Etch frustration in congruent lithium niobate single crystals induced by femtosecond ultraviolet laser irradiation

Modification of the chemical etching behaviour of undoped congruent z-cut lithium niobate single crystals is achieved by pre-illumination of the –z face of the crystal with sub-ps ultraviolet laser radiation at 248 nm, at energy fluences below the threshold for ablation. A systematic study of the effect of the energy fluence and total exposure on the etch frustration is presented. Received: 23 July 2001 / Accepted: 28 September 2001 / Published online: 20 December 2001     

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.     

Threshold fluence for domain reversal directly induced by femtosecond laser in lithium niobate

Fabricating domain reversal directly induced by femtosecond laser is a novel and promising method to induce micron-period or even submicron-period inverted domain structure for it averts the domain spreading and mergence which is hard to avoid by traditional electric-field poling method. In this paper, the domain reversal process in lithium niobate crystal by irradiation of femtosecond pulses whose spatial and temporal distributions are taken into consideration is numerically simulated in the framework of Fahy’s model. The simulation results manifest the domain inversion window theory and predict the threshold reversal fluence. The experiment to form domain reversal via direct illumination with femtosecond laser in Y-cut lithium niobate samples was conducted at room temperature. The multi-ring-like structures on the processed samples tally with the inversion window theory and the calculated threshold reversal fluence is well within the scope obtained by simulation, which serves as a corroborative evidence to prove the domain reversals can be formed by direct irradiation with femtosecond laser in lithium niobate.     

Ablation induced by single-and multiple-femtosecond laser pulses in lithium niobate

We present results on the surface-damage threshold of lithium niobate after single-and multiple-femtosecond laser pulse irradiation at the pulse duration of 80 fs, with a 800-nm wavelength, and a repetition rate of 1 kHz. The surface-ablation threshold was found to decrease significantly with an increase in the pulse number applied to the surface until reaching an almost constant level due to an incubation effect, which is attributed to the laser-induced defect formation. The threshold of lithium niobate under a single shot is found to be 2.82 J/cm², and the threshold fluence for an infinite number of pulses F _th(∞) converges to a common value of 0.52 ± 0.06 J/cm² for N > 80. The results have the potential for application in laser micromachining and the fabrication of related optical devices and applications in frequency conversion by a femtosecond laser in lithium niobate.     

Femtosecond laser ablation of DAST

High rate femtosecond (fs) laser ablation of the organic salt 4-N,N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST), an organic crystal with very high optical nonlinearities has been demonstrated. The threshold fluence and the ideal fluence range for damage free ablation for the wavelengths 550, 600, and 775 nm have been determined and the quality of the produced grooves has been investigated. The threshold fluences are in the order of 10–70 mJ/cm² and the ideal fluence range for damage free ablation is ranging from 30 to 300 mJ/cm², depending on the wavelength. The optimal focussing for ablation has been investigated and first results towards the structuring of a ridge waveguide are presented. We conclude that this method is most promising for waveguide patterning of DAST surfaces for integrated optics applications.     

Laser-induced preferential domain nucleation in hafnium-doped congruent lithium niobate crystal

The focused visible laser-induced preferential domain nucleation effect is investigated in 3 mol% hafnium-doped congruent lithium niobate crystal. The local phase variation is in-situ monitored during laser-induced preferential domain nucleation. The variations of phase distributions are reconstructed by digital holographic interferometry. The nucleation field decreases exponentially with increasing irradiation intensity. The space charge field along the z direction is thought to be an important mechanism for the laser-induced preferential domain nucleation. Laser-induced hafnium-doped congruent lithium niobate crystals appear to be a promising candidate for further development of ferroelectric domain engineering.     

Periodic poling of magnesium-oxide-doped stoichiometric lithium niobate grown by the top-seeded solution method

Magnesium-oxide-doped stoichiometric lithium niobate has been produced using the technique of top-seeded solution growth from a lithium-rich melt. Optical tests, performed with a combination of argon-ion laser lines, have confirmed a previously published result (at 532 nm) that this material has superior resistance to photorefractive damage. This material has been shown, for the first time, to be amenable to periodic poling. Optical parametric oscillator tests have shown that this material maintains the advantages of periodically poled, congruent, un-doped lithium niobate while showing no evidence of photorefractive damage under typical operating conditions. Operating wavelengths as a function of quasi-phase-matching period and temperature have been measured for the optical parametric oscillator, providing useful new information about refractive-index dispersion in this material. This work establishes periodically poled, magnesium-oxide-doped stoichiometric lithium niobate as a viable material for nonlinear optics. Received: 28 June 2000 / Revised version: 12 September 2000 / Published online: 7 February 2001     

