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ABSTRACT: We have studied ultra-fast laser ablation of Si and a metal via the neutral and ion yield, the energy distribution of emitted
neutrals and the charge distribution as a function of the laser pulse width. Two processes, one leading to the ejection of
fast (3–7eV), the other to slow thermal particles, can be identified. The origin of the first process can be correlated with
laser pulse widths (or pump–probe delays) and processes on a time scale below 100fs. Results for Si confirm recent findings
for Coulomb explosion (CE) and we show for the first time that CE exists as a mechanism of material removal from metals for
ultra-short laser pulses.
Applied Physics A 04/2012; 83(2):333-336. · 1.63 Impact Factor
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ABSTRACT: Zur Herstellung von hochpräzisen Transplantaten aus menschlicher Hornhaut eignet sich die Abtragung mittels Excimer Laser
bei 193 nm Wellenlänge besonders gut. Darauf basiert das Excimer Laser Corneal-Shaping-System (ELCS-S), mit dem aus Spenderhornhäuten
in vitro durch Laserablation eine breite Palette verschiedener Dickenprofile herstellbar ist. Durch Verwendung eines unlängst
implementierten Abtragungsalgorithmus (OSLA, Optimized Scanning Laser Ablation) konnte die Qualität der bearbeiteten Oberfläche
wesentlich verbessert werden, wie anhand von elektronenmikroskopischen Aufnahmen demonstriert wird. Dadurch rückt die Verwendbarkeit
von refraktiven Transplantaten (Lentikeln), insbesonders zur Korrektur von Weitsichtigkeit, in greifbare Nähe.
To produce highly accurate transplants of human donor corneal tissue, Excimer Laser Ablation using 193 nm wavelength (as employed
by the ELCS-System) is a well suited method. By using a recently developed ablation algorithm (Optimized Scanning Laser Ablation,
OSLA), the surface smoothness has improved considerably as shown by scanning EM. Excimer laser treated surfaces can now be
produced with deviations in smoothness of less than 10 μm.
e & i Elektrotechnik und Informationstechnik 04/2012; 117(4):265-268.
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ABSTRACT: Using the molecular-dynamics technique, cluster emission for 5 keV Ar bombardment of a Cu (111) surface has been investigated using a many-body (tight binding) potential for the Cu-Cu interaction. The calculations allow us to analyse the basic processes underlying cluster emission. It is found that two distinct processes can be distinguished which lead to cluster emission under energetic ion bombardment. The first process causes the emission of small clusters, which are emitted by a collective motion during the development of the collision cascade within the first picosecond after impact. Thus, emission times of such clusters agree with the emission times of atoms in sputtering. Such a process can be envisioned if, for example, a few layers below the surface, an energetic recoil causes the development of a subcascade. Energy transferred by this event to the surface is strongly directional and can lead to the simultaneous emission of a group of neighbouring surface atoms, which in some cases will remain bounded and form a cluster after emission. Typically, clusters emitted by this mechanism consist of atoms, which are neighbouring in the target and are almost exclusively surface atoms, similar to all sputtered atoms. Emission of large clusters (cluster sizes of 10 or more atoms), as observed experimentally, is a puzzling phenomenon. From our calculations we conclude that the emission of such large clusters does not occur during the collisional phase of sputtering, but happens much later (5-10 ps after ion impact). Emission can occur for spike events, where all the energy of the impinging ion is deposited locally in a small volume near to the surface, and the sputtering yield is 3-5 times the average yield. Such events are rare, but we have found a few cases in our calculations where stable clusters consisting of more than 20 atoms were emitted. Melting of the spike volume occurs, and the high temperatures and pressures produced can cause emission of large fragments during the thermal phase. The composition of such large clusters is quite different from that of small clusters. They consist of atoms from different layers and the constituents are also generally not next-neighbour atoms. This change in origin of the cluster atoms reflects the mixing and diffusion processes occurring in the melted zone before emission. The calculations indicate that hydrodynamical phenomena might play a role in the emission of large fragments. Additional calculations, where the energy was distributed 'thermally' in a three-dimensional volume under the surface for 500 fs, give very similar results, even in such cases where the kinetic phase of the collision-cascade development was absent.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 02/2004; 362(1814):177-94. · 2.77 Impact Factor
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ABSTRACT: Area laser lathing and trephination of donor corneas is used to produce different types of grafts for human transplantation. 193 nm (ArF excimer) laser radiation is used, since this is known to give a non-thermal laser-tissue interaction with a minimal zone of tissue damage. To guarantee the highest degree of flexibility concerning the overall shape of the grafts as well as their thickness profiles, we use a small (compared with the area to be ablated) scanning laser spot. For area lathing of the tissue we have developed a new ablation algorithm (optimized scanning laser ablation, OSLA) that can be applied to lathe and perforate any tissue--with concave (as in this application), convex or plane surface geometry--where surface precision and smoothness are key issues. Using OSLA with the Excimer Laser Corneal Shaping System (a tool for in vitro fabrication of all kinds of corneal transplants like donor buttons for keratoplasty, lamellar grafts for epikeratoplasty and refractive lenticules) enabled us to produce all types of corneal grafts with very high precision. This is considered to be a major improvement towards the production of refractive lenticules.
