Mihai Stafe

Pulsed Laser Ablation of Solids

Here we give an overview on our main experimental and theoretical work on pulsed laser ablation (PLA) on several targets: metals (Al, Ti, Fe and Cu) and dielectrics (Er:Ti:LiNbO3). PLA is demonstrated to be effective when the laser fluence is larger than a threshold value which depends mainly on the optical properties of the material and wavelength. The experiments indicate that the ablation rate decreases approximately linearly with wavelength and with the inverse of beam diameter, whereas an increasing fluence leads to logarithmic increase of the ablation rate. Optodynamics and laser spectroscopy are used for analyzing the ablation plasmas and the ablated structures in real-time. We further characterize theoretically the laser ablation and present the photo-thermal model for ablation with short pulses. We show that PLA in ns/ps regimes can be considered as a superposition of heating, melting, vaporization and melt ejection under the action of the plasma recoil pressure, the plasma shielding effect being also accounted for. The ablation rate is calculated numerically by solving through finite-differences method a heat-type equation in an iterative algorithm and indicates, like the experiments do, that the ablation rate is first constant during the first pulses and decreases strongly afterward with increasing pulse number.

Impact of the lasr wavelength and fluence on the ablation rate of aluminium

The dependence of the ablation rate of aluminium on the fluence of nanosecond laser pulses with wavelengths of 532 nm and respectively 1064 nm is investigated in atmospheric air. The fluence of the pulses is varied by changing the diameter of the irradiated area at the target surface, and the wavelength is varied by using the fundamental and the second harmonic of a Q-switched Nd-YAG laser system. The results indicate an approximately logarithmic increase of the ablation rate with the fluence for ablation rates smaller than ∼6 μm/pulse at 532 nm, and 0.3 μm/pulse at 1064 nm wavelength. The significantly smaller ablation rate at 1064 nm is due to the small optical absorptivity, the strong oxidation of the aluminium target, and to the strong attenuation of the pulses into the plasma plume at this wavelength. A jump of the ablation rate is observed at the fluence threshold value, which is ∼50 J/cm2 for the second harmonic, and ∼15 J/cm2 for the fundamental pulses. Further increasing the fluence leads to a steep increase of the ablation rate at both wavelengths, the increase of the ablation rate being approximately exponential in the case of visible pulses. The jump of the ablation rate at the threshold fluence value is due to the transition from a normal vaporization regime to a phase explosion regime, and to the change of the dimensionality of the hydrodynamics of the plasma-plume.

Real-Time Monitoring of the Pulsed Laser Ablation of Metals Using Ablation Plasma Spectroscopy

Here we demonstrate that the emission spectra of the ablation-plasma produced by nanosecond laser pulses on metallic Al targets may be directly connected to the ablation rates and the dimensions of the ablated craters. We show that the variation of the individual spectral-lines intensities with pulse number gives direct, real-time information on the crater depth, whereas the relative intensities of the lines and their widths enable us to study the variation of the electron temperature and density with pulse number and laser fluence in direct connection to the ablation rates. To interpret these results we use a simple model in which the plasma-plume is treated as an ideal gas expanding away from the target with a velocity given by the electron-temperature, and exerting a recoil pressure determined by the electron temperature and density. The model correlates the plume hydrodynamic-length to the crater dimensions and succeeds in predicting the rims heights.

Laser-Matter Interaction Above the Plasma Ignition Threshold Intensity

In this chapter we present the process of laser-matter interaction above the plasma ignition threshold intensity. The physics of the pulsed laser ablation process at high intensities is very complex since it involves, besides direct laser-solid interactions, the process of plasma formation and expansion, and the laser-plasma interaction. Inverse Bremsstrahlung and photoionization processes is considered to be the main absorption mechanisms of the laser light within the ablation plumes produced on metallic targets. Plasma kinetics including electron impact excitation/ionization and recombination processes, as well as the energy transfer from electrons to ions and neutral species are considered. Section 4.1 presents the main phenomena involved in production of the ablation plasma and in laser-plasma interaction during PLA: plasma formation and evolution. In this section, plasma heating, self focusing, critical density, shielding, and plume expansion is discussed. Interaction of plasma plume with obstacles is also treated in Sect. 4.1.3. Experimental methods for analyzing the main phenomena involved in laser-plasma interaction (i.e. optical and mass spectroscopy, high speed imaging) are presented in Sect. 4.2. The most important parameters which characterize the laser-ablated plumes (density and the temperature) are usually determined by optical techniques (i.e. interferometry, Thomson-scattering and plasma spectroscopy) which can be used to reveal the characteristic features of plasma, as well as to estimate and describe qualitatively and quantitatively its properties. The theoretical models for describing the laser-plasma interaction allow one to estimate the spatial–temporal distribution of the plasma parameters such as temperature, density and pressure. Among the models describing the dynamics of the expanding ablation vapour/plasma plume, Monte Carlo simulations and hydrodynamic equations approaches have been widely used. The numerical results on the ablation plasma were validated by comparison to the experimental data obtained by using optical emission and absorption spectroscopy, mass spectrometry, time-of-flight and charge collection measurements. Section 4.3 presents in more detail theoretical results obtained within the photo-thermal model on the characteristics of the ablation plasma in relation to the ablation rate in nanosecond irradiation regime.

