Wet bone ablation with mechanically Q-switched high-repetition-rate CO2 laser

2 laser using a miniature water spray is demonstrated. An ablation threshold of 1.4 J/cm², an optimal energy density of 9–10 J/cm², and a corresponding specific ablation energy of 25–30 J/mm³ are found for pig thighbone compacta at λ=9.57 μm and a beam waist diameter of 0.5 mm. The water spray alleviates tissue carbonization even at high laser pulse repetition rates and increases ablation efficiency. Received: 9 March 1998/Revised version: 6 July 1998     

Silver nanoparticles generated by pulsed laser ablation in supercritical CO2 medium

Pulsed laser ablation (PLA) has been widely employed in industrial and biological applications and in other fields. The environmental conditions in which PLA is conducted are important parameters that affect both the solid particle cloud and the deposition produced by the plume. In this work, the generation of nanoparticles (NPs) has been developed by performing PLA of silver (Ag) plates in a supercritical CO₂ medium. Ag NPs were successfully generated by allowing the selective generation of clusters. Laser ablation was performed with an excitation wavelength of 532 nm under various pressures and temperatures of CO₂ medium. On the basis of the experimental result, both surface of the irradiated Ag plate and structure of Ag NPs were significantly affected by the changes in supercritical CO₂ pressure and temperature. With increasing irradiation pressure, plume deposited in the surrounding crater created by the ablation was clearly observed. In Field Emission Scanning Electron Microscopy (FE-SEM) the image of the generated Ag NPs on the silicon wafer and the morphology of Ag particles were basically a sphere-like structure. Ag particles contain NPs with large-varied diameter ranging from 5 nm to 1.2 μm. The bigger Ag NPs melted during the ablation process and then ejected smaller spherical Ag NPs, which formed nanoclusters attached on the molten Ag NPs. The smaller Ag NPs were also formed around the bigger Ag NPs. Based on the results, this new method can also be used to obtain advanced nano-structured materials.     

Femtosecond pulsed laser ablation with spatial filtering

With the rise in demand for miniaturized features with better acute edge acuity and negligible thermal damage zone, one of the key vital areas lies in the refinement of the quality of the laser beam itself. Spatial filter is routinely used in optical micromachining systems to smoothen the Gaussian profile of the machining spot in order to obtain a feature of the desired quality. However, its profile smoothening effect has never been investigated for femtosecond pulsed laser micromachining process since the extremely high peak power of femtosecond pulses will cause damage on the filtering aperture of spatial filter. During the development of an acousto-optical micromachining system using femtosecond pulses, we found that if the damage of the filtering aperture can be circumvented, spatial filter can improve the machining quality of femtosecond pulse ablation, especially when ablation is conducted at low-fluency range (just above the ablation threshold fluency). In this paper, we investigate and demonstrate both the improvement and potential that beam refinement can bring about. In our experiment, a series of test patterns were ablated with a 400 nm second-harmonic Ti:Sapphire femtosecond laser of 150 fs duration at varying pulse energy ranging from 31 to 39 nJ. The specimen used in the experiment is a platinum- (Pt) sputtered coating of 100 nm thickness on a quartz substrate. The results show a significant improvement in the constancy of the shape as well as the size of ablated feature, revealing an improved beam profile and beam energy distribution due to spatial filtering.     

Influence of scanning velocity on femtosecond laser direct writing lines on FOTURAN glass

Lines are induced on the surface of a photosensitive (FOTURAN) glass by focused femtosecond laser transverse writing with scanning velocity in a wide range of 40- 1800μm/s. The formed lines are analyzed using scanning electron microscope (SEM) and optical microscope (OM). It is observed that three distinct morphologies of lines are produced depending on the scanning velocity. Lines written in low velocity level (40 - 100 μm/s) and high velocity level (1000 - 1800 μm/s) are uniform and regular, while those written in moderate velocity level (150 - 600 μm/s) are rough. The influence of scanning velocity is explained based on different pulses overlapping or cumulative dose of laser exposure in irradiated area. Fabrication of shallow groove on the surface is also demonstrated.     

High power repetitive TEA CO2 pulsed laser

A high power repetitive spark-pin UV-preionized TEA CO₂ laser system is presented. The discharge for generating laser pulses is controlled by a rotary spark switch and a high voltage pulsed trigger. Uniform glow discharge between two symmetrical Chang-electrodes is realized by using an auto-inversion circuit. A couple of high power axial-flow fans with the maximum wind speed of 80 m/s are used for gas exchange between the electrodes. At a repetitive operation, the maximum average output laser power of 10.4 kW 10.6 ??m laser is obtained at 300 Hz, with an electro-optical conversion efficiency of 15.6%. At single pulsed operation, more pumping energy and higher gases pressures can be injected, and the maximum output laser energy of 53 J is achieved.     