Physics in Medicine and Biology 06/1999; 44(5):1169-80. · 2.83 Impact Factor
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ABSTRACT: Das schon vor Jahren in einer Zusammenarbeit mit dem Institut für Allgemeine Physik der Technischen Universität Wien und der
Universitätsklinik für Augenheilkunde entwickelte System (ELCS-System), welches eine universelle Bearbeitung von Spenderhornhäuten
mittels eines Excimer-Lasers erlaubt, ist seit kurzem als Teil des Schwind-Labor-Excimer-Lasers erhältlich. Es können damit
sowohl Hornhaut-Lentikel für die lamelläre und refraktive Keratoplastik als auch Transplantate für die penetrierende Keratoplastik
angefertigt werden. Neuerliche Laboruntersuchungen nach der Bearbeitung zeigten eine sehr gute Oberflächenqualität mit lediglich
geringen konzentrischen Stufenbildungen, welche den Abtragekreisen entsprechen. Die Schädigungszone im Gewebe unter der Abtragezone
war kleiner als 0,3 μm. Dickemessungen wiesen eine Schwankungsbreite zum Sollwert von ± 53 μm auf (Ultraschall-Pachymeter
mit Hornhaut-Kontakt). Die Schnittfläche der penetrierenden Transplantate, bei deren Präparation der Durchmesser, die Form,
sowie der Schnittwinkel frei gewählt werden können, zeigte eine regelmäßige Oberfläche und eine minimale Protrusion der Descemet’schen
Membran. Eine Endothelzellschädigung erfolgte lediglich in einem Bereich von ca. 50 μm von der Kante an gemessen. Das ELCS-System
wird eine wesentliche Verbesserung der Präparation gegenüber den bisher verwendeten Methoden (Kryodrehbank, Mikrokeratom,
Rotorkeratom) zur Herstellung von lameliierenden Hornhauttransplantaten ermöglichen, da einerseits eine mechanische Schädigung
der Spenderhornhaut entfällt und andererseits eine höhere Präzision gegeben ist.
The commercially available Schwind-excimer-laser has been equipped with an additional device, the “Excimer Laser Corneal Shaping
system”, that allows a variable UV-laser preparation of donor corneas. The technique has been developed some years ago in
a collaboration between the Institute for General Physics, Vienna University of Technology and the Second Department of Ophthalmology.
With its use both, lenticules for planolamellar keratoplasty and refractive epikeratoplasty, as well as donor buttons for
penetrating keratoplasty can be designed and cut according to the surgeons specifications. Recent laboratory results showed
an excellent quality of the centrally ablated surface as well as surface of the penetrating cut with only narrow concentric
“notches” corresponding to the lathing steps starting close to the central area. Higher stromal magnification on TEM revealed
a damage zone of less than 0.3 μm in close approximation to the treated surface. The resulting thickness revealed a difference
of less than ± 53 μm from the intended value (measured by using contact ultrasound pachymetry). In penetrating keratoplasty,
where the shape, the diameter and the cutting angle can be choosen independently, ultrastructural studies showed the perpendicular
stromal surface to be smooth with a minimal protrusion of Descemet’s membrane. Endothelial injury was only observed in a narrow
zone of about 50 μm adjacent to the cutting edge. The ELCS-system will offer a considerable improvement for the preparation
of corneal grafts when compared to presently available techniques like the cryolathe, the mikrokeratome or rotorkeratome,
because it can avoid mechanical damage to the donor cornea and potentially allows for a much higher precision.
