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.     

Parametric study on femtosecond laser pulse ablation of Au films

Ablation process of 1 kHz rate femtosecond lasers (pulse duration 148 fs, wavelength 775 nm) with Au films on silica substrates has been systemically studied. The single-pulse threshold can be obtained directly. For the multiple pulses the ablation threshold varies with the number of pulses applied to the surface due to the incubation effect. From the plot of accumulated laser fluence N × ?_th(N) and the number of laser pulses N, incubation coefficient of Au film can be obtained (s = 0.765). As the pulse energy is increased, the single pulse ablation rate is increasing following two ablation logarithmic regimes, which can be explained by previous research.     

Formation of amorphous sapphire by a femtosecond laser pulse induced micro-explosion

We report on structural characterization of void-structures created by a micro-explosion at the locus of a tightly focused femtosecond laser pulse inside the crystalline phase of Al₂O₃ (R3c space group). The transmission electron microscopy (TEM), micro-X-ray diffraction (XRD) analysis, and Raman scattering revealed a presence of strongly structurally modified amorphous regions around the void-structures. We discuss issues of achieving the required resolution for structural characterization and assignment of newly formed phases of nano-crystallites by TEM, XRD, and Raman scattering from micro-volumes of modified materials enclosed inside the bulk of the host phase.     

Experimental investigation of recoil-momentum-generation efficiency under near-IR femtosecond laser ablation of refractory metals in vacuum

The use of ultrashort laser pulses is a way to increase recoil momentum under laser ablation of materials, because, in this case, the energy deposition per unit volume of the target material is substantially higher due to reduced heat dissipation. By using methods of combined interferometry, we estimated the specific impulse (～200–900 s), momentum coupling coefficient (～2 × 10^?5?3 × 10^?4 Ns/J), laser-energy conversion efficiency to kinetic energy of the gas-plasma flow (～0.05–0.82), and degree of the gas-plasma flow monochromaticity (～0.72–0.92) under femtosecond (τ ～ 45 fs, λ ～ 800 nm) ablation of refractory metals (Ti, Zr, Mo, and Nb) in vacuum.     

Superresolved femtosecond laser ablation

The determinist behavior of the femtosecond ablation process allows morphing features well under the diffraction limit by utilizing the thresholding effect, down to the nanometer scale. Because there are a vast range of applications where scaling down the size of the features is a major concern, we investigate the use of superresolving pupil plane filters. As is well known, these filters redistribute the focused optical intensity for a narrower bright spot and, as a trade-off, increase the sidelobes. However, this drawback can be rendered insignificant if all the outer optical power is kept under the determinist threshold value. Two types of pure absorbing binary filter have been tried, giving credence to a size reduction of the ablations in fused silica.     

Single and multiple shot near-infrared femtosecond laser pulse ablation thresholds of copper

The single-shot ablation threshold and incubation coefficient of copper were investigated using an amplified near-infrared, femtosecond Ti:sapphire laser. To date, the near-infrared femtosecond ablation threshold of copper has been reported in the range of several hundred millijoules per cm² based primarily on multiple shot ablation studies. A careful study of the single shot ablation threshold for copper was carried out yielding an incident single-shot ablation threshold of (1.06±0.12) J/cm² for a clean copper foil surface. This was determined by measuring the diameters of the ablation spots as a function of the laser pulse energy using scanning electron microscopy for spatially Gaussian laser spots. When multiple shots were taken on the same spot, a reduction in ablation threshold was observed, consistent with a multiple shot incubation coefficient of 0.76±0.02. Similar experiments on 250 nm and 500 nm copper thin films sputtered on a silicon substrate demonstrated that scaling the threshold values with the absorbance of energy at the surface yields a consistent absorbed fluence threshold for copper of (59±10) mJ/cm². This absorbed threshold value is consistent with the expected value from a two-temperature model for the heating of copper with an electron-lattice coupling constant of g=10¹⁷ Wm^-3 K^-1. Single-shot rippling of the surface in the threshold ablation intensity regime was also observed for the foil target but not for the smooth thin film target. PACS 61.80.Ba; 61.82.Bg     

Lithographical laser ablation using femtosecond laser

Lithographical laser ablation was demonstrated using a femtosecond laser with a lithographical optical system. In this method, a femtosecond laser beam passes through a mask and the pattern is imaged on a film by a coherent optical system. As a result, the film is lithographically ablated, and a micron-sized pattern can be generated in a single shot. The resolution of generation was 13 m, and the narrowest width of a generated line was about 4 m. Moreover, the system was applied to transmission gratings as masks, and nano-sized periodic structures such as nano-sized hole matrices and nano-meshes were generated in a single shot. PACS 52.38.Mf; 42.25.Hz; 42.82.Cr; 81.16.-c     

