Appl. Phys. A 81, 65–70 (2005)
Applied Physics A
DOI: 10.1007/s00339-005-3211-7
Materials Science & Processing
w. kautek1,2,u p. rudolph2,∗ g. daminelli2 2 ¨ j. kruger
Physico-chemical aspects of femtosecond-pulse-laser-induced surface nanostructures 1 Institute
for Physical Chemistry, University of Vienna, Waehringer Strasse 42, 1090 Vienna, Austria for Thin Film Technology, Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany
2 Laboratory
Received: 12 December 2004/Accepted: 20 December 2004 Published online: 12 April 2005 • © Springer-Verlag 2005
Near-ablation threshold investigations focusing on the generation of periodic nanostructures and their correlation with physico-chemical properties of the solid phase such as e.g., the material-dependent surface energy, were conducted. Molecular dynamic modelling in the sub-picosecond time domain was used to consider ultrafast opto-electronic processes triggering surface reorganization reactions. Fluid containment of solid interfaces showed strong influence on the resulting micro- and nanostructures due to its drastic reduction of the surface energy. The phenomena are discussed in respect to the minimization of the surface free energy in dependence of material composition and interfacial structure.
ABSTRACT
PACS 78.70.-g;
1
81.07.-b; 68.35.Md
Introduction
Ablation craters and ditches machined by repetitive laser pulsing exhibit ordered surface microstructures such as so-called ripples, which can limit the machining precision inherently. This classical ripple formation is known since the early days of laser application [1]. Their origin was found in the interference of the incident, reflected or refracted laser radiation with the scattered or diffracted light travelling near the surface. Their period depends on the wavelength, the angle of incidence, and the polarization of the laser beam [2]. The interaction of femtosecond laser pulses results in ultimate high-precision processing not accessible with pulses in the picosecond, nanosecond or even longer region [2–7]. Intensive research in femtosecond pulse laser micromachining of polymers and metallic materials has been initiated much more than a decade ago [2, 3]. Early work on human tissue, various other biological materials, and dielectric samples followed soon after in this laboratory [4–7]. The material interaction of near-infrared ultrashort laser pulses down to durations of 5 fs have been investigated and their processing potentials were established [8–10]. Experiences with top-down u Fax: +43-1-4277-52449, E-mail:
[email protected] ∗ Present address: Federal Ministry of Economics and Labour, Scharnhorststraße 34–37, 10115 Berlin, Germany
structuring strategies for functional microstructures leading to regular ripples with sub-micrometer period not only on dielectrics [11–16], but also on silicon [17–22], indium phosphide [23, 24], and titanium nitride [17, 18, 25] preceded the present study. Recent investigations on the femtosecond pulse laser interaction with high-performance ceramics like silicon carbide, aluminium nitride, and a composite compound SiC-TiCTiB2 , indicated that a direct correlation between chemical composition and ripple character exists [26, 27]. Moreover, femtosecond laser interaction with the silicon-water interface showed that also a contacting condensed phase has a strong physico-chemical influence on these phenomena [28]. In this context, near-ablation threshold phenomena, such as the generation of nanostructures, and correlations of their morphology with the composition of the substrate and the nature of the interface are discussed on the basis of the thermodynamic concepts of surface energy changes and timeresolved molecular dynamic modelling of non-thermal melting processes in the femtosecond time regime [20]. 2
Experimental
Ti:Sapphire femtosecond lasers (Spectra Physics, Tsunami and Spitfire) were utilized for ablation experiments at an 800 nm central wavelength with bandwidth-limited pulse durations of 130 fs at a repetition rate of 1 kHz. The pulse duration was determined by means of an autocorrelator. Laser pulse energies were varied using a rotatable half-wave plate in front of the compressor unit. The pulse energy was measured employing a power meter and a pyroelectric detector in the multi- and single-pulse case, respectively. The number of laser pulses per spot was controlled by a pulse counter. The target was mounted on a motorized x - y-z translation stage. The surface of the sample was positioned perpendicular to the direction of the incident laser beam. Optical investigations were done with an optical microscope. More experimental details can be found in the respective cited publications [27, 28]. 3 3.1
Results and discussion Ceramics
Femtosecond laser irradiation in the multi-pulse regime of various compound ceramic materials led to peri-