Spektrum der Augenheilkunde 04/1997; 11(2):53-58. · 0.26 Impact Factor
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ABSTRACT: A femtosecond pulse laser in the visible spectral region shows promise as a potentially new powerful corneal sculpting tool. It combines the clinical and technical advantages of visible wavelengths with the high ablation quality observed with nanosecond-pulse excimer lasers at 193 nm. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs and 7 ns, and centre wavelengths at 615 nm and 600 nm, respectively, both focused to an area of the order of 10–5 cm2, have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30–100 m range from the excision edge at all fluences above the ablation threshold of F
th60 J cm–2 (I
th9 GW cm–2). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterised by damage zones of less than 0.5 m were observed at all fluences above ablation threshold of F
th1 J cm–2 or I
th3 TW cm–2 (31012 W cm–2). The transparent cornea material can be forced to absorb ultrashort pulses of extremely high intensity. The fs laser generates its own absorption by a multiphoton absorption process.
Applied Physics A 01/1994; 58(5):513-518. · 1.63 Impact Factor
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ABSTRACT: Es wird ein neues System vorgestellt, welches es ermöglicht, mittels Laserablation aus Spenderhornhäuten Lentikeln für die
aphake und myope Epikeratophakie sowie für die lamellierende Keratoplastik herzustellen. Unter Verwendung eines computergesteuerten
Positioniersystems wird die Spenderhornhaut in einem fokussierten Excimer-Laserstrahl (ArF, 193 nm) bewegt. Der hier verwendete
Photoablationsprozeß ermöglicht eine effektive Abtragung mit Schädigungszonen kleiner 0,3 um im Gewebe, das die Abtragezone
umgibt.
Die Computersoftware ermöglicht eine benutzerfreundliche Auswahl verschiedenster Parameter, so wie den Durchmesser der optischen
Zone, die Form der Randzone, die Brechkraft, die zentrale und minimale Dicke sowie die Umrißform des Transplantates.
Es werden Daten über die Stabilität der Hornhautdicke während des Bearbeitungsprozesses sowie eine Methode zur Kontrolle der
Qualität und Reproduzierbarkeit des Ablationsprozesses unter Verwendung von PMMA-Scheiben präsentiert. Histologische und elektronenmikroskopische
Ergebnisse von Lentikeln, die mit dem „Excimer-Laser-Corneal Shaping System“ hergestellt wurden, werden gezeigt.
A new system is presented for variable laser-ablation of donor corneas into lenticules used for aphakic and myopic epikeratophakia,
as well as for lamellar keratoplasty. With the help of a computer-controlled positioning system using high-precision micropositioning
elements the donor cornea is moved in a holding device in front of a focused excimer laser beam (ArF, λ = 193 nm). This photo-ablation
lathing process is known to cause a damage zone of less than 0.3 urn which assures an optimal viability of the stromal cells
in close approximation to the treated surface.
The computer software allows the fast and convenient selection of a variety of parameters, such as the diameter of the optical
zone, the shape of the wing zone, the refractive power, the central and the minimal thickness, and the overall contour of
the lenticule. The selection of parameters for lamellar transplantation encompasses not only the thickness of the graft but
also the overall configuration using a scanning device for selection of the shape and size.
Laboratory data on the stability of the corneal thickness during the lasing process, as well as a method for quality control
and reproducibility of the ablation using PMMA discs are presented. Furthermore, histological and electron microscopical results
of lenticules prepared from human donor corneas with this “Excimer-Laser Corneal Shaping System” are shown.
Spektrum der Augenheilkunde 04/1992; 6(4):176-179. · 0.26 Impact Factor
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ABSTRACT: Photoablation studies of biological material (human cornea) with UV and visible laser light show that effective, apparently non-conventional thermal photoablation can be achieved by introducing energy absorbing dopants in the tissue. Previously unknown high ablation rates of 80 Gmm/pulse have been observed. The results allow one to clearly postulate different ablation mechanisms for increasing laser fluence. The results are compared with the photoablation rates observed with 193 nm UV laser light on undoped human cornea. Explosive desorption has been found the dominant process involved.
