Determination of the laser-induced damage threshold of bulk polymer samples at 1.06 mu m using the pulsed photothermal deflection technique

A pulsed Nd-YAG laser beam is used to produce a transient refractive index gradient in air adjoining the plane surface of the sample material. This refractive index gradient is probed by a continuous He-Ne laser beam propagating parallel to the sample surface. The observed deflection signals produced by the probe beam exhibit drastic variations when the pump laser energy density crosses the damage threshold for the sample. The measurements are used to estimate the damage threshold for a few polymer samples. The present values are found to be in good agreement with those determined by other methods.

Femtosecond laser-induced damage in reflector

We have investigated the damage for ZrO2/SiO2 800 nm 45 degrees high-reflection mirror with femtosecond pulses. The damage morphologies and the evolution of ablation crater depths with laser fluences are dramatically different from that with pulse longer than a few tens of picoseconds. The ablation in multilayers occurs layer by layer, and not continuously as in the case of bulk single crystalline or amorphous materials. The weak point in damage is the interface between two layers. We also report its single-short damage thresholds for pulse durations ranging from 50 to 900 fs, which departs from the diffusion-dominated tau(1/2)(p) scaling. A developed avalanche model, including the production of conduction band electrons (CBE) and laser energy deposition, is applied to study the damage mechanisms. The theoretical results agree well with our measurements. (c) 2005 Elsevier B.V. All rights reserved.

Mechanisms of femtosecond laser-induced breakdown and damage in MgO

Single-shot laser damage threshold of MgO for 40-986 fs, 800 nm laser pulses is reported. The pump-probe measurements with femtosecond pulses were carried out to investigate the time-resolved electronic excitation processes. A theoretical model including conduction band electrons (CBE) production and laser energy deposition was applied to discuss the roles of multiphoton ionization (MPI) and avalanche ionization in femtosecond laser-induced dielectric breakdown. The results indicate that avalanche ionization plays the dominant role in the femtosecond laser-induced breakdown in MgO near the damage threshold. (c) 2005 Elsevier B.V. All rights reserved.

Laser conditioning and multi-shot laser damage accumulation effects of HfO2/SiO2 antireflective coatings

Laser conditioning effects of the HfO2/SiO2 antireflective (AR) coatings at 1064 nm and the accumulation effects of multishot laser radiation were investigated. The HfO2/SiO2 AR coatings were prepared by E-beam evaporation (EBE). The singleshot and multi-shot laser induced damage threshold was detected following ISO standard 11254-1.2, and the laser conditioning was conducted by three-step raster scanning method. It was found that the single-shot LIDT and multi-shot LIDT was almost the same. The damage mostly > 80% occurred in the first shot under multi-shot laser radiation, and after that the damage occurring probability plummeted to < 5%. There was no obvious enhancement of the laser damage resistance for both the single-shot and multi-shot laser radiation of the AR coatings after laser conditioning. A Nomarski microscope was employed to map the damage morphology, and it found that the damage behavior is defect-initiated for both unconditioned and conditioned samples. © 2004 Elsevier B.V. All rights reserved.

Evaluation of laser ablation threshold in polymer samples using pulsed photoacoustic technique

The acoustic signals generated in solids due to interaction with pulsed laser beam is used to determine the ablation threshold of bulk polymer samples of teflon (polytetrafluoroethylene) and nylon under the irradiation from a Q-switched Nd:YAG laser at 1.06µm wavelength. A suitably designed piezoelectric transducer is employed for the detection of photoacoustic (PA) signals generated in this process. It has been observed that an abrupt increase in the amplitude of the PA signal occurs at the ablation threshold. Also there exist distinct values for the threshold corresponding to different mechanisms operative in producing damages like surface morphology, bond breaking and melting processes at different laser energy densities.

Measurement of laser ablation threshold on doped BiSrCaCuO high temperature superconductors by the pulsed photothermal deflection technique

Laser‐induced damage and ablation thresholds of bulk superconducting samples of Bi2(SrCa)xCu3Oy(x=2, 2.2, 2.6, 2.8, 3) and Bi1.6 (Pb)xSr2Ca2Cu3 Oy (x=0, 0.1, 0.2, 0.3, 0.4) for irradiation with a 1.06 μm beam from a Nd‐YAG laser have been determined as a function of x by the pulsed photothermal deflection technique. The threshold values of power density for ablation as well as damage are found to increase with increasing values of x in both systems while in the Pb‐doped system the threshold values decrease above a specific value of x, coinciding with the point at which the Tc also begins to fall.  