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odic nanostructures [27]. A cavity on aluminium nitride generated by N = 100 pulses with a fluence of F = 5.6 J/cm2 , 10 times higher than the ablation threshold, Fth = 0.5 J/cm2 , showed ripples perpendicularly oriented to the field vector (Fig. 1 [27]). Two periodicities could be discriminated: at the edge of the cavity 200 – 300 nm, and in the centre 610 nm. This observation indicates a correlation between the order of periodicity and the laser fluence due to the Gaussian profile of the laser beam. This phenomenon has been described also on barium borosilicate glass [11], fused silica [11], wide bandgap insulators (BaF2 and CaF2 ) [14–16] and on silicon [22] calling the classical interpretation for ripples formation in relation to interference processes [2] into question. Actually, only two types of structures were observed and were correlated with the electric field of the incident beam. No transient index gratings were created as a result of the intensity variation. Silicon carbide plays an important role in many industrial applications because of its hardness, high melting temperature, chemical and thermal resistance, combined with its low weight. A composite ceramic compound SiC-TiC-TiB2 was designed for tribological applications. Addition of TiC and TiB2 to the SiC matrix reduces the wear rate. Mechanical machining of these materials is difficult due to their hardness up to 22 GPa. Short and ultra-short pulse laser machining was investigated as a solution to avoid mechanical tool wear and minimize thermal and mechanical stress [26]. µ-Raman measurements after fs-treatment of SiC-TiC-TiB2 showed rutile (TiO2 ) coverage of the Ti-containing grains, whereas SiC was covered by unoriented graphite and a reduced Si species,
but negligible oxide. The melted regions were converted to resolidified nanocrystalline and amorphous phases. Also XPS analyses on SiC proofed the absence of silicon oxide in contrast to nanosecond treatment, which resulted in an oxide coverage. Pronounced ripples were also observed at the composite ceramic compound (SiC-TiC-TiB2 ). 10 pulses with F = 0.9 J/cm2 ( Fth = 0.24 J/cm2 ) caused widespread ripples with a periodicity of Λ ∼ 500 nm, a finer type with 200 nm arranged in clusters, and a third disk-like feature with a period of 600 – 650 nm (Fig. 2 [27]). Energy dispersive X-ray analysis (EDX) showed that the cluster-like ripple structures with a periodicity of 200 nm consisted of SiC. The widespread ripple structures contained mainly TiC with a periodicity of 500 nm. The disks consisted of TiB or TiB2 with dimensions of 600 – 650 nm. That means that the ripples with the largest periodicity were the one with the highest share of TiC. The two other components, SiC and TiB2 , were localized only in small regions (Fig. 3). In conclusion, there are two ripple types as a function of fluence F . At higher F , structures with a periodicity of 600– 650 nm near to the laser wavelength of 800 nm were observed on all investigated ceramic compounds. At lower F near the ablation threshold, periods of 200 – 500 nm occurred depending on the chemical composition. In the latter case, material-dependent parameters determine the ripple formation process. Far above the ablation threshold fluence, the nature of the material becomes less important and laser parameters control the morphology. The 600 – 650 nm structures
FIGURE 1 Femtosecond laser ablation of aluminum nitride (AlN) in air. 800 nm, 130 fs, N = 100, 5.6 J/cm2 . (a) Overview and (b) detail [27]
FIGURE 2 Femtosecond laser ablation of the composite ceramic compound (SiC-TiC-TiB2 ) in air. 800 nm, 130 fs, N = 10, 0.9 J/cm2 . Formation of periodic structures. (a) Overview and (b) detail [27]
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Physico-chemical aspects of femtosecond-pulse-laser-induced surface nanostructures
67 FIGURE 3 EDX analysis of the composite ceramic compound (SiCTiC-TiB2 ) after femtosecond laser ablation. 800 nm, 130 fs, N = 10, 0.9 J/cm2 . Λ: periodicity [27]
can be explained in the classical ripple model, which corresponds to a non-uniform energy input into the sample, modulated by the interference between the incident wave and an induced surface wave [2]. It needs roughening of the surface by the first laser pulses. Laser ablation can also lead to the formation of noncoherent structures as e.g. at a peak fluence of 0.20 Jcm−2 near the ablation threshold (Fig. 4b [28]). Such features may be due to spatio-temporal ordering requiring at least two degrees of freedom, as e.g. the temperature and the melt film thickness. This behaviour can be understood from the behaviour of zero isoclines of a particular variable (e.g. melt thickness) where the time-derivative is zero [2]. On the other hand, transitions of fluid-vapour interfaces on heating and cooling through the binodal and spinodal regions can involve oscillatory instabilities that may result in self-assembled surface features. Actually, interfacial oscillatory instabilities have also been observed in classically heated fluid systems such as a binary metallic fluid, under the influence of two competing forces with different time scales [29]. Such phenomena are based on wetting-dewetting transitions at the interface driven by spinodal decomposition. Surface freezing and wetting transitions may parallel thermal melting. Surface freezing is an interfacial phase transition where an ordered solid-like film forms at the liquid- vapour interface at temperatures above bulk freezing. It may occur by nucleation of a strongly undercooled liquid wetting film. In any case, the driving force of wetting and freezing films formation is the
