The creation of microporous surface modification of chitosan thin films irradiated by ultrashort laser pulses are studied. For this purpose, chitosan substrates were treated by using an amplified Ti:sapphire laser system at 800 nm central wavelength with 30 fs and 150 fs pulse duration and repetition rate 1 kHz and 50 Hz, respectively. Formation of surface modifications for both cases (30 fs and 150 fs) after femtosecond laser irradiation were observed. The threshold values for single-pulse (N = 1) and multi-pulse (N > 1) modification were evaluated by studying the linear relationship between the squared crater diameter D2 and the logarithm of the laser fluence (F) for N = 1, 2, 5, 10, 20, 30 and 50 number of laser pulses. The coefficient of incubation ξ, a major parameter in the process of surface modification and ablation of materials also was calculated for multi - pulse fluence threshold estimation by power - law relationship Fth (N) = Fth (1) Nξ-1, where N is the number of applied laser pulses. The surface properties of chitosan based thin films before and after femtosecond laser irradiation were investigated. The aim of this work is to determine the optimal morphological characteristics of the created structures for tailoring of protein adsorption and cell behavior.
The collagen – elastin biopolymer thin films treated by CPA Ti:Sapphire laser (Femtopower – Compact Pro) at 800nm central wavelength with 30fs and 1kHz repetition rate are investigated. A process of surface modifications and microporous scaffold creation after ultrashort laser irradiation has been observed. The single–shot (N=1) and multi–shot (N<1) ablation threshold values were estimated by studying the linear relationship between the square of the crater diameter D2 and the logarithm of the laser fluence F for determination of the threshold fluences for N=1, 2, 5, 10, 15 and 30 number of laser pulses. The incubation analysis by calculation of the incubation coefficient ξ for multi – shot fluence threshold for selected materials by power – law relationship form Fth(N)=Fth(1)Nξ-1 was also obtained. In this paper, we have also shown another consideration of the multi – shot ablation threshold calculation by logarithmic dependence of the ablation rate d on the laser fluence. The morphological surface changes of the modified regions were characterized by scanning electron microscopy to estimate the generated variations after the laser treatment.
We have developed a low cost apparatus for open- and closed-aperture Z-scan measurements of multi-photon absorption
(MPA) cross-sections of solid and liquid samples. The experimental setup uses simple diodes for light detection. The
signals are recorded with a low-cost two-channel PC-scope. We have developed a LabView based software, which
analyzes single laser pulses and allows averaging over several shots. First measurements on a CR-39 polymer
demonstrated the functioning of the method. Furthermore, we have shown that for 25fsec ultra short pulses three-photon
absorption (ThPa) must be considered in addition to two-photon absorption (TPA). The appropriate nonlinear absorption
(TPA-, ThPA-) coefficients and the nonlinear refractive index can be obtained via a best fit of the data to theoretical
curves, which have been derived and adapted for ThPA from formulas for TPA accessible in the literature.
Sputtering of Cu/Ti layers was performed by Ar+ions. Analysis of the atomic and molecular composition of the sputtered
plume was performed by means of Ultra-short Laser Neutral Mass Spectrometry (US-LSNMS) and Secondary Ion Mass
Spectrometry (SIMS). Several ionic masses were observed and systematically studied with respect to the exposure time,
laser fluence and target composition. The obtained data for complex layers indicate generally a good agreement between
SNMS and SIMS. SIMS is more sensitive for many elements however the mass interferences can limit the analytical
applicability. US-LSNMS mass spectra of Cu(Ti) sample have been acquired for different exposure times. In order to
determine the effect of different laser ionization energies over the mass distribution of the elements, mass spectra of
Cu(Ti) multilayers at several laser ionization energies were acquired. The elements interdiffusion was analyzed also by
US-LSNMS, demonstrating the sensitivity, the limits and the future potential of the SNMS method for material
characterization.
