Laser-induced damage threshold at different wavelengths of Ta2O5 films annealed over a wide temperature range

The laser-induced damage of Ta₂O₅ films annealed at a wide range of temperature (473-1273 K) at the laser wavelengths of 1064 and 355 nm was investigated. The relations between microstructure, optical properties, chemical composition, absorption and laser-induced damage threshold (LIDT) were studied. The dependence of a damage mechanism on laser wavelengths was discussed. It was found that the LIDT either at 1064 or 355 nm first increased and then decreased with increase of annealing temperature. The LIDT at 1064 nm was influenced by the substoichiometric defects, structural defects and thermal diffusion, whereas at 355 nm it was affected mainly by the intrinsic absorption and structural defects. Both the maximum LIDT at the two wavelengths were obtained at the annealing temperature of 873 K, which could be attributed to the lowest defect density in films.     

Superhydrophobic antireflective silica films: fractal surfaces and laser-induced damage thresholds

Several superhydrophobic antireflective silica films have been prepared by a solgel method that uses hexamethyl-disilizane (HMDS) as a modifier. In a high-power laser, laser-induced damage thresholds (LIDTs) of 23-30 J/cm2 were obtained at 1064-nm wavelength with 1-ns pulse duration. By atomic-force microscopy and optical microscopy, the fractal surfaces of films were studied, and multifractal spectra (MFSs) were calculated both before and after laser damage. The two-sided effect of HMDS on particle growth determined the surface fractal of a particle and the multifractal structure of a film's surface. The bigger deltaalpha was, both before and after laser damage, the lower the LIDT was. The effect of methyl groups should be included in the determination of the MFS of the LIDT.     

Investigation of damage threshold to TiO2 coatings at different laser wavelength and pulse duration

Laser-induced damages to TiO₂ single layers and TiO₂/SiO₂ high reflectors at laser wavelength of 1064 nm, 800 nm, 532 nm, and pulse width of 12 ns, 220 ps, 50 fs, 8 ns are investigated. All films are prepared by electron beam evaporation. The relations among microstructure, chemical composition, optical properties and laser-induced damage threshold (LIDT), have been researched. The dependence of damage mechanism on laser wavelength and pulse width is discussed. It is found that from 1064 nm to 532 nm, LIDT is mainly absorption related, which is determined by film's extinction coefficient and stoichiometric defects. The rapid decrease of LIDT at 800 nm is due to the pulse width factor. TiO₂ coatings are mainly thermally by damaged at long pulse (τ ≥ 220 ps). The damage shows ablation feature at 50 fs.     

单晶硅材料的1064nm Nd:YAG脉冲激光损伤特性研究

作为微电子和光电子系统中普遍使用的一种结构材料,单晶硅一直是人们研究的焦点.长期以来,人们对其电学性质进行了非常深入细致的研究,却疏于对其抗击强激光辐照特性的研究.随着激光通讯和光电对抗技术的发展,对光学材料的激光破坏特性和加固技术进行研究的需求也显得越来越迫切.本文主要对单晶硅的抗激光损伤特性进行研究,研究了单晶硅材料在1064nm Nd:YAG激光自由脉冲输出模式和单脉冲输出模式作用下的损伤特性,通过对两种激光作用下单晶硅损伤形貌的分析,在热效应与热力耦合模型的基础上,对单晶硅的激光损伤机制进行了探索.     

毫秒激光致光学薄膜损伤阈值的测试与分析

根据ISQ-11254分别测量了脉宽1 ms,波长1 064 nm激光作用下TiO2/SiO2高反膜、增透膜的损伤阈值,结合高分辨率CCD和光学显微镜观测了损伤形貌,分析了毫秒量级激光损伤光学薄膜的损伤机理.结果表明:脉宽1 ms激光作用下TiO2/SiO2增透膜的损伤阈值为高反膜的2.4倍,损伤区域为若干分离的损伤点...     