lowering of surface free energy [30]: ∆E s ∼ ∆E lv − (∆E sv + ∆E sl )
(1)
where ∆E lv , ∆E sv , and ∆E sl are the surface free energies of the liquid-vapour, the solid-vapour, and the solid-liquid interfaces. The sign of ∆E s determines the tendency. Regular coherent interconnected structures (labyrinth pattern) have been observed recently [31] also with the electrochemical metal phase formation in the underpotential and overpotential deposition region typical for spinodal decomposition mechanisms [32]. The influence of chemistry, i.e. interatomic interactions, on surface phase transitions such as surface freezing is widely an open question so far. The regular spinodal structures described above are formed near the thermodynamic equilibrium on long time scales (up to minutes) dissimilar to the present phase changes in the femtosecond and picosecond time regime [34]. A spinodal decomposition upon laser heating of a melt phase, however, involves a massive pressure rise. The whole volume undergoes a so-called phase explosion under these conditions. It involves the rapid spontaneous growth of small density fluctuations extending over large spatial scales, i.e., the entire liquid volume [33], and nanoscale ripples cannot be formed. In conclusion of the so-far described experimental results, the phenomenon of the fs-laser-induced generation of regular nanostructures can be correlated with surface energy changes
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Applied Physics A – Materials Science & Processing FIGURE 4 Femtosecond laser irradiation of silicon. 800 nm, 130 fs, N = 1000, repetition rate 1 kHz. (a) F = (0.24 ± 0.02)Jcm−2 (1 × Fth ); (b) F = (0.20 ± 0.03)Jcm−2 (2 × Fth ) [28]
triggered upon femtosecond pulse laser irradiation. Processes far from equilibrium states, such as strong electron heating coupled with lattice destabilization, may play a major role in changing the kinetic paths from the initial to final thermodynamic state of the surface. The high-intensity laser-triggered destabilization thus only affects the kinetic conditions without affecting the thermodynamic final state. The equilibrium shape of a crystalline solid’s surface is correlated to the tendency to attain a minimum surface free energy. This has to take into account that the surface free energy for different faces is usually different. In a good approximation the total surface enthalpy, Hs and the surface energy, E s are not distinguished [35] E s ∼ Hs = G s + TSs
(2)
with G s the surface free enthalpy and Ss the surface entropy. In this context, chemical influences can be rationalized. The surface energy, E s , of a covalently bonded crystal, in the simplest assumption, is one-half of the energy to rupture bonds passing through the unit area [36, 37] E S = 1/2E cohesion
(3)
That implies that higher bonding strengths in a solid, i.e., a higher E cohesion , lead to higher surface energy, E s . An analogous proportionality between E s and lattice energy of ionic crystals exists as well [38]. It could be shown that the equilibrium surface for any given plane is not smooth [39]. Also the total surface energy, E S , is a minimum for a given apparent plane area, that requires an improbably ordered arrangement, and the minimum surface free energy, G s,relax , is attained for a saw-toothed surface with teeth and waves as much a several tens nanometers in height representing an optimum energy entropy balance. This simplified “saw-tooth” structure can well be correlated with the observed ripple structures of less than 200 nm periodicity. It also has been pointed out that this resulting equilibrium roughness may involve a surface-melting step [40]. The total surface energy, E S , generally is larger than the surface free energy G S . If rearrangement and relaxation is allowed e.g., by non-thermal surface melting due to strong electronic excitation after repetitive femtosecond laser pulses [20], saw-toothanalogous or wavy features may be probable. Accordingly, the probability and the rate of forming relaxed nanostructured surfaces should depend on the value of
the surface free energy gain, ∆G , which always has a negative sign ∆G = G s,relax − G s
(4)
