Surface texturing of aluminum alloy 2024-T3 via femto- and nanosecond pulse excimer laser irradiation

Surface texturing effects of aluminum alloy 2024-T3 using femtosecond and nanosecond pulse laser irradiation were studied. The micrographs of the scanning electron microscopy (SEM) were characterized as a function of incident laser fluence. Results indicated that the surface features, ranging from nano to microdimension, can be developed through variation in laser fluence intensities. Two ablation regimes in the logarithmic fluence dependence of the ablated depth for the 500-fs-pulse irradiation were observed. The theoretical analysis for ablation processes is in good agreement with the experimental results.     

Free electron laser ablation of urinary calculi: an experimentalstudy

Infrared laser ablation of urinary calculi was investigated as a function of wavelength to determine the relation of ablation threshold fluences, ablation depths, and optical absorption. A simple photothermal ablation model was employed to examine this relationship. Human urinary calculi composed of >95% uric acid, >95% cystine, >95% calcium oxalate monohydrate (COM), and >90% magnesium ammonium phosphate hexahydrate (MAPH) were used. Various wavelengths between 2.1 and 6.5 μm were selected to perform threshold fluence and ablation depth measurements. The laser source for this study was the tunable pulsed infrared free electron laser (FEL) at Vanderbilt University. Experimental results indicated a correlation of threshold fluence and ablation depth to the optical absorption properties of the calculi. When calculus optical absorption increased, the threshold fluences decreased. Although the ablation depths increased with calculus optical absorption, results indicated that in certain calculi the ablation depth was affected by optical attenuation through the ablation plume. These observations were in agreement with the photothermal ablation model, but fractures in striated calculi at higher optical absorptions indicated the contribution of a photomechanical mechanism     

Plasma mediated ablation of biological tissues withnanosecond-to-femtosecond laser pulses: relative role of linear andnonlinear absorption

Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range of 1 ns-300 fs at 1053-nm wavelength. It was found that pulsed laser ablation of transparent and weakly absorbing gels is always mediated by plasma. On the other hand, ablation of strongly absorbing tissues is mediated by plasma in the ultrashort-pulse range only. Ablation threshold along with plasma optical breakdown threshold decreases with increasing tissue absorbance for subnanosecond pulses. In contrast, the ablation threshold was found to be practically independent of tissue linear absorption for femtosecond laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond range of laser pulse duration, optical breakdown proceeds via avalanche ionization initiated by heating of electrons contributed by strongly absorbing impurities at the tissue surface. In the ultrashortpulse range, optical breakdown is initiated by multiphoton ionization of the irradiated medium (six photons in case of tissue irradiated at 1053-nm wavelength), and is less sensitive to linear absorption. High-quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with subpicosecond laser pulses. Experimental results suggest that subpicosecond plasma mediated ablation can be employed as a tool for precise laser microsurgery of various tissues     

Enhanced polymer ablation rates using high-repetition-rateultraviolet lasers

Etch rates (μm/pulse) for glycol-modified polyethylene terephthalate (PETG) under pulsed UV (255 nm) laser processing are measured as a function of pulse repetition frequency in the range 0.7-15 kHz. Materials removal rates (μm/s) scale approximately linearly with pulse repetition frequency at a fluence of 0.59 J/cm², and there appears to be no attenuation of the ablating laser beam by the ejected material plume for pulse rates up to 15 kHz. The instantaneous etch rate for pulses in a sequence increases markedly (~40%) for long pulse sequences (>100 pulses) at high PRF (15 kHz), an effect which can be used to increase machining rates while operating at a moderate laser fluence     

Mechanism of dye-enhanced pulsed laser ablation of hard tissues:implications for dentistry

Alexandrite laser ablation of enamel enhanced by an indocyanine green dye was studied. A microjet system was employed to deliver precisely measured small amounts of absorbing dye solution to the site of irradiation. A sequence of physical phenomena involved in dye-enhanced laser ablation of dental enamel was revealed when laser pulse profiles were compared with the profiles of laser-induced pressure, laser-induced plasma, and ablation plume kinetics. To understand photomechanical effects on the enamel ablation, the absolute values of pressure waves were measured by a calibrated wide-band acoustic transducer. Absolute amplitude and temporal profile of pressure waves, plasma emission, ablation plume kinetics, ablation efficiency, and crater quality under free-running and and Q-switched ablation of enamel were studied. It was found that there is an optimal dye solution volume (100-200 nL) when the maximum ablation efficiency (30 μm/pulse) can be obtained. It was shown that the ablation efficiency under Q-switched laser irradiation is approximately one order of magnitude lower than that under free-running ablation. It was shown that Q-switched enamel ablation with dye solution is caused by the powerful recoil pressure wave with an amplitude 3-6.5 kbar. In contrast, dye-enhanced free-running enamel ablation is caused by plasma-mediated evaporation of enamel and accompanied by recoil pressure waves of lower amplitude (0.5-1 kbar) that is below mechanical damage threshold in enamel. Uneven crater walls after Q-switched ablation were observed by scanning electron microscopy (SEM). Free-running ablation makes precise craters with smooth and even crater walls     

