Nanosecond and femtosecond laser ablation of brass: particulate and ICPMS measurements

Femtosecond and nanosecond lasers were compared for ablating brass alloys. All operating parameters from both lasers were equal except for the pulse duration. The ablated aerosol vapor was collected on silicon substrates for particle size measurements or sent into an inductively coupled plasma mass spectrometer. The diameters and size distribution of particulates were measured from scanning electron microscope (SEM) images of the collected ablated aerosol. SEM measurements showed that particles ablated using nanosecond pulses were single spherical entities ranging in diameter from several micrometers to several hundred nanometers. Primary particles ablated using femtosecond ablation were approximately 100 nm in diameter but formed large agglomerates. ICPMS showed enhanced signal intensity and stability using femtosecond compared to nanosecond laser ablation.     

Modeling optical breakdown in dielectrics during ultrafast laser processing

Laser ablation is widely used in micromachining, manufacturing, thin-film formation, and bioengineering applications. During laser ablation the removal of material and quality of the features depend strongly on the optical breakdown region induced by the laser irradiance. The recent advent of amplified ultrafast lasers with pulse durations of less than 1 ps has generated considerable interest because of the ability of the lasers to process virtually all materials with high precision and minimal thermal damage. With ultrashort pulse widths, however, traditional breakdown models no longer accurately capture the laser-material interaction that leads to breakdown. A femtosecond breakdown model for dielectric solids and liquids is presented that characterizes the pulse behavior and predicts the time- and position-dependent breakdown region. The model includes the pulse propagation and small spatial extent of ultrashort laser pulses. Model results are presented and compared with classical breakdown models for 1-ns, 1-ps, and 150-fs pulses. The results show that the revised model is able to model breakdown accurately in the focal region for pulse durations of less than 10 ps. The model can also be of use in estimating the time- and position-resolved electron density in the interaction volume, the breakdown threshold of the material, shielding effects, and temperature distributions during ultrafast processing.     

Depth analysis of polymer-coated steel samples using near-infrared femtosecond laser ablation inductively coupled plasma mass spectrometry

The viability of near-infrared femtosecond laser ablation (fs-LA) inductively coupled plasma mass spectrometry (ICPMS) for the in-depth analysis of polymer coatings over galvanized steel substrates has been studied. A good depth resolution was obtained modifying the femtosecond Gaussian beam to a flat-top beam by using a liquid-crystal display. In order to avoid mixing of information coming from successive shots, a low repetition rate was accomplished and signals were monitored shot by shot. Different kinds of coatings were used to demonstrate the capability of femtosecond ablation for depth-profiling analysis. Ablation was conducted under He atmosphere, after sample cell Ar was admixed. The depth profiles obtained by LA-ICPMS are in good agreement with those obtained by GD-OES for the three analyzed samples. In cases where due to averaging over several millimeter sample roughness determines the depth resolution of GD-OES, it was found that LA-ICPMS achieves better depth resolution due to the better lateral resolution. The depth resolution obtained by LA-ICPMS was found to be 240 nm and 2.3 microm, for a hot-dip galvanized steel (HDGS) and a polymer-polymer-coated HDGS, respectively, compared to the 2.2 and 4.5 microm achieved with GD-OES for the same samples.     

Imaging of copper, zinc, and other elements in thin section of human brain samples (hippocampus) by laser ablation inductively coupled plasma mass spectrometry

Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was used to produce images of element distribution in 20-microm thin sections of human brain tissue. The sample surface was scanned (raster area approximately 80 mm(2)) with a focused laser beam (wavelength 213 nm, diameter of laser crater 50 microm, and laser power density 3 x 10(9) W cm(-2)) in a cooled laser ablation chamber developed for these measurements. The laser ablation system was coupled to a double-focusing sector field ICPMS. Ion intensities of 31P+, 32S+, 56Fe+, 63Cu+, 64Zn+, 232Th+, and 238U+ were measured within the area of interest of the human brain tissue (hippocampus) by LA-ICPMS. The quantitative determination of copper, zinc, uranium, and thorium distribution in thin slices of the human hippocampus was performed using matrix-matched laboratory standards. In addition, a new arrangement in solution-based calibration using a micronebulizer, which was inserted directly into the laser ablation chamber, was applied for validation of synthetic laboratory standard. The mass spectrometric analysis yielded an inhomogeneous distribution (layered structure) for P, S, Cu, and Zn in thin brain sections of the hippocampus. In contrast, Th and U are more homogeneously distributed at a low-concentration level with detection limits in the low-nanogram per gram range. The unique analytical capability and the limits of LA-ICPMS will be demonstrated for the imaging of element distribution in thin cross sections of brain tissue from the hippocampus. LA-ICPMS provides new information on the spatial element distribution of the layered structure in thin sections of brain tissues from the hippocampus.     

