Femtosecond pulsed laser ablation of thin gold film

Laser micromachining on 1000 nm-thick gold film using femtosecond laser has been studied. The laser pulses that are used for this study are 400 nm in central wavelength, 150 fs in pulse duration, and the repetition rate is 1 kHz. Plano-concave lens with a focal length of 19 mm focuses the laser beam into a spot of 3 μm (1/e² diameter). The sample was translated at a linear speed of 400 μm/s during machining. Grooves were cut on gold thin film with laser pulses of various energies. The ablation depths were measured and plotted. There are two ablation regimes. In the first regime, the cutting is very shallow and the edges are free of molten material. While in the second regime, molten material appears and the cutting edges are contaminated. The results suggest that clean and precise microstructuring can be achieved with femtosecond pulsed laser by controlling the pulse energy in the first ablation regime.     

Metal thin film ablation with femtosecond pulsed laser

Micromachining thin metal films coated on glass are widely used to repair semiconductor masks and to fabricate optoelectrical and MEMS devices. The interaction of lasers and materials must be understood in order to achieve efficient micromachining. This work investigates the morphology of thin metal films after machining with femtosecond laser ablation using about 1 μm diameter laser beam. The effect of the film thickness on the results is analyzed by comparing experimental images with data obtained using a two-temperature heat transfer model. The experiment was conducted using a high numerical aperture objective lens and a temporal pulse width of 220 fs on 200- and 500-nm-thick chromium films. The resulting surface morphology after machining was due to the thermal incubation effect, low thermal diffusivity of the glass substrate, and thermodynamic flow of the metal induced by volumetric evaporation. A Fraunhofer diffraction pattern was found in the 500-nm-thick film, and a ripple parallel to the direction of the laser light was observed after a few multiple laser shots. These results are useful for applications requiring micro- or nano-sized machining.     

A simple model for high fluence ultra-short pulsed laser metal ablation

The ultra-short laser metal ablation is a very complex process, the complete simulation of which requires applications of complicated hydrodynamics or molecular dynamics models, which, however, are often time-consuming and difficult to apply. For many practical applications, where the laser ablation depth is the main concern, a simplified model that is easy to apply but at the same time can also provide reasonably accurate predictions of ablation depth is very desirable. Such a model has been developed and presented in this paper, which has been found to be applicable for laser pulse duration up to 10 ps based on comparisons of model predictions with experimental measurements.     

Femtosecond laser ablation of polypropylene for breathable film

A polypropylene (PP) film was ablated using a femtosecond laser with a center wavelength of 785 nm, a pulse width of 184 fs and a repetition rate of 1 kHz. Increments of both the pulse energy and the shot number of pulses lead to co-occurrence of photochemical and thermal effect, demonstrated by the spatial expansion of rim on the surface of PP. The shapes of the laser-ablated PP films were imaged by a scanning electron microscope (SEM) and measured by a 3D optical measurement system (NanoFocus). And, the gas and water vapor transmission rate, mechanical properties of PP film micropatterned by fs laser pulses was characterized. Our results demonstrate that a femtosecond pulsed laser is an efficient tool for breathable packaging films in modifying the flow of air and gas, where the micropatterns are specifically tailored in size, location and number of which is easily controlled by laser processing conditions.     

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.     

A simplified predictive model for high-fluence ultra-short pulsed laser ablation of semiconductors and dielectrics

Ultra-short pulsed laser ablation is a very complicated process and a predictive model is very desirable for process design and optimization in practical applications. However, the molecular dynamics or hydrodynamic models, although they are powerful and necessary tools for the study of the fundamental physics, are time-consuming and difficult to apply for practical applications. In this paper, a predictive, simplified and easy to apply model has been developed for high-fluence ultra-short laser ablation of semiconductors and dielectrics. Unlike many other simplified models, this model does not involve any free adjustable variables. The model predictions agree well with experimental measurements for femtosecond laser ablation, while the model is not very applicable for pulse durations more than ∼10 ps.     

Surface ablation of lithium tantalate by femtosecond laser

We report measurements of the laser induced breakdown threshold in lithium tantalate with different number of pulses delivered from a chirped pulse amplification Ti: sapphire system. The threshold fluences were determined from the relation between the diameter D² of the ablated area and the laser fluence F₀. The threshold of lithium tantalite under single-shot is found to be 1.84 J/cm², and the avalanche rate was determined to be 1.01 cm²/J by calculation. We found that avalanche dominates the ablation process, while photoionization serves as a free electron provider.     

