is related to a break-up mechanism of Landau-Ginzburg two-well potential to a single well, as the driving force is increased.     

Observation of ultra-long lifetime UV-induced infrared absorption in undoped and magnesium-doped congruent lithium niobate crystals

We report experimental observation of ultra-long lifetime UV-induced absorption in undoped and 5 mol.% Mg-doped congruent lithium niobate crystals between 400 nm and 2500 nm wavelength range. The saturated absorption coefficient change in undoped congruent lithium niobate was about 3 to 7 times that in Mg-doped congruent lithium niobate in the mid-infrared spectrum. The UV-illuminated LiNbO₃:Mg was fully recovered in the dark after 2 days, whereas the undoped congruent LiNbO₃ still exhibited nontrivial infrared absorption. We attribute the cause of the UV-induced absorption to the creation of hole-trapped intermediate states O⁻ near lithium vacancies.