Applied Physics B 01/1989; 49(5):463-467. · 2.19 Impact Factor
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ABSTRACT: The temperature dependence of the sputtering yield and velocity distribution of sputtered neutral Ca and Cr atoms has been studied by Doppler shift laser fluorescence spectroscopy (DSLFS) for 15‐keV‐Ar<sup>+</sup> ion bombardment.These investigations have been performed in the search for thermal spike effects and radiation‐enhanced sublimation in sputtering at elevated temperatures. Ca targets have been studied up to about 270 °C and Cr targets up to 750 °C. The shapes of the velocity distributions do not change in the respective temperature ranges for both target materials. Furthermore, the absolute yield of sputtered neutral ground state atoms does not increase with temperature.This indicates that no radiation‐enhanced thermal contribution is of relevant importance. Moreover, we can explain the velocity distributions in both cases by the linear cascade theory of sputtering. The thermal contribution due to thermal evaporation increases drastically above 230 °C for Ca and 550 °C for Cr. In both cases the corresponding velocity distribution can be fitted by a Boltzmann distribution.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 10/1985; · 1.34 Impact Factor
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ABSTRACT: The recently much debated problem of excited state sputtering has been investigated for Cr and Ca targets using Doppler‐shift laser‐fluorescence spectroscopy (DSLFS). Using this sensitive and level selective method, sputtered densities for Ca and Cr atoms in the electronic ground and excited metastable states have been investigated as a function of oxygen background pressure. Furthermore, their velocity distributions have been measured for increasing oxygen coverage. In general the population of the ground state during sputtering of Cr and Ca decreases drastically by a factor greater than 50 for Cr or 20 for Ca, respectively, when the clean target is exposed to an oxygen partial pressure between 5×10<sup>-</sup><sup>8</sup> and 1×10<sup>-</sup><sup>6</sup> Torr. At the same time the maximum of the velocity distribution shifts from about 2 km/s to values between 3 and 5 km/s for Cr as well as for Ca. Thus, the velocity distribution of the Ca ground state becomes similar to the one obtained for metastable states of Ca. In addition, the light of short‐lived excited states produced during the sputtering process has been monitored for different transition lines of Ca and Cr for different oxygen pressures. A quartz microbalance was used to monitor relative sputtering yields as a function of the oxygen partial pressure. For high oxygen coverage the total sputtering yield for Ca and Cr decreases by a factor of about 3. The contribution of charged particles remains below 20%. The majority of sputtered products are molecules or clusters.
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 05/1984; · 1.25 Impact Factor
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ABSTRACT: To extend the time scale in molecular dynamics (MD) calculations of sputtering and ion assisted deposition we have coupled our MD calculations to a kinetic Monte Carlo (KMC) calculation. In this way we have studied surface erosion of Cu(1 0 0) under 200–600 eV Cu ion bombardment and growth of Cu on Cu(1 0 0) for deposition at thermal energies up to energies of 100 eV per atom. Target temperatures were varied from 100 to 400 K. The coupling of the MD calculation to a KMC calculation allows us to extend our calculations from a few ps, a time scale typical for MD, to times of up to seconds until the next Cu particle will impinge/be deposited on the crystal surface of about 100 nm2 in size. The latter value of 1 s is quite realistic for a typical experimental sputter erosion or deposition experiment. In such a calculation thermal diffusion processes at the surface and annealing of the surface after energetic ion bombardment can be taken into account.To achieve homo-epitaxial growth of a film the results clearly show the important influence of deposition at non-thermal energies like in sputter or beam assisted deposition as compared to a purely thermal deposition. The optimum energy per atom for homo-epitaxial growth of Cu on Cu (at 100 K) was found to be around 20 eV. In sputter erosion for 600 eV Cu bombardment of Cu(1 0 0) at 400 K we found layer by layer erosion at very low ion fluxes changing into the development of a pronounced surface structure at high ion fluxes.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 193:352-358. · 1.21 Impact Factor
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ABSTRACT: We have used the molecular dynamics technique employing many body potentials to study the interaction of Al clusters of different sizes (60, 210, 504 and 1080 atoms) with a Cu (111) surface at room temperature in the energy range between 0.1–30 eV per cluster atom. This energy region covers the full range from deposition without any sputtering of target or reflection of cluster atoms up to bombarding energies, where reflection, sputtering and crater formation become dominant. One main motivation was to investigate the influence of cluster size on thin film deposition with cluster beams. From the results we conclude, that the mean kinetic energy (“temperature”) of the target atoms near the impact point only depends on the bombarding energy per cluster atom. However, the time this region remains at an elevated temperature increases strongly with cluster size. A deposition “temperature” can be chosen by varying the bombarding cluster energy. Thus, the “temperature” can be tuned to a suitable value, at which the duster melts at the surface, recrystallises and at most weak mixing with the first few surface layers takes place, but the “temperature” is not high enough for evaporation (sputtering), pronounced mixing or penetration. Sufficient time to complete the melting and re-crystallisation can be provided by using sufficiently large clusters.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms.