TEA CO2 laser ablation of coronary artery

Summary One of the prerequisites for successful laser angioplasty is the ablation of the atherosclerotic lesions, without thermal or shock-wave damage of the healthy tissue. In this study was evaluated the effectiveness of a TEA CO₂ laser, emitting pulses of the lower TEM mode, 100 ns duration, at a repetition rate of 2.4 Hz, for the ablation of cardiovascular tissue. Normal and atherosclerotic human arteries (post mortem) were irradiated for a range of fluences up to 10J pulse⁻¹ cm⁻². After irradiation, the samples were prepared for histologic examination. The results showed that controlled ablation of normal and atherosclerotic coronary artery can be accomplished with the TEA CO₂ laser, with minimal thermal damage.     

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.     

真空中脉冲激光烧蚀铝等离子体发射光谱及粒子速度研究

利用Q开关Nd:YAG激光器产生的1.06 μm倍频后532 nm、脉宽10 ns的脉冲激光聚焦在置于真空室中铝靶上,观测激光诱导的铝等离子体发射光谱.采用不同的激光能量,分析了波长范围为350 nm到400 nm的空间、时间分辨发射光谱.在局部热力学平衡(LTE)条件近似下,根据谱线的相对强度,计算得到等离子体电子温度,给出了靶面附近电子温度的空间、时间演化规律;根据谱线半宽,计算等离子体电子密度,并给出了靶面附近电子密度的空间与时间演化规律;在靶面正前方处放置动能探测器,记录粒子飞行时间信号,观测不同激光强度烧蚀铝靶产生等离子体中三种粒子到达探测器的时间,计算得出等离子体中三种观测到的粒子喷射速度.     

Enhancement of pulsed laser ablation assisted with continuous wave laser irradiation

Continuous wave(CW) laser irradiation is employed to enhance the pulsed laser ablation of silicon and stainless steel(316 L)samples. Different surface temperatures generated by the CW laser irradiation are set as the initial working circumstance for the pulsed laser ablation. The diameter and depth of laser-ablated craters are measured to study threshold fluence, pulse incubation coefficient and ablation rate under different surface temperatures. Numerical simulation employing Heat Transfer in Solid and Deformed Geometry Interface modules in COMSOL is performed to estimate ablation rate theoretically based on Hertz-Knudsen equation. The realized crater-related data are analyzed to further obtain their dependences on surface temperature. The parametric and morphological studies indicate that the weakened plasma shielding effect and thermal diffusion in the ablated region induced by the CW laser irradiation lead to the enhanced pulsed laser ablation significantly.     

KrF pulsed laser ablation of polyimide

In the present paper, polyimide surfaces were processed with pulsed KrF laser radiation at fluences near the ablation threshold. The morphology of the processed surfaces was studied by scanning electron microscopy and chemical analyses performed by electron dispersive spectroscopy. The formation of conical structures was observed for radiation fluences lower than 0.5 J/cm². The areal density of cones increases with the number of pulses and decreases with the radiation fluence. At low fluences (<150 J/cm²), cones are formed due to shadowing by calcium phosphate impurities while for higher fluences the main mechanism of cones formation is believed to be radiation hardening.     

A simple current pulsed low-pressure CO2 laser with passive Q-switching

We report on a simple and stable pulse tunable CO₂ laser suitable for many investigations in the region around 10m. The pulsed discharge when combined with the passiveQ-switching technique provides pulses of 100 ns duration with a peak power of few kilowatts.Due to the interest in pulsed low-pressure CO₂ lasers as useful irradiation sources, extensive investigations have been carried out. In particular, various Q-switching techniques have been developed to generate short CO₂ laser pulses [1–4].     

Micromachining of microchannel on the polycarbonate substrate with CO2 laser direct-writing ablation

Low-power CO₂ laser direct-writing ablation was used to micromachine a microchannel on the polycarbonate substrate in this work. The influence of the process parameters (the laser power, the moving velocity of the laser beam and the scanning times) on the micromachining quality (the depth, the width and their aspect ratio) of the microchannel was experimentally studied. The depth and width of microchannel both increase with the increase of the laser power and the decrease of the moving velocity of the laser beam. When higher laser power and slower moving velocity were used, the polycarbonate surface bore more heat irradiated from the CO₂ laser for longer time which results in the formation of deeper and wider molten pool, hence the ability to fabricate bigger microchannel. Because of the effect of the laser power on the depth and width of microchannels, higher aspect (depth/width) ratio could be achieved using slower moving velocity and higher laser power, and it would reach a steady state when the laser power increases to 9.0 W possibly caused by the effect of laser power on the different directions of microchannel. The polycarbonate–polycarbonate chip was bonded with hot-press bonding technique.     