Analysis of polyacrylamide gels for trace metals using diffusive gradients in thin films and laser ablation inductively coupled plasma mass spectrometry

A simple method for the analysis of polyacrylamide diffusive gradients in thin film (DGT) gels by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), employing a novel use of (115)In internal standardization, has been developed. This method allows the determination of Co, Ni, Cu, Zn, Cd, and Pb concentrations (at the DGT filter face) or fluxes in sediments at a spatial resolution of 100 microm. Single-layered gels, using an optimized laser defocus of 4000 microm at 400 mJ power, showed high precision (generally approximately 10%) and a linear response during solution deployment. Of the elements Sc, In, Ba, La, Ce, and Tb, Ba most closely tracked variations in laser energy and showed the highest analytical precision but could not be used as an internal standard due to its elevated presence in natural sediments. Therefore, internal standardization, necessary to normalize data collected on different days, was carried out using (115)In contained within a second layer of backing gel and dried along with the analyte layer as a dual-gel disk. This multilayered gel standard required a laser defocus setting of 1000 microm and a laser power of approximately 800 mJ. Analytical precision for a 64-spot ablation grid at 100-microm spacing was approximately 10%. Verification of this method was carried out on DGT sediment probes deployed in Priest Pot (English Lake District). Results obtained by conventional slicing techniques and aqueous elution agreed with laser ablation results when the different sampling areas were considered. The elution results varied by a factor of <2, whereas the laser ablation technique showed a variability of approximately 4, indicating localized elevated concentrations of Co. This higher resolution LA-ICPMS method could ultimately lead to an improved understanding of the geochemical processes responsible for metal uptake and release in sediments.     

Determination of phosphorus-, copper-, and zinc-containing human brain proteins by LA-ICPMS and MALDI-FTICR-MS

Human brain proteins containing phosphorus, copper, and zinc were detected directly in protein spots in gels of a human brain sample after separation by two-dimensional gel electrophoresis using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). A powerful laser ablation system with cooled laser ablation chamber was coupled to a double-focusing sector field ICPMS. The separated protein spots in 2D gels were fast screened using the optimized microanalytical LA-ICPMS technique measured at medium mass resolution with a focused laser beam (wavelength, 213 nm; diameter of laser crater, 50 mum; and laser power density, 3 x 10(9) W cm(-2)) with respect to selected three essential elements. Of 176 protein spots in 2D gel from a human brain sample, phosphorus, copper, and zinc were detected in 31, 43, and 49 protein spots, respectively. For the first time, uranium as a naturally occurring radioactive element was found in 20 selected protein spots. The detection limits for P, S, Cu, Zn and U were determined in singular protein spots with 0.0013, 1.29, 0.029, 0.063, and 0.000 01 mg g(-1), respectively. A combination of LA-ICPMS with matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FTICR-MS) was applied for the identification of selected protein spots from human brain protein separated by 2D gel electrophoresis. Combining MALDI-FTICR-MS for the structure analysis of metal- and phosphorus-containing human brain proteins with LA-ICPMS, the direct analysis of heteroelements on separated proteins in 2D gels can be performed. For quantification of analytical LA-ICPMS data, the number of sulfur atoms per protein (and following the sulfur concentration) determined by MALDI-FTICR-MS was used for internal standardization. From the known sulfur concentration in protein, the concentration of other heteroelements was calculated. In addition, the number of phosphorylation and the phosphorylation sites of phosphorylated proteins in the human brain sample detected by LA-ICPMS were determined by MALDI-FTICR-MS. This technique allows the study of posttranslational modifications in human brain proteins.     

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.     

Development of tunable picosecond dye laser for multiphoton ionization of dioxin precursors in supersonic jet/time-of-flight mass spectrometry

A distributed-feedback dye laser has been developed for achieving the efficient multiphoton ionization of chlorobenzene and dichlorobenzene, that is, precursor molecules of dioxins. This tunable picosecond laser with a narrow spectral line width, that is, a nearly transform-limited pulse, provides a more efficient ionization than the nanosecond laser, which is currently in use in supersonic jet spectrometry. The advantage of picosecond over nanosecond and femtosecond lasers is also discussed on the basis of the theoretical model reported in a previous paper.     