Laser damage properties of TiO2/Al2O3 thin films grown by atomic layer deposition

Research on thin film deposited by atomic layer deposition (ALD) for laser damage resistance is rare. In this paper, it has been used to deposit TiO(2)/Al(2)O(3) films at 110 °C and 280 °C on fused silica and BK7 substrates. Microstructure of the thin films was investigated by x-ray diffraction. The laser-induced damage threshold (LIDT) of samples was measured by a damage test system. Damage morphology was studied under a Nomarski differential interference contrast microscope and further checked under an atomic force microscope. Multilayers deposited at different temperatures were compared. The results show that the films deposited by ALD had better uniformity and transmission; in this paper, the uniformity is better than 99% over 100 mm Φ samples, and the transmission is more than 99.8% at 1064 nm. Deposition temperature affects the deposition rate and the thin film microstructure and further influences the LIDT of the thin films. As to the TiO(2)/Al(2)O(3) films, the LIDTs were 6.73±0.47 J/cm(2) and 6.5±0.46 J/cm(2) at 110 °C on fused silica and BK7 substrates, respectively. The LIDTs at 11 °C are notably better than 280 °C.     

Effect of nanosecond laser pre-irradiation on the femtosecond laser-induced damage of Ta2O5/SiO2 high reflector

The effect of nanosecond laser pre-irradiation on the femtosecond laser-induced damage behaviors of 800 nm 0° AOI Ta(2)O(5)/SiO(2) high reflectors fabricated by e-beam evaporation was explored. Laser pre-irradiation was carried out by Raster-scanning with scanning mode of 1-on-1 and scanning velocities timed such that there was a beam overlap at 70% of the peak fluence, utilizing 5 Hz 1064 nm 12 ns Nd:YAG fundamental lasers. Femtosecond laser damage was investigated by 1 kHz 800 nm 135 fs Ti: sapphire laser system with 1-on-1 mode test. The results indicated that nanosecond laser pre-irradiation did not promote the femtosecond laser-induced damage threshold of reflectors. Instead, the thresholds of all the samples with various fluence steps for pre-irradiation were reduced by about 20%. Furthermore, the damage morphologies were analyzed by optical microscope, SEM and AFM, which displayed deterministic field induced breakdown characteristics. To explain these phenomena, a theoretical model including photoionization, avalanche ionization, and decays of electrons was built to simulate the evolution of electron density in the conduction band. Field ionization mechanism was considered to dominate the femtosecond laser damage process, while the electronic defects induced by nanosecond laser pre-irradiation accelerated the femtosecond laser damage evolution.     

Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress

We report on a technique utilizing time-resolved detection of laser-induced stress transients for the measurement of optical properties in turbid media specifically suitable for biological tissues. The method was tested initially in nonscattering absorbing media so that it could be compared with spectrophotometry. The basis of this method is provided by the conditions of temporal stress confinement in the irradiated volume where the pressure generated in tissues heated instantly by laser pulses is proportional to the absorbed laser energy density, and the exponential profile of the initial stress distribution in the irradiated volume corresponds to the z-axial distribution of the absorbed laser fluence. Planar thermoelastic waves can propagate in water-containing media with minimal distortion, and their axial profiles can be detected by an acoustic transducer with sufficient temporal resolution. The acoustic waves induced by 14-ns laser pulses in nonscattering media, turbid gels, and tissues were measured by a piezoelectric transducer with a 3-ns response time. Temporal profiles of stress transients yielded z-axial distributions of the absorbed laser energy in turbid and opaque media, provided that the speed of sound in these media was known. The absorption and effective scattering coefficients of beef liver, dog prostate, and human aortic atheroma at three wavelengths, 1064 nm (in near infrared), 532 nm (visible), and 355 nm (near UV), were deduced from laser-induced stress profiles with additional measurements of total diffuse reflectance.     

Laser-induced bulk damage in various types of vitreous silica at 1064, 532, 355, and 266 nm: evidence of different damage mechanisms between 266-nm and longer wavelengths

Laser-induced-damage thresholds (LIDT's) with various types of vitreous silica at 1064, 532, 355, and 266 nm are investigated. At 1064 nm no difference in the LIDT was observed in any sample. At 1064-355 nm the wavelength dependence of the LIDT of synthetic fused silica (SFS) can be described well by the relation I(th) = 1.45lambda(0.43), where I(th) is the LIDT in J/cm(2) and lambda is the wavelength in nanometers. At 266 nm, however, LIDT's were smaller than half of the calculated value from the relation above. This difference can be explained by the damage mechanism; at 266 nm two-photon absorption-induced defects lower the LIDT as in the case of KrF-excimer-laser-induced defects, whereas at longer wavelengths the two-photon process does not occur. LIDT's of fused quartz (FQ) at 532 and 355 nm and that of SFS containing ~1000 ppm of Cl and no OH at 355 nm were a little lower than those of the other SFS's. This lower LIDT may be related to the absorption of metallic impurities in FQ and dissolved Cl(2) molecules in SFS. At 266 nm, on the other hand, LIDT's of FQ's were higher than those of most SFS's.     

