Showing papers by "Arkadi Rosenfeld published in 2005"

A general continuum approach to describe fast electronic transport in pulsed laser irradiated materials : The problem of Coulomb explosion

Abstract: We present a continuum model, based on a drift-diffusion approach, aimed at describing the dynamics of electronic excitation, heating, and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond and nanosecond pulsed laser irradiation. The laser-induced charging of the targets is investigated at laser intensities above the material removal threshold. It is demonstrated that, for near-infrared femtosecond irradiation, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. On the other hand, application of the model to UV nanosecond pulsed laser interaction with bulk silicon has pointed out the possibility of Coulomb explosion in semiconductors. For such regimes a simple analytical theory for the threshold laser fluence of CE has been developed, showing results in agreement with the experimental observations. Various related aspects concerning the possibility of CE depending on different irradiation parameters (fluence, wavelength and pulse duration) and material properties are discussed. This includes the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the reflection and absorption characteristics.

Theoretical investigations of material modification using temporally shaped femtosecond laser pulses

Abstract: We present a two-dimensional model, based on a drift–diffusion approach, developed to describe the dynamics of electronic excitation and lattice heating in several dielectric materials with different electron–phonon coupling properties (e.g. fused silica and sapphire) under the action of femtosecond near-infrared laser pulse trains with variable separation time between pulses. The modeling approach was aimed to describe the mechanisms that enable the spatial modulation of the structures induced by temporally modulated laser excitation and ablation of wide-band-gap dielectric materials. The possible geometric contours of the laser-induced craters on the target surfaces are discussed on the basis of the lattice-temperature profiles obtained by modeling. It was found that the observed difference in the crater shapes generated in fused silica and sapphire is conditioned by the difference in dynamics of electron excitation and recombination channels characteristic of these two materials. This effect can be used to convert a given temporal pulse modulation into spatial modulation, opening up new perspectives for material processing in order to obtain desired structure profiles.

Optimization of ultrafast laser generated low-energy ion beams from silicon targets

Abstract: We demonstrate the possibility to manipulate the kinetic properties of ion beams generated by ultrafast laser ablation of silicon. The versatility in regulating the sub-keV ion flux is achieved by implementing adaptive control of the temporal shape of incident laser pulses. Tunable characteristics for the charged beams are obtained using excitation synchronized with the phase-transformation dynamics, exploiting transitions to volatile fluid states with minimal energetic expenses.

Temporal pulse manipulation and consequences for ultrafast laser processing of materials

Abstract: Following advances in ultrafast laser technology as a reliable tool for material probing and processing, we discuss various options for control and optimization. The possibility to tailor the temporal shape of ultrashort laser pulses enables extended opportunities for material processing. The concept of optimizing laser interactions is based on the possibility to regulate the energy delivery so that control of laser-induced phenomena can be achieved and quality structures can be realized. An experimental demonstration of the possibility to design excitation sequences tailored with respect to the material response is described, laying the groundwork for adaptive optimization in materials structuring. We show that under particular irradiation conditions involving modulated excitation, the energy flow can be controlled and the material response can be guided to improve processing results. This is particularly important for processing brittle materials. Further examples are given to illuminate the possibility to optimize the kinetic properties of ions emitted from laser-irradiated semiconductors, using excitation sequences synchronized with the solid-to-liquid transformation time. Versatile sub-kilo-electron-volt ion beams are obtained, exploiting transitions to supercritical fluid states with minimal energetic expenses. Temporally selective irradiation can thus open up efficient thermodynamic paths, unfolding interesting perspectives for "intelligent," feedback-assisted processing of materials.

Surface charging under pulsed laser ablation of solids and its consequences: studies with a continuum approach

Abstract: Dynamics of electronic excitation, heating and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond pulsed laser irradiation is studied based on a unified continuum model. A simplified drift-diffusion approach is used to model the energy flow into the sample in the first hundreds of femtoseconds of the interaction. The laser-induced charging of the targets is investigated at laser intensities slightly above the material removal threshold. It is demonstrated that, under near-infrared femtosecond irradiation regimes, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. Various related aspects concerning the possibility of CE for different irradiation parameters (fluence, wavelength and pulse duration) as well as the limitations of the model are discussed. These include the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the optical (reflection and absorption) characteristics. A controversial topic concerning CE probability in laser irradiated semiconductor targets is also a subject of this work.