Electron drift velocity in CO2 laser mixtures

Summary A modified Bradbury-Nielsen grid system has been used to measure the electron drift velocityW in two gas mixtures of special interest when modelling h.p. CO₂ lasers. The results are compared with the theoretical values obtained with the conventional two-term theory in order to assess the limits of the theoretical predictions and have indications on the accuracy of the available cross-sections. G.N.E.Q.P.-C.N.R.     

Nanosecond laser ablation for pulsed laser deposition of yttria

A thermal model to describe high-power nanosecond pulsed laser ablation of yttria (Y₂O₃) has been developed. This model simulates ablation of material occurring primarily through vaporization and also accounts for attenuation of the incident laser beam in the evolving vapor plume. Theoretical estimates of process features such as time evolution of target temperature distribution, melt depth and ablation rate and their dependence on laser parameters particularly for laser fluences in the range of 6 to 30 J/cm² are investigated. Calculated maximum surface temperatures when compared with the estimated critical temperature for yttria indicate absence of explosive boiling at typical laser fluxes of 10 to 30 J/cm². Material ejection in large fragments associated with explosive boiling of the target needs to be avoided when depositing thin films via the pulsed laser deposition (PLD) technique as it leads to coatings with high residual porosity and poor compaction restricting the protective quality of such corrosion-resistant yttria coatings. Our model calculations facilitate proper selection of laser parameters to be employed for deposition of PLD yttria corrosion-resistive coatings. Such coatings have been found to be highly effective in handling and containment of liquid uranium.     

Theoretical considerations regarding pulsed CO2 laser annealing of silicon

We present a calculation of the surface temperature and investigate the “thermal runaway” phenomenon during pulsed CO₂ laser (λ = 10.6 μm) annealing of silicon. In calculating the temperature variation of free carrier absorption in n-Si, we have taken into account acoustic deformation potential scattering, optical deformation potential scattering, and ionized impurity scattering. The deformation potentials are adjusted to fit the experimentally observed values at 300°K. Also, we discuss the contribution of free carrier absorption during annealing with a Nd:glass laser (λ = 1.06 μm).     

Minimization of cavitation effects in pulsed laser ablation illustrated on laser angioplasty

Cavitation effects in pulsed laser ablation can cause severe deformation of tissue near the ablation site. In angioplasty, they result in a harmful dilatation and invagination of the vessel walls. We suggest to reduce cavitation effects by dividing the laser pulse energy into a pre-pulse with low and an ablation pulse with high energy. The pre-pulse creates a small cavitation bubble which can be filled by the ablation products of the main pulse. For suitable energy ratios between the pulses, this bubble will not be enlarged by the ablation products, and the maximal bubble size remains much smaller than after a single ablation pulse. The concept was analyzed by numerical calculations based on the Gilmore model of cavitation dynamics and by high-speed photography of the effects of single and double pulses performed with a silicone tube as vessel model. The use of double pulses prevents the deformation of the vessel walls. The concept works with an energy ratio of up to about 1:30 between the pulses. For the calculated optimal ratio of 1:14.6, the bubble volume is reduced by a factor of 17.7. The ablation pulse is best applied when the pre-pulse bubble is maximally expanded, but the timing is not very critical.     

Theoretical model of CO2 laser ablation of soft-tissue phantoms

Summary A model of thermal laser ablation of soft tissues is developed taking into consideration two mechanisms: evaporation and liquid moving, due to vapour pressure gradient. Usually a soft tissue is modelled as a single-component material with thermal and optical properties very similar to those of water. We examined the non-stable kinetics of the evaporation process, for short-pulse infrared laser ablation of soft tissues, and we also calculated the average liquid velocity and the ablation rates under vapour pressure gradient. The theoretical results are in good agreement with previous reported experimental data on gelatin and polyacrylamide tissue phantoms The authors of this paper have agreed to not receive the proofs for correction     

Laser induced medium perturbation in a pulsed CO2 laser

Laser induced medium perturbation (LIMP) has been identified by time-resolved interferometry observations of the lasing/non-lasing boundaries in a pulsed, kilojoule CO₂ electron beam laser, working at atmospheric pressure. This fundamental effect destroys the quality of the laser output beam and therefore has serious implications in laser design.