Femtosecond laser ablation of crystals SiO2 and YAG

Damage threshold of crystals SiO2 and YAG against 60-900 fs, 800 nm laser pulses are reported. The breakdown mechanisms were discussed based on the double-flux model and Keldysh theory. We found that impact ionization plays the important role in the femtosecond laser-induced damage in crystalline SiO2, while the roles of photoionization and impact ionization in YAG crystals depend on the laser pulse durations. (C) 2007 Elsevier B.V. All rights reserved.

Modified porous silica antireflective coatings with laser damage resistance for Ti:sapphire

Porous SiO2 antireflective (AR) coatings are prepared from the colloidal silica solution modified with methyltriethoxysilane (MTES) based on the sol-gel route. The viscosity of modified silica suspensions changes but their stability keeps when MTES is introduced. The refractive indices of modified coatings vary little after bake treatment from 100 to 150 Celsius. The modified silica coatings on Ti:sapphire crystal, owning good homogeneity, display prominent antireflective effect within the laser output waveband (750-850 nm) of Ti:sapphire lasers, with average transmission above 98.6%, and own laser induced damage thresholds (LIDTs) of more than 2.2 J/cm2 at 800 nm with the pulse duration of 300 ps.

Growth and physical properties of a new chiral open-framework crystal for NLO applications: Cesium hydrogen l-malate monohydrate

Cesium hydrogen l-malate monohydrate, CsH(C4H4O5)·H2O, is a new chiral open-framework semi-organic crystalline material with a second-harmonic generation efficiency one order of magnitude greater than KDP. Single crystals of this new material have been grown by the conventional slow cooling technique from aqueous solution. Grown crystals display both platy and prismatic morphologies depending on the imposed supersaturation. Hardness values measured using Vickers hardness indenter show considerable anisotropy. The resistivity behavior at room temperature and above, places the crystal between an ionic conductor and a dielectric. The single-crystal SHG efficiency estimated through Maker fringes experiment gives deff which is 4.24 times that of KDP. Single and multiple shot experiments performed on the grown crystals for the fundamental and second harmonic of pulsed Nd:YAG laser (1064 and 532 nm) show that it exhibits a high laser damage threshold which is a favorable property for nonlinear optical applications.

钛宝石强激光负载的有机硅复合凝胶增透膜研究

以硅酸四乙酯(TEOS)和甲基三乙氧基硅烷(MTEOS)为前驱材料，用溶胶-凝胶(sol-gel)方法在钛宝石表面制备得到均匀性良好且具有高激光损伤阈值的有机硅复合凝胶增透膜。膜层在钛宝石激光器输出波段(750～850nm)的增透效果显著，其平均透过率超过98．6％；激光破坏阈值为2．2J／cm^2(800nm，300ps)；膜层表面均匀性达到激光波面的要求，在皮秒、飞秒超短脉冲高功率激光领域具有应用价值。溶胶的性能测试结果表明，溶胶粘度和成膜折射率均随溶液中CH3SiO1.5溶胶体含量的增加而增大，而膜

溶胶-凝胶多孔二氧化硅减反膜稳定性研究

多孔SiO2膜层经热处理后，具有很高的激光破坏阈值，但是结构中有许多Si-OH亲水基团，导致光学透过率受环境相对湿度的影响很大。实验目的是改善膜层内部结构，使膜层结构中的亲水基团转变为疏水基团。提高膜层的疏水性，增强膜层的透过率稳定性。系统地研究了膜层透过率随时间变化的规律，在氨气和六甲基二硅氮烷(HMDS)混合气氛下热处理膜层，处理后生成Si-O-Si(CH2)3非极性疏水基团，使膜层的疏水性大大提高，因而膜层的透过率稳定性有大幅度提高。稳定性的提高延长了膜层的寿命。处理后膜层的表面粗糙度良好，均方根表

透红外含碲氟铝基玻璃的激光损伤阈值

在氟铝酸盐玻璃组分中加入适量声子能量低的重金属氧化物TeO2，得到一种新的氧氟化物玻璃。该材料具有良好的成玻璃性能，适合制作大尺寸红外窗口镜。研究了TeO2含量对玻璃特征温度、阿贝数和红外透过性能的影响。同时测试了这种玻璃的抗DF激光能力，结果表明：TeO2含量为15％的玻璃，DF激光破坏阈值达14．95kW·cm^-2。分析显示，由于玻璃基质的多声子吸收，对激光能量的吸收而引起的热冲击是导致玻璃破坏的主要原因。进一步降低玻璃中水分，可以提高玻璃抗激光破坏性能。