Sub-part-per-billion analysis of aqueous lead colloids by ArF laser induced atomic fluorescence

Highly sensitive analysis of aqueous lead carbonate colloids was demonstrated by two-pulse laser-induced atomic fluorescence. The first laser pulse at 1064 nm ablated the sample solution to create an expanding plume. The colloids, being heavier, trailed behind and became concentrated. They were then intercepted by an ArF laser pulse that induced prompt atomic fluorescence at 405.8 nm from the lead atoms. The detection limit for lead was 0.24 ppb. Tap water was analyzed, and lead emissions were clearly observed. Time-resolved spectroscopy revealed that the efficient 193-nm excitation of the analytes was more universal than expected. That was confirmed by the successful application of the technique to colloids and alloys other than lead.     

Fabrication of mitigation pits for improving laser damage resistance in dielectric mirrors by femtosecond laser machining

Femtosecond laser machining is used to create mitigation pits to stabilize nanosecond laser-induced damage in multilayer dielectric mirror coatings on BK7 substrates. In this paper, we characterize features and the artifacts associated with mitigation pits and further investigate the impact of pulse energy and pulse duration on pit quality and damage resistance. Our results show that these mitigation features can double the fluence-handling capability of large-aperture optical multilayer mirror coatings and further demonstrate that femtosecond laser macromachining is a promising means for fabricating mitigation geometry in multilayer coatings to increase mirror performance under high-power laser irradiation.     

外加偏置电场对类金刚石薄膜激光损伤形貌影响研究

研究了使用非平衡测控溅射技术沉积的类金刚石(DLC)薄膜其抗激光损伤能力;通过对比施加偏置电场前后薄膜的损伤情况,发现:DLC薄膜施加偏置电场后,薄膜的激光损伤区域内有大量丝状薄膜,损伤形貌存在明显不同,损伤面积减小;薄膜的激光损伤情况得到改善。实验结果显示,外加偏置电场对DLC薄膜的损伤有影响。认为:激光在DLC薄膜中激励产生的光生电子在电场的作用下产生快速漂移,间接降低了激光辐照区域内的局部能量密度,减缓了薄膜的石墨化,提高了DLC薄膜的抗激光损伤能力。     

Analysis of optical damage mechanisms in hollow-core waveguides delivering nanosecond pulses from a Q-switched Nd:YAG laser

Hollow-core waveguides consisting of a glass capillary tube with an internal reflective coating are capable of delivering pulse energies of tens of millijoules with improved focusability compared to step index fibers of similar core diameter. We demonstrate the capability of these fibers to deliver high-power Q-switched pulses at the fundamental (1064 nm), second (532 nm), and third (355 nm) harmonics of a Nd:YAG laser, both in terms of peak power and beam quality delivered. In terms of peak power delivery, the primary limitation is the occurrence of bend-induced optical damage to the reflective coating. The damage mechanism and the influential factors are analyzed, in particular, the dependence upon the number of guided modes, core diameter, coating thicknesses, and input polarization alignment.     

Temporal evolution of the laser-induced breakdown spectroscopy spectrum of aluminum metal in different bath gases

The spectral emission of gas-phase aluminum and aluminum oxide was measured during and immediately after exposure of a bulk-aluminum sample to a laser-induced spark produced by a focused, pulsed laser beam (Nd:YAG, 10-ns pulse duration, 35 mJ/pulse, lambda = 1064 nm). The spectral emission was measured as a function of time after the onset of the laser pulse, and it was also measured in different bath gases (air, nitrogen, oxygen, and helium).     