Analysis of cavitation dynamics during pulsed laser tissue ablationby optical on-line monitoring

Flashlamp pumped mid-IR laser systems emitting in the 2-3-μm wavelength range are widely used for various medical applications, especially for tissue ablation. Explosive evaporation is inevitably associated with this process due to the short pulse durations of these laser systems and the high absorption of tissue and water in this spectral regime. Tissue displacement and dissection occur in liquid environment as a consequence of the induced cavitation. Depending on the application these processes might enhance the tissue ablation but can also cause adverse tissue effects. The ablation dynamics were investigated by evaluating the change in reflected probe-light intensity re-emitted from the application fiber tip. The ablated cavity and the signal was correlated to fast-flash photographs of the event. Based on this reflection signal a water/tissue discrimination system is introduced which can widely support medical laser applications. In laser sclerostomy ab externo, for example, this approach can be used as a feedback system to automatically control the ablation process. With such a system, adverse effects to adjacent tissue in the anterior chamber of the eye can be minimized     

Preparation of PbTiO3 thin films by laser ablation

Abstract

PbTiO₃ thin films have been deposited on Si or Pt substrates by laser ablation method using ArF excimer or YAG laser. Species evaporated in the vacuum have been studied by mass analysis, and the measured Pb and Ti are not oxidized. Spacial distributions of the deposition rates on the substrate have been studied as a parameter of laser fluence. The excimer laser gives better films than the YAG. Perovskite films have been obtained at substrate temperature ～ 400 °C and appropriate ambient O₂ gas pressure.     

Low Temperature Process for Synthesis of (100) Textured Pb(Zr0.48Ti0.52)O3 Thin Films on Si Substrate by Laser Lift-Off Transferring Technique

(100) textured Pb(Zr_0.48Ti_0.52)O₃ (PZT) films were prepared on silicon substrates by MOD process and laser lift-off technique. Textured PZT films were first grown on (001) Sapphire substrate, using Ba(Mg_1/3Ta_2/3)O₃ (BMT) materials as buffer layer. The (100) textured PZT/BMT/Sapphire films were attached to Si substrate using a transient-liquid-phase Pd-In bonding process, and then were separated from Sapphire substrates by a laser lift-off process, in which, a 38 ns pulse from excimer laser (248 nm) at 600 mJ/cm² fluence melted BMT buffer layer, expelling the Sapphire. The crystallinity of the surface of films was further improved by laser annealing. X-ray diffraction analysis of the PZT films showed that the crystallographic structure of films is maintained during laser lift-off process. Electrical testing of the films after laser lift-off process followed by laser annealing demonstrated that the ferroelectric properties are retained for the transferred films (P_r = 9μ C/cm² and E_c = 74 kV/cm).     

Photothermal-induced temperature changes in a model inner ear: acomparison of visible, infrared, and ultraviolet lasers

The temperature change in a model cylindrical vestibule (90 mm³) was measured following irradiation by argon (488-514 nm), CO₂ (10.6 μm), KTP (Nd:YAG) (532 nm), Er:YAG (2.9 μm), and XeCl (308 nm) lasers. Otic capsule bone was used to simulate the otosclerotic stapes footplate, and the thickness of each specimen was machined to variable thicknesses (0.20-0.90 mm). Thermocouples were used to measure the temperature below the air-bone surface at depths of 1.0, 3.0, and 5.0 mm. The time-dependent temperature change, thermocouple position, and bone thickness were measured following single pulse application from argon, CO₂, and KTP (Nd-YAG) lasers. The effect of infrared and ultraviolet lasers on vestibule fluid temperature changes were studied with several fluence and pulse sequences. The temperature change in the vestibule following pulsed laser irradiation decreased with increasing bone thickness and thermocouple depth. Laser irradiation from CO₂, argon, KTP-532, XeCl, and Er:YAG lasers produced minimal (less than 5°) vestibule temperature changes. Measured temperatures were in good agreement with an analytic model, based on a solution to the bio-heat equation in semi-infinite media. The results are discussed with relevance to ear surgery     