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ABSTRACT: To present the excimer laser corneal shaping system (ELCS-S), an add-on device to the Keratom, a commercially available 193-nm excimer laser built by Schwind.
The system is designed for the preparation of donor corneas under sterile conditions using the ultraviolet laser to offer greatest possible flexibility. Lenticules for planolamellar grafting and refractive epikeratoplasty, as well as donor buttons for penetrating keratoplasty can be computer-designed by the surgeon or technician and lathed with the system.
Using the excimer laser corneal shaping system (ELCS-S) on human donor corneas, the central surface of the epikeratoplasty lenticule exhibited only narrow, flat concentric notches corresponding to the single lathing steps. Transmission electron microscopy revealed a damage zone of less than 0.3 microm in close approximation to the treated surface. The final thickness revealed a difference of less than +/-53 microm from the intended, initially programmed value. Ultrastructural studies showed the perpendicular stromal surface of the penetrating keratoplasty buttons to be smooth with minimal protrusion of Descemet's membrane. Endothelial injury was observed in a zone averaging between 40 and 100 microm adjacent to the cutting edge only.
The excimer laser corneal shaping system (ELCS-S) allows a computer-controlled, surgeon-designed, sterile preparation of lamellar and penetrating corneal grafts with the use of the excimer laser. This could offer significant advantages in comparison to presently available systems for lamellar dissection and trephination.
Journal of refractive surgery (Thorofare, N.J.: 1995) 16(1):23-31. · 2.54 Impact Factor
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ABSTRACT: A new system is presented that allows a variable trephination of donor corneas for the preparation of corneal buttons used in penetrating keratoplasty. With the help of a computer-controlled positioning system that uses high-precision micropositioning elements (both translation and rotational stages) the donor cornea is removed, epithelial side up, in a fixation device in front of a focused excimer laser beam (ArF, lambda = 193 nm). User friendly computer software allows the surgeon to select a variety of parameters (diameter, shape, angle of trephination) of the corneal graft. Histological and electron microscopical data of human corneas trephined with this "Excimer Laser Corneal Shaping System" are presented.
Refractive & corneal surgery 7(5):385-9.
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ABSTRACT: Precise lathing of epikeratoplasty lenticules is difficult to achieve with the cryolathe due to unpredictable expansion of the lathing tools and the corneal tissue during the freezing process. In addition, the procedure destroys all viable cells in the transplant thereby possibly contributing to the prolonged period of visual rehabilitation. Non-freezing techniques using the microkeratome or the rotor-trephine, on the other hand, are technically demanding, can cause mechanical damage during cutting or fixation, and, have not given consistently reproducible refractive results. A new system is presented that allows a variable laser ablation of donor corneas into lenticules for aphakic and myopic epikeratoplasty, as well as for lamellar keratoplasty. With the help of a computer-controlled positioning system that uses high-precision micropositioning elements (both translation and rotational stages) the donor cornea is moved, epithelial side down, in a holding device in front of a focused excimer laser beam (ArF, lambda = 193 nm). This photoablation lathing process assures the viability of the stromal cells in the lenticule in close approximation to the treated surface. The user friendly computer software allows the fast and convenient selection of a variety of parameters, such as the diameter of the optical zone, the shape of the wing zone, the refractive power, the central thickness of the lenticule and the overall contour of the transplant. The first laboratory data of lenticules prepared from human corneas with this "Excimer Laser Corneal Shaping System" are presented.
Refractive & corneal surgery 7(5):377-84.
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ABSTRACT: The dynamics of laser ablation from metallic surfaces (Ag, Al, Fe and Ni) induced by the combined effect of two 30 fs sub-threshold laser pulses has been examined. In a pump–probe setup the yield of the emitted secondary ions and neutrals has been determined as a function of the delay between the two laser pulses. The instantaneous generation of highly excited (ballistic) electrons by the laser pulse and the thermal properties of the metal, which have been modified to be valid into the regime of high electron temperatures have been found to be determining factors for the ablation process. Unexpectedly, two distinct maxima for particle emission have been observed as a function of the time separation of the pump and the probe pulse. The energy relaxation is discussed within the frame of the two-temperature model (TTM) and it is shown that the measured behavior (in the time domain) of ablated particles can only be explained by taking into account a general expression for the thermal conductivity, valid for a wide range of electron temperatures and in addition a substantial role of hot, ballistic electrons.
Applied Surface Science.