飞秒激光作用下异质结的线性吸收谱研究

以二奈嵌苯分子吸附在TiO₂表面所组成的异质结构为例,介绍了在飞秒激光作用下由染料分子和半导体组成的异质结构中从分子基态到半导体导带的超快电子传输过程,在理论上分析了分子内部传输和直接电子传输过程对线性吸收谱的贡献.与分子内部传输过程项相比较分析了电子的超快直接传输在不同的分子及半导体结构下对线性吸收谱的影响. 关键词： 飞秒激光 线性吸收谱 超快电子转移     

Dynamics of ablation plasma induced by a femtosecond laser pulse in electric fields

Direct observations of ablation plasma dynamics in electric field is presented. A time-resolved spatial profile of the ablation plasma induced by femtosecond laser ablation (fsLA) with high fluence is visualized using a planar-laser-induced fluorescence (P-LIF) method. The external electric field is produced by installing a mesh electrode at 6 mm from a Samarium solid target. The Sm ion plasma created by the fsLA showed collective motion regardless of the external electric field, until they reached close to the electrode. When the accelerating and decelerating field was applied, the ions almost disappeared behind the electrode from the field of view. The observations are understood utilizing a SIMION simulation with a conceivable potential gradient caused by Debye shield effect, which is that the ablation plasma keeps the same potential as the target voltage and follows electric potential gradient near the mesh electrode. It is also revealed that this effect degrades time-of-flight resolution at high fluence irradiation. This work gives a new direction for further developments of a fsLA time-of-flight spectrometer.     

Experimental investigation of the dynamics of laser-induced gas-plasma flows under femtosecond laser ablation of copper in vacuum

Thermophysical and gas-dynamic characteristics of gas-plasma flows induced by ultrashort laser pulses interacting with a thin-film copper target in vacuum were studied experimentally. Using combined laser interferometry and complex processing of experimental data, we estimated the momentum coupling coefficient and the efficiency of laser-energy conversion to kinetic energy, spatiotemporal distributions of the number density and velocities of particles, pressure, and temperature in the gas-plasma flow. We provide comparative analysis of presented data with those found in the literature, which were obtained by other methods.     

Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon

Femtosecond laser (Ti:sapphire, 100 fs pulse duration) ablation of silicon in air was compared with nanosecond laser (Nd:YAG, 3 ns pulse duration) ablation at ultraviolet wavelength (266 nm). Laser ablation efficiency was studied by measuring crater depth as a function of pulse number. For the same number of laser pulses, the fs-ablated crater was about two times deeper than the ns-crater. The temperature and electron number density of the laser-induced plasma were determined from spectroscopic measurements. The electron number density and temperature of fs-induced plasmas decreased faster than ns-induced plasmas due to different energy deposition mechanisms. Images of the laser-induced plasma were obtained with femtosecond time-resolved laser shadowgraph imaging. Plasma expansion in both the perpendicular and the lateral directions were compared. PACS 52.38.Mf; 52.30.-q     

超连续谱注入飞秒光参量放大实验研究

提出了一种很有特色的基于超连续谱注入的飞秒光参量放大技术,该种技术方法不仅克服了常规1 053 nm脉冲在前端小能量放大时的增益窄化现象,而且可实现800 nm激光和1 053 nm激光的零时间同步。通过系统的实验研究,获得了大于4 mJ的1 053 nm宽带信号光能量     

超连续谱注入飞秒光参量放大实验研究

 提出了一种很有特色的基于超连续谱注入的飞秒光参量放大技术,该种技术方法不仅克服了常规1 053 nm脉冲在前端小能量放大时的增益窄化现象,而且可实现800 nm激光和1 053 nm激光的零时间同步。通过系统的实验研究,获得了大于4 mJ的1 053 nm宽带信号光能量     

Electron dynamics and prompt ablation of aluminum surface excited by intense femtosecond laser pulse

Thin aluminum film homogeneously heated by intense IR femtosecond laser pulses exhibits on the excitation timescale consequent fluence-dependent rise and drop of the IR-pump self-reflectivity, followed by its final saturation at higher fluences F > 0.3 J/cm². This prompt optical dynamics correlates with the initial monotonic increase in the accompanying laser-induced electron emission, which is succeeded by its non-linear (three-photon) increase for F > 0.3 J/cm². The underlying electronic dynamics is related to the initial saturation of IR resonant interband transitions in this material, followed by its strong instantaneous electronic heating via intraband transitions during the pump pulse resulting in thermionic emission. Above the threshold fluence of 0.3 J/cm², the surface electronic heating is balanced during the pump pulse by simultaneous cooling via intense plasma removal (prompt ablation). The relationship between the deposited volume energy density in the film and its prompt electronic temperature derived from the self-reflection measurements using a Drude model, demonstrates a kind of electron “liquid–vapor” phase transition, driven by strong cubic optical non-linearity of the photo-excited aluminum.     