Adaptive control of ion beams produced by ultrafast laser ablation of silicon (Invited Paper)

Abstract: In a context where ultrafast lasers have become ideal tools for material probing and processing we present various concepts for process control and optimization. Temporal tailoring of ultrashort laser pulses enables synergies between radiation and material and, therefore, new opportunities for optimal processing of materials. The concept of optimizing laser interactions is based on the possibility to adjust energy delivery so that control of laser-induced processes can be achieved and particular states of matter can be accessed. We present recent results related to the implementation of adaptive feedback loops based on temporal shaping of ultrafast laser pulses to control laser-induced phenomena for practical applications. The chosen example indicates the possibility to manipulate the kinetic properties of ions emitted from ultrafast laser irradiated semiconducting samples, using excitation sequences synchronized with the phase-transformation characteristic times. Versatile sub-keV ion beams are obtained exploiting transitions to supercritical fluid states with minimal energetic expenses, while achieving very efficient energy coupling and thermodynamic paths towards highly volatile states. Temporally selective irradiation can thus open up efficient thermodynamic paths towards critical points, delivering at the same time an extended degree of control in material processing.

Verfahren zum Feinpolieren/-strukturieren wärmeempfindlicher dielektrischer Materialien mittels Laserstrahlung

Abstract: Zum Feinpolieren/-strukturieren warmeempfindlicher dielektrischer Materialien, insbesondere mit einem geringen Warmeausdehnungskoeffizienten, mittels Laserstrahlung wird erfindungsgemas ein Verfahren angegeben, bei dem intensive ultrakurze Laserstrahlung auf eine zu bearbeitende Oberflache des Materials gerichtet wird und die Einwirkzeit der Laserstrahlung auf die Oberflache im Bereich von 10 -13 s bis 10 -11 s und die Energie der Laserimpulse unterhalb der Ablationsschwelle aber ausreichend fur das Entstehen einer Coulombexplosion eingestellt wird. Mit dem erfindungsgemasen Verfahren wird ein Materialabtrag im Nanometerbereich mit ultrakurzen Laserpulsen im Pikosekunden- und Subpikosekunden-Bereich realisiert, wobei wahrend eines praablativen Verfahrensschritts (Abtrag unterhalb der Ablationsschwelle) die Materialoberflache fein poliert wird. Aufgrund der extrem kurzen Einwirkzeit der Laserstrahlung auf die zu bearbeitende Oberflache findet eine sehr kleine Erwarmung statt, die nur im Bereich von einigen 10 Grad liegt.

Verfahren zum Feinpolieren/-strukturieren wärmeempfindlicher dielektrischer Materialien mittels Laserstrahlung A method for fine polishing / -strukturieren heat-sensitive dielectric materials by laser radiation

Abstract: Zum Feinpolieren/-strukturieren warmeempfindlicher dielektrischer Materialien, insbesondere mit einem geringen Warmeausdehnungskoeffizienten, mittels Laserstrahlung wird erfindungsgemas ein Verfahren angegeben, bei dem intensive ultrakurze Laserstrahlung auf eine zu bearbeitende Oberflache des Materials gerichtet wird und die Einwirkzeit der Laserstrahlung auf die Oberflache im Bereich von 10 -13 s bis 10 -11 s und die Energie der Laserimpulse unterhalb der Ablationsschwelle aber ausreichend fur das Entstehen einer Coulombexplosion eingestellt wird. For fine polishing / -strukturieren heat sensitive dielectric materials, in particular with a low coefficient of thermal expansion, by means of laser radiation is specified according to the invention a method is directed at the intensity ultra-short laser radiation on a surface to be worked of the material and the time of action of the laser radiation on the surface in the range of 10 - 13 s, however, set to 10 -11 s and the energy of the laser pulses below the ablation threshold sufficient for the emergence of a Coulomb explosion. Mit dem erfindungsgemasen Verfahren wird ein Materialabtrag im Nanometerbereich mit ultrakurzen Laserpulsen im Pikosekunden- und Subpikosekunden-Bereich realisiert, wobei wahrend eines praablativen Verfahrensschritts (Abtrag unterhalb der Ablationsschwelle) die Materialoberflache fein poliert wird. With the inventive method, material removal is realized in the nanometer range with ultra-short laser pulses in the picosecond and sub-picosecond range, while a praablativen process step (cut below the ablation threshold) the material surface is finely polished. Aufgrund der extrem kurzen Einwirkzeit der Laserstrahlung auf die zu bearbeitende Oberflache findet eine sehr kleine Erwarmung statt, die nur im Bereich von einigen 10 Grad liegt. Because of the extremely short contact time of the laser radiation on the surface to be processed a very small heating takes place, which is only in the range of some 10 degrees.

A method for fine polishing / -strukturieren heat-sensitive dielectric materials by means of laser radiation

Abstract: For fine polishing / -strukturieren heat-sensitive dielectric materials, in particular with a low coefficient of thermal expansion, by means of laser radiation is shown according to the invention a method is directed to a surface to be worked of the material in the intense ultrashort laser radiation and the exposure time of the laser radiation on the surface in the range of 10 With the inventive method, a material removal is carried out in the nanometer range with ultrashort laser pulses in the picosecond and subpicosecond range, wherein the material surface is finely polished during a praablativen process step (removal below the ablation threshold). Due to the extremely short exposure time of the laser radiation on the surface to be treated a very small warming takes place, which is only in the range of some 10 degrees.