高压电缆缓冲层烧蚀缺陷的超声检测实验

高压电缆缓冲层烧蚀故障是近年来频发的电缆故障类型，然而目前的烧蚀缺陷检测手段难以满足存量电缆的检测需求。本文首次研究了基于铝护套内表面粗糙度的高压电缆缓冲层烧蚀缺陷的超声检测方法。首先，开展了潮湿条件下的缓冲层烧蚀模拟实验，并对烧蚀后的铝片开展了激光共聚焦显微镜测试以及电化学阻抗谱分析，发现随着烧蚀时间的增加，铝片的表面粗糙度逐渐增大，同时铝片表面的腐蚀程度逐渐加深，对应的缓冲层烧蚀缺陷逐渐加重，表明铝片的表面粗糙度与潮湿条件下缓冲层的烧蚀程度存在关联。其次，对烧蚀后的铝片开展了超声检测实验，并通过相邻超声回波信号的幅值比推算出了铝片腐蚀面的粗糙度，与实验测得的粗糙度具有相同的变化趋势。本文结果表明超声检测可用于检测缓冲层烧蚀缺陷的严重程度，为高压电缆缓冲层烧蚀缺陷超声检测方法的应用奠定了研究基础。     

Preparation of organic semiconductive thin films by laser ablation: Control of structure and electric properties by selecting wavelength,fluence, and substrate temperature

Amorphous organic semiconductive thin films with electric conductivities ranging between 10⁻⁵ and 10¹ Scm⁻¹ are prepared on several temperature-controlled substrates by excimer laser ablation (ELA) of 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) with 193 nm (ArF), 248 nm (KrF) and 308 nm (XeCl) beams. The structure, electric conductivity, and carrier species of the prepared films depend strongly on the ablation wavelength, fluence, and substrate temperature. Thermoelectromotive force measurements demonstrate conversion of carrier species from n-type to p-type with increasing fluence of a 308-nm beam from 0.2 to 4.0 Jcm⁻²pulse⁻¹. A film prepared on a substrate at 300 °C by ELA with a 308-nm beam partially contains a polyperinaphthalene (PPN) structure with electric conductivity of 10⁻² to 10⁻¹ Scm⁻¹. © 1998 Scripta Technica, Electr Eng Jpn, 125(2): 19–26, 1998     

Pulse-extended excimer laser annealing of lead zirconate titanate thin films

Abstract

Excimer laser annealing is a promising method for the crystallisation of ferroelectric layers, such as PZT, in low thermal budget integrated device fabrication processes. A technique is described whereby the problem of very high surface temperatures in PZT is overcome by the use of temporal pulse extension, where the effective laser pulse length is increased from 24ns to 374ns. Modelled temperature profiles through a PZT thin film structure during laser irradiation illustrate the benefit of pulse extension due to enhanced heat propagation into the PZT. The modelling also shows that underlying silicon is not heated significantly even with pulse extension. Initial results show that PZT can be crystallised into the perovskite phase from the top downwards with minimal surface damage.     

Ce3+:LiCaAlF6 crystal for high-gain orhigh-peak-power amplification of ultraviolet femtosecond pulses and newpotential ultraviolet gain medium: Ce3+:LiSr0.8Ca0.2AlF6

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce³⁺:LiCaAlF₆ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 μJ output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce³⁺ :LiSr_0.8Ca_0.2AlF₆, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module     

脉冲激光清洗瓷式绝缘子表面污秽温度场和应力场分析

脉冲激光清洗技术具有运行成本低和可控性好等优点,已开始应用于瓷式绝缘子表面的污秽清除。脉冲能量密度太大,会造成瓷绝缘子的损伤。能量密度过小,直接影响清洗效率。文中以瓷式绝缘子及表面污秽为对象,通过建立有限元模型,分析不同能量密度脉冲激光下瓷式绝缘子表面温度和应力随距离变化的规律,研究脉冲激光清洗瓷式绝缘子的表面污秽清洗机制,确定最佳清洗能量密度。结果表明,能量密度为1.18 J/cm^2到2.01 J/cm^2的脉冲激光清洗瓷式绝缘子时,绝缘子表面温度远小于其气化温度;脉冲能量密度为1.41 J/cm^2时,表面最大的应力小于且接近瓷式绝缘子的抗拉强度,在不损伤基底的前提下此脉冲能量密度清污效率最高。研究成果为脉冲激光清洗瓷式绝缘子能量密度选择提供了重要依据。     

Semiconductor saturable absorber mirrors (SESAM's) for femtosecondto nanosecond pulse generation in solid-state lasers