This thermodynamic relation holds independently from the mechanistic path, even when the kinetics is modified by faroff equilibrium states such as hot electrons which can change the transition state of the lattice on the way to the end product (see above). The experiment showed that the tendency to relax and form self-assembled nanostructures decreased in the order SiC, TiC, TiB2 (Fig. 3 [27]). Assuming a correlation between tensile strength (SiC: 390 MPa; TiC: 258 MPa [41]), lattice energy, and E S , the relatively stronger tendency of the SiC surface to relax to a nanostructured surface in contrast to the TiC and TiBx surfaces exhibiting less surface energy could thus be rationalized. This strong chemical influence can also be supported by a greater experimental E S value of 5.3 J/m2 for β-SiC [34] than of 4.6 J/m2 of γ-TiC [34]. It is well established that the removal of atoms and clusters from an ordered surface leading to various surface defects rather causes the surface energy to decrease, so that the tendency to reassemble (∆G) would be lessened. The idea that ripples may arise from relaxation more than from interference effects as in the classical model [14–16, 22] agrees with the energy reduction picture (4). The concept that surface (coulomb) explosion may be the cause of instability and therefore lead to self-assembly [14–16, 22] would contradict the above surface energy approach. It should be further noted that bonding of foreign ligands e.g., in a surface conversion reaction like e.g., oxidation of the TiC or TiBx grains (see above) [26, 27], or a ligand addition/exchange to the surface atoms in fluid contact (see below) may affect this mechanism drastically. 3.2
Silicon-fluid interface
Material laser processing in the presence of water has attracted interest due to many motivations. Higher plasma pressure and longer duration of the shock waves may be advantageous for laser shock processing, where changes in the material structure and stress state result in improved surface hardness, fatigue strength, and corrosion resistance of the material. Generation of bubbles as well as water explosion have been employed in steam laser cleaning for the removal of particles from surfaces [42]. Water convection and bubble motion
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Physico-chemical aspects of femtosecond-pulse-laser-induced surface nanostructures
contribute to the removal of debris redeposition. The high heat capacity of water provides a better heat sink, cooling effectively heat sensitive substrates and the ejected material. Formation of nanoparticles by laser ablation of solids in liquids has been achieved thanks to the confinement effects on vaporized material within the liquid layer. A water layer during material laser processing may also allow the coupling of electrochemical techniques for in-situ monitoring of laser machining on differently conducting multilayers [43, 44]. Femtosecond laser ablation of silicon in water contact has been studied only recently [28]. Its behaviour in dry femtosecond laser ablation is well known [19]. The plasma breakdown threshold of water is 0.58– 1.11 J/cm2 for pulse duration of 100 fs at 580 nm [45], somewhat above the ablation threshold of silicon of 0.26 J/cm2 (800 nm, 130 fs). Under water confinement, near the ablation threshold, and well below the water breakdown fluence, no regular ripples but chaotic nanostructures occurred under fluid contact (Fig. 4a [28]), whereas well-oriented ripples were formed without the water phase (Fig. 4b [28]). In the case of maximum fluences above the threshold (near-Gauss-shaped beam profile) one can observe [28] also the 100 nm-ripples at a maximum fluence leading to the “large” ripples which are formed not in correspondence with the chemical composition of the solid and the interface. This suggests according to the findings on ceramic samples that different chemical surface conditions lead to deviating relaxation features. One can expect that fluid contact decreases the surface energy and surface tension, respectively, so that the driving force of the surface near regions to relax and self-organize (∆G , (4)) is drastically reduced. Actually, the wet surface did not show regular self-assembly structures in contrast to the dry surface (Fig. 4a [28]). The effect of the fluid contact on the laser-treated surface is based on the physico-chemical changes due to ligand exchange versus the dry surface. If a clean solid surface is immersed in a liquid, there generally is liberation of heat, qimm [34]: qimm = E sl − E s
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surface melt layer. Those single-pulse features, however, contrasted from the observed multi-pulse ripples in their size and orientation. 3.3
Femtosecond perturbation of bonds