掺杂对铌酸锂晶体非挥发全息存储性能的影响

通过研究掺镁、掺锌和掺铟同成分铌酸锂晶体的紫外-红光双色全息存储性能,发现双色记录响应时间均比单色记录时明显缩短,最多的可减小3个数量级;双色记录灵敏度大幅度提高,在掺镁5 mol.%的晶体中可达到11 cm/J.在掺杂浓度超过抗光损伤阈值的铌酸锂晶体中,均可实现非挥发全息存储.但是,在掺镁、锌样品中,深、浅能级中心上的光栅反相,而在掺铟样品中则表现为同相.这是由于掺杂离子的种类不同,在铌酸锂晶体中形成的缺陷中心也不同所引起的. 关键词： 掺杂 铌酸锂晶体 非挥发 全息存储     

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.     

飞秒激光的波长对SiC材料烧蚀的影响

利用10倍的显微物镜将近红外飞秒激光脉冲汇聚到宽带隙半导体材料6H SiC的前表面,研究样品的烧蚀及诱导微细结构。用扫描电镜(Scanning electron microscope,SEM)及光学显微镜测量烧蚀斑。利用烧蚀面积与激光脉冲能量的关系确定SiC的烧蚀阈值。给出了SiC样品的烧蚀阈值与飞秒激光波长的依赖关系。实验结果表明,可见光区随波长增加,烧蚀阈值从0.29J/cm2增加到0.67J/cm2;而在近红外区,SiC的烧蚀阈值为0.70J/cm2左右,基本上不随激光波长变化而改变。结合计算结果,可以认为在飞秒激光烧蚀SiC的过程中,在近红外区,光致电离和碰撞电离均起到了重要的作用;而在可见光区,光致电离的作用相对大一些。     

Experimental evidence of a non-relationship between photorefractive inhibition and photoconductivity increase in LiNbO3:Mg

By means of measurements of both photoconductivity and two-wave mixing using cw 532 nm laser light, a direct relationship between optical damage resistance and photoconductivity coefficient for several congruent magnesium-doped lithium niobate crystals, with concentrations below and above the threshold of around 4.6 mol% MgO in melt, has not been observed. Specifically, when the polar axis is parallel to the photorefractive grating vector formed by two-interference beams, an increase of optical-damage resistance above the threshold is obtained. However, the photoconductivity coefficient is of the same magnitude of those samples below the threshold. On the other hand, when the optical axis is perpendicular to the grating vector, a decrease of the refractive index grating for crystals below the threshold could be observed, but even for this case the photoconductivity coefficient is unchanged, except for only one specimen with high magnesium level which exhibits simultaneously photorefractive response and high photoconductivity. These results suggest that the increase of photoconductivity is not very essential in the process of photorefractive inhibition; rather, the distribution of magnesium ions with respect to polar axis is an important parameter in the mechanism of optical-damage resistance.     

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。     

超短脉冲照射下氟化锂的烧蚀机理及其超快动力学研究

研究了超短脉冲激光照射下LiF晶体的破坏机理及其超快动力学过程,利用扫描电镜和原子力显微镜等测试手段,观测了飞秒激光照射下LiF晶体的烧蚀形貌。利用烧蚀面积与激光脉冲能量的对数关系确定了LiF晶体的破坏阈值,并利用非线性玻璃棒展宽脉宽,得到了800nm激光作用下LiF破坏阈值对激光脉宽（50～1000fs）的依赖关系;利用抽运一探针超快探测平台,探测了LiF烧蚀过程中反射率的变化。采用雪崩击穿模型,并根据晶体材料反射率与材料的介电常量的依赖关系,通过数值计算,模拟了材料烧蚀阈值与脉宽的依赖关系及材料激发过程中反射率的变化关系。结果表明,理论结果与实验结果符合较好。讨论了飞秒激光照射下LiF晶体中导带电子数密度的变化规律,并解释了相应的实验结果。     

Doping transition metal ions as a method for enhancement of ablation rate in femtosecond laser irradiation of silicate glass

We present a doping method to improve the femtosecond laser ablation rate and promote ablation selectivity. Doping transition metal ions, Co2＋ or Cu2＋, in silicate glass apparently change absorption spectroscopy and induce resonant absorption at wavelengths of 600 and 800 nm, respectively. Comparing with femtosecond laser processing of the same glass without doping, we find that the threshold fiuenee decreases and the ablation rate increases in resonant absorption in doped silicate glass. Resonant absorption effectively increases multiphoton ionization for seed-free electron generation, which in turn enhances avalanche ionization.     