Experimental investigation of the dimensions and quality of laser-drilled holes in metals

We investigated the process of laser micro-drilling of copper and iron by using nanosecond laser-pulses at 532nm wavelength in atmospheric air. We analyzed the ablated volume, ablation rate, crater diameter, and craters quality as functions of laser-fluence and beam-diameter. The fluence was varied between 10 and 6000 J/cm2 by changing the laserenergy. The results indicate that the ablated volume increases linearly with fluence, whereas the ablation rate and crater diameter increase linearly with the fluences square root. The ablated volume, ablation rate, and crater diameter, increase with thermal diffusivity of the materials. Additionally, the ablation threshold-fluence is demonstrated to be directly related to the optical penetration depth. The ablated volume, ablation rate, and crater diameter were further assessed for beam-diameters in the range of 10-50 microns by translating the targets away from the focal plane while keeping a constant fluence. The results indicate that the ablated volume increases linearly with beam-diameter, whereas the ablation rate and crater diameter increase linearly with the inverse of the beam-diameters square root. To investigate the craters quality we measured the dimension of the thermally affected zone (TAZ) around the craters as a function of fluence. At fluences up to 400 J/cm2, where strong ionization occurs within the plume, the crater diameter is <15 microns (comparable with beam diameter) and there is small TAZ around the craters. Further increase of the fluence leads to a significant increase of TAZ, indicating that the expanding plasma plays a major role in metals ablation in this fluence domain.

Material Removal and Deposition by Pulsed Laser Ablation and Associated Phenomena

This chapter starts with the presentation of some micro and nano requirements by describing some influences of the ambient gas and target material on the ablation process in general and on the ablation rate in particular. Ablation in liquids is also briefly presented as an alternative ablation technique. Nanoparticle formation from the ablation plume is further explained based on basic homogeneous condensation theory. In the last part the PLD process is presented through its basic components: ablation, plume propagation and particle deposition. Special attention is given to the plume propagation and filtering techniques. ‘Classical’ filtering techniques as axe-off, back-side and ‘eclipse’ techniques are described starting from the plume propagation. Also some more advanced multi-element masks and a more recent filtering technique based on the plume reflection developed by the authors are presented and described in this chapter.

Numerical simulations of surface plasmon resonances in metal-chalcogenide waveguides

In this paper we present several numerical simulations of the surface plasmon resonance for Kretschmann type configuration in a metal-chalcogenide waveguide. We assume that the chalcogenide (GaLaS) waveguide layer have finite thickness, whereas the gold film layer and the air cover layer are semi-infinite layers (from an optical point of view). We determined the thickness of the chalcogenide film for which plasmonic resonant coupling of the incident radiation to the waveguide occurs. We calculated the propagation constant for the TE- and TM- modes (both for visible and IR domain), the attenuation coefficient and the electromagnetic field distribution within the waveguide. The obtained results provide the conditions for design an optical memory device 2D based on light-light interaction in plasmonic configuration.

Experimental Techniques for Analyzing the Material Removal and Deposition Rates in Real Time

In this chapter, three real time techniques for ablation and deposition rates monitoring are presented. First one is the ‘optoacoustic and interferometric’ approach. This method relay on the laser-mater interaction produced waves and their propagation through the material and through the surrounding atmosphere for determination of the depth of formed crater. The second one is the spectroscopy of the ablation plume. Spectroscopic analysis of the ablation plasmas is based on the direct connection between the plasma characteristics with different pulse numbers and laser fluences and the dimensions of the craters ablated on metallic targets. The third one is the deposited thickness real time measurements using microbalances. This technique relay on the single crystal quartz resonance frequency shift while additional layers of various materials are deposited on it’s surface. Few basic equations and setup schemes are presented for these techniques.

Lasers for Pulsed Laser Ablation

In this chapter, several lasers (e.g. Nd-YAG laser, Ti-sapphire laser, excimer laser, \(\text {CO}_{2}\) laser) are analysed because they are versatile sources of energy in a highly concentrated form, being attractive tools and research instruments for a large variety of research and production fields, and for laser ablation in particular. We present the fundamentals on the operation modes of these lasers (pulsed, CW, tunable), pulse duration (nano, pico, femtosecond range), power and operation wavelength (from near UV to far IR range), emphasizing their best capabilities. As current technology is pushed to ever smaller dimensions, lasers become a truly enabling solution, reducing thermomechanical damage and facilitating heterogeneous integration of components into functional devices. Also, the Q-switched and mode locked lasers are analysed together with the most used methods for their practical operation: mechanical, electro-optical, acousto-optical, methods with saturable absorbers and thin films etc. The knowledge of the laser operation and the parameters is used to control PLA process for different types of solid materials in different ambient conditions. At the end of this chapter several combined irradiation methods (exposure to intense surface plasmon optical near field, the 3D laser lithography etc.) and some effects of pulse duration on the ablation rate are discussed together the projection through microlens array and laser trepanning.

A spectroscopic and theoretical study of the laser ablation rate of Al

We investigated experimentally and theoretically the laser ablation of Al by using nanosecond laser-pulses at 532 nm wavelength in atmospheric air. We analyzed experimentally the dependence of the ablation rate on the laser fluence and pulse number. The fluence was varied between 3 and 3000 J/cm2 by changing the laser-energy, while pulse number was varied between 4 and 60 in steps of 4. The optical microscopy data indicate that the ablation rate increases approximately linearly with the 1/3 power of the fluence. For high fluences (hundreds of J/cm2) the ablation rate is demonstrated to be very large (~2 micron/pulse) and approximately constant during 30 consecutive pulses and much smaller during the next pulses. The dependence of the ablation rate on pulse number was further addressed by spectrometric analysis of the ablation-plasmas produced at high fluences. We found that the plasma temperature varies similarly to the ablation rate when increasing the pulse number. The ablation rate in the low fluence regime was addressed theoretically within the frame of a photo-thermal model which accounts for the material heating, melting and evaporation upon laser radiation. The theoretical and experimental results are in good agreement indicating the validity of the model for low laser fluences.