The small periodic features depend strongly on the composition (Sect. 3.1) and the interfacial chemistry (Sect. 3.2). A possible explanation may be a self-organization structure formation during the relaxation of a highly nonequilibrium surface including non-thermal melting processes [20]. Support for this mechanism comes from a theoretical simulation of femtosecond ablation. Without ablation, i.e. without bond-breaking, rapid excitation of electrons can cause massive instability in the crystal lattice within a few 10 fs due to a perturbation of the interatomic bonds. Below threshold fluences, however above a modification threshold (comp. study on Si modification thresholds [19]), this instability, then, relaxes on a several 100 fs time scale by surface reorganization in regular, periodic structures. A typical signature of such structures are bifurcations. Molecular dynamics (MD) simulations on the basis of an electronic tight-binding Hamiltonian in real-space took into account all atomic degrees of freedom. Nonequilibrium occupation numbers for the energy levels of the system which, being time-dependent, lead to lattice dynamics on time-dependent potential energy surfaces, were calculated. Thus, a theoretical framework is provided for the treatment of strong nonequilibrium situations in materials where atomic and electronic degrees of freedom play an equally important role. Figure 5 [20] shows snapshots of the lattice dynamics due to excitation with a laser pulse of duration 20 fs. During the first 100 fs after the peak maximum only a moderate expansion of the system occurs. If sufficient energy above the ablation threshold is delivered, strong bond breaking (200 fs) and ablation (500 fs) starts to take place. This may also be followed by classical thermal melting when the electron energy has been dissipated to the lattice leading to
(5)
and the surface energy attains a smaller value E sl . In water contact, this energy reduction is practically independent of the nature of the solid [34]. Actually, water causes an extremely high surface energy reduction due to its high dipole moment. Water contact, therefore, releases the need of surface rearrangement so that no self-assembled regular ripples occur at near-threshold radiant exposures (Fig. 4a [28]) in contrast to the dry surface at comparable fluence (Fig. 4b [28]). At higher maximum fluences above the threshold (e.g., 7 × Fth ) [28], ablation processes do not allow interfacial chemical conditions to control the surface relaxation characteristics so that even there self-assembled regular ripples may occur (not shown here). This aspect is not yet fully understood, and needs further investigation. Single fs-pulse could yield melted silicon surfaces either generating Si bubbles, which left behind round holes after resolidification, [19], or heating adjacent water, which then formed water bubbles [46]. These could oscillate and emit an acoustic wave imposing a wavy circular ripple structure on the
FIGURE 5 Snapshots of the ultrashort ablation of silicon as a response to excitation with a laser pulse of Gaussian shape. 20 fs, 4.0 eV per atom, 64 atoms [20]
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Applied Physics A – Materials Science & Processing
the well-known wavelength-scale ripples independent of the chemistry and the bond strength. 4
Conclusions
Controlled chemical and morphological surface conversions may be attractive for surface technological and nanotechnological purposes. The remaining roughnesses are self-assembled being not only detrimental to micromachining applications but also opening a range of nanotechnological applications. A ripple type with periodicities around 200 nm and less could be generated near the ablation threshold on SiC and Si, but not on TiC and TiB2 phases in air contact. This phenomenon can be correlated with surface energy changes triggered by femtosecond-laser-induced non-thermal melting processes on the 100 fs scale. A clear dependence on the chemical composition of the bulk and/or the conversion layer could be detected. Materials with high surface energy (SiC, Si) exhibited a strong tendency to relax to the nano-ripples, whereas materials with low surface energies (TiC and TiB2 ) did not show this process. At higher fluences, these chemical differences were masked and classical ripples with periodicities approaching the laser wavelength (800 nm) occurred. Bonding of foreign ligands e.g., in a surface conversion reaction like oxidation of the TiC or TiB2 grains or a ligand addition/exchange to the surface atoms in fluid contact, may affect this mechanism drastically. Water contact reduces the surface energy practically independent of the nature of the solid and releases the need of surface rearrangement so that no self-assembled regular nano-ripples occur at threshold fluences. ACKNOWLEDGEMENTS Partial financial support was provided by the European Community, in the BRITE-EURAM III project BRP-CT96-0265, the TMR Project “Modelling and Diagnostic of Pulsed Laser–Solid Interaction: Applications to Laser Cleaning”, FMRX-CT980188, by the German Ministry for Research and Technology (BMBF) in the framework of LASER 2000 (Laserinduzierte Fertigungsverfahren, Verbundprojekt ABLATE, # 13N 7048/7) and the project “Safety for Applications of Femtosecond Laser Technology” - SAFEST (BMBF-Projektverband Femtosekundentechnologie).
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