Intracavity semiconductor saturable absorber mirrors (SESAM's) offer unique and exciting possibilities for passively pulsed solid-state laser systems, extending from Q-switched pulses in the nanosecond and picosecond regime to mode-locked pulses from 10's of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAM's for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAM's provide sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence, in a compact structure with low insertion loss. We have been able to demonstrate, for example, passive modelocking (with no Q-switching) using an intracavity saturable absorber in solid-state lasers with long upper state lifetimes (e.g., 1-μm neodymium transitions), Kerr lens modelocking assisted with pulsewidths as short as 6.5 fs from a Ti:sapphire laser-the shortest pulses ever produced directly out of a laser without any external pulse compression, and passive Q-switching with pulses as short as 56 ps-the shortest pulses ever produced directly from a Q-switched solid-state laser. Diode-pumping of such lasers is leading to practical, real-world ultrafast sources, and we will review results on diode-pumped Cr:LiSAF, Nd:glass, Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO₄     

Unique Growth and Optical Behaviors of Chromium Doped ZnSe Nanoparticles Induced by ns Laser Pulse

We have synthesized chromium doped ZnSe nanocrystalline particles (Cr²⁺:ZnSe NCPs) by using nanosecond pulsed laser ablation of polycrystalline Cr²⁺:ZnSe micron-sized powder in liquid environment. The scanning electron micrscope and X-ray diffraction results reveal that the products are ZnSe cubic sphalerite structure with an average size around 60 nm. Based on the Cr²⁺:ZnSe nanoparticles, typical random laser emissions centering at 2175 nm with a threshold of 0.4 mJ/pulse were observed. Compared to the Cr²⁺:ZnSe bulk laser, the central wavelength shows a ～175 nm blue-shift. The photoluminescence lifetime of Cr²⁺:ZnSe NCPs is shorter than that of Cr²⁺:ZnSe bulk.     

Deposition of diamond-like carbon films by excimer lasers usingfrozen acetylene

Diamond-like carbon (DLC) films were deposited by laser ablation with the target made of frozen acetylene, instead of conventional graphite. An ArF laser and a KrF laser were used to dissociate frozen acetylene and to deposit DLC films on quartz substrates below 300°C. The C-H bond was more abundant in the films deposited by the KrF laser. It was possible to deposit particle-free films in contrast to laser ablation of graphite targets. Experimental results suggested the importance of energetic and charged species ejected from frozen acetylene     

Fabrication of saturable absorbers in InGaAsP-InP bulksemiconductor laser diodes by heavy ion implantation

Several semiconductor Fabry-Perot laser diodes with InGaAsP-InP bulk active layers have been implanted with oxygen and phosphorus ions to form saturable absorbers. The characteristics of the lasing threshold current increase and the change in the optical spectrum have been investigated as a function of the ion fluence. Based on existing models for the formation of point defects in solids, a theory has been derived that effectively describes these laser parameters, as well as radiation-induced losses, as function of the ion fluence. The lasing threshold current of the laser diodes increased up to more than four times due to ion implantation, accompanied by a wavelength shift of more than 30 nm to the blue. Bistability for optical injection is observed     

Microsecond laser ablation of thrombus and gelatin under clearliquids: Contact versus noncontact

Laser thrombolysis is a procedure for removing blood clots in occluded arteries using pulsed laser energy. The laser light is delivered through an optical fiber to the thrombus. The ablation process is profoundly affected by whether the optical fiber tip is inside a catheter or is in contact with the thrombus. This study measured ablation efficiency of 1-μs laser pulses to remove a porcine clot confined in a silicone tube. The cavitation process was investigated by visualizing laser-induced bubble formation on gelatin targets with flash photography and measuring the acoustic transients with a pressure transducer. The laser spot size did not affect the mass of material removed. The efficiency of the contact ablation was at least three times greater than that of the noncontact ablation. Finally, the mass removed was closely correlated with the measured bubble expansion pressure     

Development of dicing technique for thin semiconductor substrate using temporally shaped femtosecond laser

Recently, semiconductor substrates for integrated circuits (ICs) have been required to be as thin as 50 µm, because many electronic devices must be miniaturized and light in weight. Machining of such thin substrates with conventional dicing techniques is very difficult. Therefore, we have proposed processing them using femtosecond laser ablation. In this work, we investigate the influence of conditions of a double pulsed laser such as the delay time and fluence on the depth and diameter in order to develop a new dicing technique for very thin ICs. A double pulsed laser (λ = 780 nm, τ = 150 fs, f = 10 Hz, Δt = 0 to 100 ps, E₁ + E₂ = 100 µJ) was focused on the Si substrate with a plano‐convex lens having a nominal focal length of 100 mm. At a delay time of 10 ps, singularly shallow and flat‐bottomed holes were obtained. When the substrates were diced under these conditions, the bottom of the processing groove was flat and very smooth, whereas many microcracks starting from the bottom of the groove formed by the conventional method have been observed. From these results, we were able to identify femtosecond laser processing conditions that were applicable to dicing of thin Si substrates. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(3): 43–48, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20028