Experimental study on reflection of high-intensity nanosecond Nd:YAG laser pulses in ablation of metals

The total reflectivity of tin and magnesium in ablation by nanosecond Nd:YAG laser pulses in air is studied. It was found that the high initial reflectivity of the studied metals undergoes a significant drop to values of 0.11 for Sn and 0.16 for Mg within a laser fluence range between about 0.8 and 11 J/cm². These reduced reflectivity values remain virtually unchanged with further increasing laser fluence. This study shows that a significant reflectivity decrease of the studied metals is caused by plasma formation in front of the irradiated surface. Below the plasma formation threshold, the reflectivity of the studied metals is observed to be virtually independent of laser fluence, indicating a small role of Drude׳s temperature effect on the reflectivity of the studied samples.     

Phase-matched nonlinear diffraction

We report on a new (to our knowledge) configuration incorporating both birefringence and quasi-phase-matching, enabling efficient phase-matched nonlinear diffraction in one-dimensional periodically poled nonlinear crystals. We demonstrate the method experimentally, showing an efficient nonlinear diffraction to the first few orders in two types of crystals, MgO doped congruent lithium niobate and congruent lithium niobate, and characterize its efficiency dependence on the fundamental power, the propagation angle, and the crystal temperature. This configuration can increase efficiencies observed in nonlinear diffraction experiments, enables ferroelectric domain characterization by nonlinear microscopy, and can be used to determine the duty cycles of periodically poled nonlinear crystals.     

Z-scan method in a study of lithium niobate with surface doping by photorefractive impurities

Doping of the lithium niobate surface significantly modifies its photorefractive characteristics. In the present work, the special features of the Z-scan response are experimentally investigated for lithium niobate with surface doping by ions of iron, copper, and their combinations. In experiments, helium-neon laser radiation with a wavelength of 633 nm and semiconductor laser radiation with a wavelength of 660 nm are used. Dependences of the response characteristics on the character of doping and the intensity and polarization state of probing radiation are obtained for the closed Z-scan scheme. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 22–29, June, 2006.     

Light scattering in single crystals of nonlinear-optical photorefractive LiNbO<Subscript>3</Subscript>:Cu and LiNbO<Subscript>3</Subscript>:Zn

Comparative studies are made of the photorefractive scattering of light in nonlinear single crystals of lithium niobate with congruent compositions (LiNbO₃) doped with "photorefractive" Cu [0.015 mass %] and "nonphotorefractive" Zn [0.5 mass %] cations. For the first time it is found that single crystals doped with "photorefractive" and "nonphotorefractive" cations have different indicatrices for photorefractive light scattering. The aperture angle for photorefractive scattering reaches its steady state value more rapidly with high laser powers than with low. However, at high powers laser induced heating of the crystal is greater, and this leads to a narrowing of the scattering indicatrix. It is also found that photorefractive scattering in these single crystals depends on the region of the boule from which a sample has been cut. This indicates that there is a nonuniform distribution over the boule of the imperfections with localized electrons which determine the magnitude of the photorefractive effect.     

Influence of short-wavelength radiation of the visible range on the optical transmission of photorefractive lithium niobate samples

Special features of the optical transmission of lithium niobate crystal samples with surface doping by photorefractive impurities at the wavelength of a helium-neon laser under short-wavelength incoherent background illumination are experimentally investigated and discussed. The possibility of optically controlled thermooptical modulation of the coherent light intensity with characteristic time of a few seconds is demonstrated for samples with surface doping by a combination of iron and copper ions. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 76–80, November, 2006.     

Investigation of the stability of electrical properties of reduced LiNbO<Subscript>3</Subscript> crystals

The instability of the electrical properties of lithium niobate single crystals of congruent composition subjected to reducing thermochemical treatment has been investigated by impedance spectroscopy. It has been shown that the subsequent heating of the reduced lithium niobate samples in dry air up to 380 K or higher is accompanied by the progressive increase in their electric resistance, which is due to the oxidization of the crystal surface layers.