Optical properties of high-density amorphous carbon films grown by nanosecond and femtosecond pulsed laser ablation

High-density tetrahedral amorphous carbon (ta-C) films have been prepared by nanosecond (17 ns) and femtosecond (150 fs) pulsed laser deposition (PLD) using fluences and repetition rates compatible with fast and homogeneous growth over large areas. Their optical properties were measured by spectroscopic ellipsometry from 1.0 to 4.7 eV and analyzed using a multi-layer Tauc-Lorentz model. In spite of very different ablation mechanisms, both PLD techniques produce high density bulk layers as revealed by a refractive index (n at 2 eV) of 2.7±0.1 for both fs-PLD and ns-PLD. Films are covered by a few nm-thick sp²-rich top layer which is denser and thicker in femtosecond PLD as compared to nanosecond PLD. The respective roles of low and high energies in the kinetic energy distribution of the incident carbon species are discussed in terms of densification and sp³↦sp² configurational relaxation predicted by the subplantation growth model. The significantly higher optical gap found in the ns-PLD films is attributed to the larger contribution of energetic species with kinetic energies E_c≥200 eV, as revealed by time-of-flight optical studies. PACS 81.40.Tv; 81.05.Uw; 81.15.Fg     

Phase transitions in femtosecond laser ablation

In this study we simulate an interaction of femtosecond laser pulses (100 fs, 800 nm, 0.1-10 J/cm²) with metal targets of Al, Au, Cu, and Ni. For analysis of laser-induced phase transitions, melting and shock waves propagation as well as material decomposition we use an Eulerian hydrocode in conjunction with a thermodynamically complete two-temperature equation of state with stable and metastable phases. Isochoric heating, material evaporation from the free surface of the target and fast propagation of the melting and shock waves are observed. On rarefaction the liquid phase becomes metastable and its lifetime is estimated using the theory of homogeneous nucleation. Mechanical spallation of the target material at high strain rates is also possible as a result of void growth and confluence. In our simulation several ablation mechanisms are taken into account but the main issue of the material is found to originate from the metastable liquid state. It can be decomposed either into a liquid-gas mixture in the vicinity of the critical point, or into droplets at high strain rates and negative pressure. The simulation results are in agreement with available experimental findings.     

Langmuir probe investigation of surface contamination effects on metals during femtosecond laser ablation

Characterisation of the plasma plume induced by femtosecond laser-metal interactions has been carried out using a Langmuir probe. A double peak distribution of ablated ions and electrons has been recorded during time of flight (TOF) experiments for three metals studied (Ag, Cu and Ni). The first peak which occurs earliest in time is attributed to a surface layer of contaminants on the metal surface as it is shown to disappear after several laser shots. The re-growth of this peak, thought to be due to a recontamination process on the surface of the metal, is the subject of this paper. Two re-contamination mechanisms were considered; adsorption of contaminants from the ambient gas, and surface diffusion effects from the surrounding contaminants. Re-contamination rates for Ag, Cu and Ni were studied under two distinct gas pressures to investigate the contamination effects from the ambient. Effects arising from surface diffusion were investigated by raising the temperature of the metal sample to increase the surface mobility of the contaminants. The total contribution of contamination species present in the ablation plume was estimated by conducting angular distribution measurements of the plume. Surface diffusion of the surrounding contaminants was found to be the dominant recontamination process.     

Thermodynamic evolution of materials during laser ablation under pico and femtosecond pulses

Using molecular-dynamics, we study the thermodynamic evolution of a simple two-dimensional Lennard–Jones system during laser ablation for pulse durations ranging from 200 fs to 400 ps. We briefly review results previously obtained for fs pulses where the evolution of the material was shown to be solely a function of the locally absorbed energy (provided that only thermal effects are important), i.e., is adiabatic. For longer pulses (100 and 400 ps) the situation becomes more complex, as the relaxation path also depends on the position in the target and on the timescale on which expansion occurs. We show that, in contrast to fs pulses, the material ejected following ps laser irradiation does not enter the liquid–vapor metastable region before ablation occurs, hence showing that phase explosion is not the dominant mechanism in this regime. Following on from previous work, we propose that trivial fragmentation is the main ablation mechanism. PACS 79.20.Ds; 79.20.Ap; 61.80.Az     

Dynamics of mid-infrared femtosecond laser resonant ablation

Resonant ablation is beneficial to avoiding uncontrollable subsurface damages in the laser ablation of polymers. In this paper the dynamics of mid-infrared laser resonant ablation of polylactic acid and toluene was calculated by using fluid dynamic equations. The merits and drawbacks of mid-infrared femtosecond laser resonant ablation of high molecular weight polymers have been discussed.     

高功率飞秒脉冲激光器的进展

概述了超短激光脉冲技术发展的历程。详尽的论述了啁啾脉冲放大技术的原理和一些关键技术 ,并对当今飞秒激光器研究发展的状况进行了综述。在分析其应用前景的基础上 ,进一步指出了这一技术领域未来的发展趋势。结果表明 ,脉冲激光放大系统以后的发展方向是 :更短脉冲 ;追求更高峰值功率 ;连续光谱调谐化 ;向小型化乃至全光纤化发展。