Micromachining of a thin film by laser ablation using femtosecond laser with masks

A gold thin film was machined by laser ablation using a femtosecond laser with mask patterns in the shape of lines and numbers. The patterns were successfully transferred with proper focusing and laser fluence. The optimal femtosecond laser fluence to keep the line width was about 5.2 mJ/cm² on the mask, and 99 mJ/cm² on the film. The processing resolution was 13 μm, and the narrowest line width was about 4 μm.     

Femtosecond laser ablation profile near an interface: Analysis based on the correlation with superficial properties of individual materials

Femtosecond laser ablation of materials is turning to be an important tool for micromachining as well as for selective removal of biological tissues. In a great number of applications, laser ablation has to process through interfaces separating media of different properties. The investigation of the ablation behavior within materials and passing through interfaces is the main aim of this study. Especially, the analysis of the discontinuity in the ablation profile close to interfaces between distinct materials can reveal some of the phenomena involved in the formation of an ablated microcavity geometry. We have used a method that correlates the ablation cross sectional area with the local laser intensity. The effective intensity ablation properties were obtained from surface ablation data of distinct materials. The application of this method allows the prediction of the occurrence of a size discontinuity in the ablation geometry at the interface of distinct media, a fact which becomes important when planning applications in different media.     

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.     

Metal absorptivity in femtosecond pulsed laser ablation

In this paper, the effect of the absorptivity of metal on femtosecond pulsed laser ablation is investigated. The formulas for the absorptivity depending on target temperature are derived from Maxwell Equations and the Lambert-Beer’s law. Based on this, a new two-temperature model is proposed to describe the femtosecond pulsed laser ablation with metal. Then, using Au as an example, a finite difference method is employed to simulate the space-dependent and time-dependent absorptivity and the target temperature. The temperature evolution of our model is compared with the result obtained form the heat conduction model taking the absorptivity as constant. It is shown that the absorptivity plays an important role in the femtosecond pulsed laser ablation. The results of this paper are helpful in choosing the best technical parameters in femtosecond pulsed laser ablation.      

Picosecond and femtosecond pulsed laser ablation and deposition of quasicrystals

A Nd:glass laser with pulse duration of 250 fs and 1.3 ps has been used to evaporate a Al₆₅Cu₂₃Fe₁₂ quasicrystalline target. The gaseous phase obtained from the ablation process has been characterised by several techniques such as emission spectroscopy, quadrupole mass spectrometry and ICCD imaging, used to study the plume composition, energy and morphology. The results show that the ablation processes in the short-pulse regimes are very different to the nanosecond one. In particular the plume angular distribution shows a characteristic high cosine exponent and the composition is completely stoichiometric and independent from the laser fluence. Furthermore the mass spectra indicate the presence of clusters, both neutral and ionised and the emission from the target suggest a rapid thermalisation leading to the melting of the surface. To clarify the ablation process some films have been deposited, on oriented silicon, at different experimental conditions and analysed by scanning electron microscopy, atomic force microscopy, energy dispersive X-ray analysis and X-ray diffraction. The analyses show the presence of nanostructured films retaining the target stoichiometry but consisting of different crystalline and non crystalline phases. In particular the nanostructure supports the hypothesis of the melting of the target during the ablation and a mechanism of material ejection is proposed for both picosecond and femtosecond regimes.     

Effect of polarization on femtosecond pulsed laser ablation of surface relief gratings using a novel interferometer

In this paper, we report the effects of polarization state of individual beams in ablation of surface relief gratings using a two-beam interferometric technique. We have carried out ablation experiments on (1 1 1) silicon to form surface relief gratings by interfering two femtosecond laser beams under different polarization combinations. Four combinations of polarization were studied, i.e., s-: s-polarization, s-: p-polarization, p-: p-polarization and circular-: circular-polarization (c-: c-polarization). A novel interferometer was used for the investigation. The grating depths, surface roughness and ablation thresholds have been shown to depend on the polarization state of the interfering beams.     

Sub-micron ablation of metallic thin film by femtosecond pulse laser

The ability to machine very small features in a material has a wide range of applications in industry. We ablated holes into thin film of 100 nm thickness made from various metals by femtosecond pulsed laser ablation. Using a Ti:Sapphire laser which supplies a laser pulse of 150 fs duration at central spectrum wavelength of 400 nm, we have produced a series sub-micron holes, whose diameters are less than 200 nm with a focused laser spot of 1.7 μm. We found that the material damage threshold has a great influence on the quality of the produced features. Experimental results shows that the heat-affected zone and the degree of being affected reduce with the increase of threshold value.     