Compact high-power, TEM(00) acousto-optics Q-switched Nd:YVO4 oscillator pumped at 888 nm

A compact, high-power, high-repetition-rate Q-switched laser oscillator is presented in this paper. Adopting the acousto-optics Q-switching mechanism, the maximum output power of 57 W of 1064 nm fundamental wave at 200 kHz is obtained under 110 W 888 nm pumping. The stable working range extends from 30 to 250 kHz, while the beam quality factor M(2) is smaller than 1.30. Subsequently, based on the 42 W of fundamental wave with the beam quality better than 1.10, 29.5 W of 532 nm green laser and 18 W of 355 nm ultraviolet wave are achieved.     

Investigation of optical damage in the nonlinear optical crystal (−)2-(α-methylbenzylamino)-5-nitropyridine

Abstract

Pulsed laser induced surface damage experiments were made on the cleaved (001) face of (—)2-(α-methylbenzylamino)-5-nitropyridine (MBANP) crystals using a pulse length of 25 ns FWHM. The highest single pulse damage threshold value was 24·2 J cm^?2 at a wavelength of 1064 nm for light polarized along the crystallographic a axis, whereas the lowest was 1·8 J cm^?2 at 532 nm for light polarized along the crystallographic b axis. The extent of the mechanical distortion of the (001) crystal plane consequent upon the absorption of a 25 ns FWHM, 10 J cm^?2 pulse from a Nd:YAG laser operating at the wavelength 1064 nm has been measured by a non-destructive and non-invasive technique.     

Light intensification modeling of coating inclusions irradiated at 351 and 1053 nm

Electric-field modeling provides insight into the laser damage resistance potential of nodular defects. The laser-induced damage threshold for high-reflector coatings is 13x lower at the third harmonic (351 nm) than at the first harmonic (1053 nm) wavelength. Linear and multiphoton absorption increases with decreasing wavelength, leading to a lower-third harmonic laser resistance. Electric-field effects can also be a contributing mechanism to the lower laser resistance with decreasing wavelength. For suitably large inclusions, the nodule behaves as a microlens. The diffraction-limited spot size decreases with wavelength, resulting in an increase in intensity. Comparison of electric-field finite-element simulations illustrates a 3x to 16x greater light intensification at the shorter wavelength.     

Wavelength dependence of repetitive-pulse laser-induced damage threshold in beta-BaB2O4

The dependence on wavelength of repetitive-pulse (10 Hz, 8-10 ns) laser-induced damage on beta barium metaborate (BBO) has been investigated. The thresholds of dielectric breakdown in bulk crystal have been found to be 0.3 GW/cm(2) at 266 nm, 0.9 GW/cm(2) at 355 nm, 2.3 GW/cm(2) at 532 nm, and 4.5 GW/cm(2) at 1064 nm. Results indicate two-photon absorption at 266 and 355 nm, which helps to produce an avalanche effect that causes breakdown at each of the four wavelengths tested. Neither the BBO refractive indices nor the absorption spectrum change until breakdown occurs.     

Spatially and temporally resolved electron number density measurements in a decaying laser-induced plasma using hydrogen-alpha line profiles

A Nd:YAG laser was operated at 1064 nm and with 6-ns pulse duration to achieve optical breakdown in gaseous hydrogen at pressures of 150 and 810 Torr. Spatially and temporally resolved laser-induced emission spectra were measured early in the plasma decay. With hydrogen-alpha line profiles, electron number density values were determined along the laser beam plasma in the range 10(19) to 10(16) cc(-1).     

Remote imaging laser-induced breakdown spectroscopy and laser-induced fluorescence spectroscopy using nanosecond pulses from a mobile lidar system

A mobile lidar system was used in remote imaging laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) experiments. Also, computer-controlled remote ablation of a chosen area was demonstrated, relevant to cleaning of cultural heritage items. Nanosecond frequency-tripled Nd:YAG laser pulses at 355 nm were employed in experiments with a stand-off distance of 60 meters using pulse energies of up to 170 mJ. By coaxial transmission and common folding of the transmission and reception optical paths using a large computer-controlled mirror, full elemental imaging capability was achieved on composite targets. Different spectral identification algorithms were compared in producing thematic data based on plasma or fluorescence light.