A high-sensitivity femtosecond to microsecond time-resolved infrared vibrational spectrometer

We describe an apparatus that provides, for the first time, a seamless bridge between femtosecond and microsecond time-resolved Raman and infrared vibrational spectroscopy. The laser system comprises an actively Q-switched sub-nanosecond pulsed kilohertz laser electronically synchronized to an ultrafast titanium sapphire regenerative amplifier to within 0.2 ns. The ultrafast amplifier provides the stable probe light source enabling high-sensitivity infrared vibrational spectroscopy of transients. Time-resolved infrared spectra of the excited-state relaxation dynamics of metal carbonyl compounds are presented to illustrate the capability of the apparatus, and transient data is resolved from 1 picosecond to over 100 microseconds. The results are compared to conventional nanosecond Fourier transform infrared (FT-IR) and laser based flash photolysis time-resolved infrared technology.     

飞秒激光烧蚀面齿轮材料的齿面表层形态研究

针对面齿轮材料18Cr2Ni4WA,研究飞秒激光辐照面齿轮材料的热力效应,建立飞秒激光烧蚀面齿轮温度-应力耦合模型,分析多脉冲时不同能量密度下电子温度、晶格温度以及热应力的变化过程。结果表明：电子温度、晶格温度以及热应力随激光能量密度的增大而增大。实验和仿真的对比结果说明,烧蚀齿面表层为残余压应力,烧蚀深度和凹坑直径随激光能量密度的增加而增大,较大的激光能量密度会产生较多的熔融物,降低飞秒激光加工质量,当能量密度为1.78 J/cm2时,齿面表层形态较好。本文为提高飞秒激光精微烧蚀面齿轮质量提供了研究基础。     

Sensitive detection of selenoproteins in gel electrophoresis by high repetition rate femtosecond laser ablation-inductively coupled plasma mass spectrometry

A laser ablation-ICPMS method using an infrared (1030 nm), low-energy (39 microJ/pulse), high repetition rate (10 kHz), femtosecond laser was developed to improve the sensitivity of detection of heteroatom-containing proteins in 1D polyacrylamide gels. A 2-mm-wide lane was ablated by ultrafast (10 cm s(-1)) back-and-forth movement of a 20-microm laser beam parallel to the protein bands while the gel advanced perpendicularly. This procedure resulted in a considerable increase in detection sensitivity (>40-fold) compared to the nanosecond 266-nm laser ablation-ICPMS, mainly because of the much larger amount of ablated material introduced into the plasma on the time scale of the dwell time of the mass spectrometer. The method was applied to the specific detection in the gel of formate dehydrogenase expressed in Escherichia coli and of selenoproteins in Desulfococcus multivorans with detection limits at the low-femtomolar levels.     

纳秒脉冲激光对水下靶材的掩模微细刻蚀加工

在激光的传输光路中加入掩模板,采用纳秒脉冲激光对水下靶材进行了掩模微细刻蚀加工,与飞秒激光微细加工相比可显著提高加工效率,加工精度由掩模保证.在所构建的试验系统中,分别在空气中、纯水中和盐溶液中进行了加工试验,在水流的作用下,激光辐照的多余热量被带走,加工边缘热影响区明显减小.同时,水下激光辐照还会在加工区域造成光学击穿,产生等离子体冲击波,在水层的约束限制作用下,加工表现出了明显的冲击波力学效应.通过这种热-力学的复合效应,使金属材料表面产生材料去除和变形的加工效果.最终在质量分数为15%的NaNO3的溶液中,利用热-力学效应和化学反应的复合作用,实现了线宽120μm左右、热影响区小、成形精度较好的微细刻蚀加工.     

Chemically assisted laser ablation ICP mass spectrometry

A new laser ablation technique combined with a chemical evaporation reaction has been developed for elemental ratio analysis of solid samples using an inductively coupled plasma mass spectrometer (ICPMS). Using a chemically assisted laser ablation (CIA) technique developed in this study, analytical repeatability of the elemental ratio measurement was successively improved. To evaluate the reliability of the CLA-ICPMS technique, Pb/U isotopic ratios were determined for zircon samples that have previously been analyzed by other techniques. Conventional laser ablation for Pb/U shows a serious elemental fractionation during ablation mainly due to the large difference in elemental volatility between Pb and U. In the case of Pb/U ratio measurement, a Freon R-134a gas (1,1,1,2-tetrafluoroethane) was introduced into the laser cell as a fluorination reactant. The Freon gas introduced into the laser cell reacts with the ablated sample U, and refractory U compounds are converted to a volatile U fluoride compound (UF6) under the high-temperature condition at the ablation site. This avoids the redeposition of U around the ablation pits. Although not all the U is reacted with Freon, formation of volatile UF compounds improves the transmission efficiency of U. Typical precision of the 206Pb/238U ratio measurement is 3-5% (2sigma) for NIST SRM 610 and Nancy 91500 zircon standard, and the U-Pb age data obtained here show good agreement within analytical uncertainties with the previously reported values. Since the observed Pb/U ratio for solid samples is relatively insensitive to laser power and ablation time, optimization of ablation conditions or acquisition parameters no longer needs to be performed on a sample-to-sample basis.     

Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses

Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply 'heats' the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.     

基于三维双温模型的飞秒激光烧蚀面齿轮材料形貌研究

在传统双温模型基础上,建立考虑变离焦量效应和随电子温度变化的动态吸收率效应的三维双温模型。分析烧蚀过程中电子、晶格温度的变化情况和烧蚀凹坑的形貌。对面齿轮材料18Cr2Ni4WA进行飞秒激光烧蚀实验结果表明：随着单脉冲飞秒激光能量密度的增大,电子的峰值温度升高,烧蚀凹坑的深度和直径增大;单脉冲飞秒激光的脉宽对烧蚀形貌的影响并不显著;烧蚀凹坑的直径和深度不会随脉冲数目一直增大,脉冲数目存在阈值,超过该阈值对于烧蚀凹坑的形貌影响反而不利。     

Near-infrared distributed feedback solgel lasers by intensity modulation and polarization modulation

Near-infrared distributed feedback (DFB) laser actions of Oxazine 725 dye in zirconia thin films and in silica bulks were investigated. Intensity modulation and polarization modulation were used to generate the DFB lasing. Wideband tuning of the output wavelength was achieved by varying the period of the modulation generated by a nanosecond Nd:YAG laser at 532 nm. Tuning ranges were 716-778 nm and 724-813 nm for the thin film lasers and the bulk lasers, respectively. The laser output showed different polarization characteristics and threshold energy variation when the feedback mechanism was changed from intensity modulation to polarization modulation.     

Plasma Mediated off-Resonance Plasmonic Enhanced Ultrafast Laser-Induced Nanocavitation

The generation of nanobubbles around plasmonic nanostructures is an efficient approach for imaging and therapy, especially in the field of cancer research. We show a novel method using infrared femtosecond laser that generates ≈800 nm bubbles around off-resonance gold nanospheres using 200 mJ/cm(2) 45 fs pulses. We present experimental and theoretical work that demonstrate that the nanobubble formation results from the generation of a nanoscale plasma around the particle due to the enhanced near-field rather than from the heating of the particle. Energy absorbed in the nanoplasma is indeed more than 11 times the energy absorbed in the particle. When compared to the usual approach that uses nanosecond laser to induce the extreme heating of in-resonance nanoparticles to initiate bubble formation, our off-resonance femtosecond technique is shown to bring many advantages, including avoiding the particles fragmentation, working in the optical window of biological material and using the deposited energy more efficiently.     

Portable laser ablation sampling device for elemental fingerprinting of objects outside the laboratory with laser ablation inductively coupled plasma mass spectrometry

Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) is a powerful method for elemental fingerprinting of solid samples in a quasi-nondestructive manner. In order to extend the field of application to objects outside the laboratory, a portable laser ablation sampling device was assembled using a diode pumped solid state laser and fiber-optics. The ablated materials were sampled on membrane filters and subsequently quantified by means of LA-ICPMS. The analytical performance of this approach was investigated for glass and gold reference materials. Accuracies of better than 20% were reached for most elements and typical limits of detection were found to be in the range of 0.01-1 μg/g. In summary, this approach combines spatially resolved sampling with the detection power of ICPMS and enables elemental fingerprinting of objects which cannot be transferred to the laboratory, e.g., archeological artifacts in museums.     

Thin-film CdS formed with pulsed-laser deposition towards optical and hybrid device applications

The paper shows various optical and optoelectronic experiments carried out on thin-film CdS on glass. The films have been prepared with nanosecond pulsed-laser deposition (PLD) on glass by using ultraviolet and infrared Nd:YAG laser emissions. The discussed topics contain the successful realization of green light emitting lasers, the properties of ultrafast two-photon excited emission, PLD controlled CdS dichroism, and organic/inorganic hybrid devices. Based on the presented results several future investigations are proposed.     

Operational stabilization of a high-power ultrashort dye oscillator-amplifiers laser chain

Stable operation of a high-power ultrashort dye laser chain is achieved with an inexpensive heating technique. The femtosecond oscillator and its pumping laser are separately covered with boxes to isolate them from external temperature fluctuations. Both laser-isolating boxes are lined with a heating pellicle that permanently and uniformly warms the lasers to preset operating temperatures. The power, spectrum, and duration of the laser-chain output pulse remain constant from day to day. This technique could be applied to other ultrafast laser devices that require long-term performance stability.