Femtosecond laser ablation of silver foil with single and double pulses

The average ablation depth per pulse of silver foil by 130 fs laser pulses has been measured in vacuum over a range of three orders of magnitude of pulse fluence up to 900 J cm⁻². In addition, double pulses with separations up to 3.4 ns have been used to probe time scales of relevance for femtosecond ablation. The double pulse ablation depth, when each pulse fluence is 0.7 J cm⁻², falls to that of a single pulse as the pulse separation is increased from 0 ps to 700 ps. This time scale decreases to only 4 ps as the fluence is increased to 11 J cm⁻². It then jumps to 500 ps across a transition fluence where the slope of the ablation depth versus logarithmic fluence characteristic changes abruptly to a higher value. In addition, for pulse separations near 1000 ps, the second pulse can cause re-deposition of ejecta from the first pulse resulting in a double pulse ablation depth only 40% that of the first pulse alone. This has important implications for the interpretation of double pulse femto-LIBS intensities. Our results suggest that the optical properties of nano or mesoparticles play a significant role in double pulse ablation with large pulse separations.     

Femtosecond laser micro-structuring of aluminium under helium

The interaction of 180 fs, 775 nm laser pulses with aluminium under a flowing stream of helium at ambient pressure have been used to study the material re-deposition, ablation rate and residual surface roughness. Threshold fluence F_th0.4 J cm⁻² and the volume ablation rate was measured to be 30<V<450 μm³ per pulse in the fluence range 1.4<F<21 J cm⁻². The presence of helium avoids gas breakdown above the substrate and leads to improved surface micro-structure by minimising surface oxidation and debris re-deposition. At 1 kHz rep. rate, with fluence F>7 J cm⁻² and >85 W cm⁻² average power density, residual thermal effects result in melt and debris formation producing poor surface micro-structure. On the contrary, surface micro-machining at low fluence F1.4 J cm⁻² with low power density, 3 W cm⁻² produces much superior surface micro-structuring with minimum melt and measured surface roughness R_a1.1±0.1 μm at a depth D50 μm. By varying the combination of fluence/scan speed during ultra-fast ablation of aluminium at 1 kHz rep. rate, results suggest that maintaining average scanned power density to <5 W cm⁻² combined with single pulse fluence <4 J cm⁻² produces near optimum micro-structuring. The debris under these conditions contains pure aluminium nanoparticles carried with the helium stream.     

脉冲激光烧蚀制备纳米Si晶粒成核区位置的确定

为了确定纳米Si晶粒气相成核的位置，采用XeCl准分子激光器，在10Pa氩气环境下，烧蚀高阻抗单晶Si靶，在距离等离子羽正下方2.0cm处、与其轴线平行放置一系列单晶Si或玻璃衬底，沉积制备了纳米Si薄膜. X射线衍射、Raman散射、扫描电子显微镜和原子力显微镜结果均显示，纳米Si晶粒只在距靶约0.5—2.8cm平行距离范围内的样品上形成，在此范围内，随着离靶平行距离的增大，所形成的纳米Si晶粒的平均尺寸逐渐减小，并且晶粒尺寸的分布也发生变化. 根据成核区起始和终止的突变特征，结合晶粒形成后的平抛运动规律，对晶粒气相成核的位置进行了估算. 关键词： 纳米Si晶粒 脉冲激光烧蚀 成核区     

Femtosecond pulsed laser deposition of nanostructured TiO2 films

Nanostructured deposits of TiO₂ were grown on Si (1 0 0) substrates by laser ablating a TiO₂ sintered target in vacuum or in oxygen using a Ti:sapphire laser delivering 80 fs pulses. The effect of the laser irradiation wavelength on the obtained nanostructures, was investigated using 800, 400 and 266 nm at different substrate temperatures and pressures of oxygen. The composition of the deposits was characterized using X-ray photoelectron spectroscopy (XPS) and the surface morphology was studied by environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). Deposits are absent of microscopic droplets in all conditions explored. The best deposits, constituted by nanoparticles of an average diameter of 30 nm with a narrow size distribution, were obtained at the shorter laser wavelength of 266 nm under vacuum at substrate room temperature.     

脉冲激光烧蚀推进技术的航天应用进展

脉冲激光烧蚀推进技术具有比冲高和推力可精确控制的特点,既可用于发射有效载荷也可用于星载动力,甚至可用小行星表面物质作为推进剂使其偏转轨道,因此,在航天领域得到越来越多关注。围绕激光单级入轨发射、同步轨道和火星轨道运输;激光微推力器用于航天器姿轨控,以及激光与电组合推进;激光烧蚀操控cm级空间碎片的轨道,以及激光烧蚀操控较大尺寸碎片的姿态;激光烧蚀偏转小行星轨道等方面,对脉冲激光烧蚀推进技术在航天领域研究现状和进展,进行了系统全面地归纳和总结,并对激光平均功率、波长、脉宽和推进剂选材等关键问题,进行了详细分析。