The majority of previous femtosecond laser ionization studies have been carried out on atomic, diatomic or small polyatomic molecules. The understanding of interaction of intense laser pulses with polyatomic molecules is at a preliminary stage. It was difficult to examine the behavior of more complex molecules in the presence of intensive laser field due to their involatility and thermal lability. In the ionization experiments performed in the current research the polyatomic molecules irradiated with ≈ 30 fs laser pulses, at wavelength 800nm do not exhibit an extensive fragmentation, and produce multiply charged ions in the intensity range of 1014W/cm2. Studies of ionization mechanisms in the case of femtosecond ionization have revealed that the dissociation paths can be avoided. To establish an intact ionization of molecules two regimes of photodissociation can be distinguished: absorption-dissociationionization (ADI) and absorption-ionization-dissociation (AID) regime. It was discovered that the crucial parameter for the post-ionization experiments is the energy necessary for ionization of the neutral species. The ionization efficiency depends strongly from the precise timing of the laser pulses and from the geometrical overlap of the focus of the postionizing laser beam with the emitted particle cloud.
The investigation of mechanism of ultra-short laser ablation process of biological tissue represents one of the challenging subjects over the last couple of years. The femtosecond laser pulses are very well suited for high precision surgery, due to its low thermal and mechanical stress. The current research was emphasized on the examination of the interaction mechanism of high intensity ultra-short femtosecond and nanosecond laser pulses with hard biological tissue material (tooth, bones). It was established that femtosecond ablation works well for ablation of complex biological molecule systems. A more detailed view was taken by investigating the ablation dynamics at several wavelengths. The precise examination of the mass spectra of laser ablation with 193 nm and 800 nm introduces the way of altering the chemical composition of the ablated tissue. In general, it was found that ablation with ultra-short (femtosecond) pulses at 800 nm radiation yields the highest number of characteristic ions. To better understand the interaction mechanism we have performed post-ionization experiments. It was of main importance to succeed to establish efficient ionization of the organic molecules with minimal fragmentation. This work demonstrates sensitivity of the time-of-flight (TOF) technique and the great potential of the laser ablation/ultra-short laser secondary neutrals mass spectroscopy (LA/US-LSNMA) method in increasing the information content of biomolecular mass spectra.
We have investigated the energy and mass distribution of particles ejected from metals and semiconductors after irradiation with intense ultra-short laser radiation. Neutral atoms and clusters have been investigated. In many cases, clearly fast particles in the eV range and slow thermal particles can be identified. The influence of the laser pulse width on the energy distribution as well as the ion to neutral ratio have been investigated.
Ultra-short laser-radiation has initiated a new era for medical laser applications as well as for laser applications in high- precision material processing. Entirely new prospects, in particular, their use for surgical applications and nano- technology, can be envisioned with ultra-short laser pulses, which are now available from pico- and femto-second laser systems. The development of broadband solid-state gain media opened new possibilities for ultra-short pulse generation. In particular, the development of all-solid-state ultra-short pulse devices promise to make such devices rigged and reduce their cost. Ultra-short laser light offers many advantages, as for instance low thermal damage and the possibility of efficient interaction of light with long wavelengths. Extremely high peak laser intensities, which can be achieved even with commercially available systems (typical values are 1015w/CM2 imply physical mechanisms, which reach beyond the classical model of (multi-) photon absorption as the principal energy transfer process. The consequences, as observed in, however still preliminary applications, result in many obvious advantages as for instance: efficient ablation; 2) minimization of collateral damage; 3) ablation thresholds and rates which are relatively insensitive to tissue type; 4) high control over ablation depth, achievable because only a small amount of tissue is ablated per pulse.
We report first IR free-electron laser experiments to compare and elucidate the effects of surface-localized vibrational excitation versus bulk vibrational excitation on the ablation of polycrystalline diamond. The measured ablation yield values as a function of laser intensity indicate the existence of two separate thresholds. The lower intensity thresholds is identified as the ablation threshold, and the higher intensity threshold is associated with the formation of a plasma plume. The wavelength dependences of both thresholds indicate that eh C-H absorption occurring at surfaces and grain boundaries does not play a significant role in the ablation process. However, both thresholds are lower when the laser is resonant with the two-phonon bulk absorption band. These findings are consistent with the model that a rapid laser- induced phase transition to graphite is responsible for the low-intensity ablation of diamond at and above the first threshold.
Previous investigations of transmural capillary ingrowth into the inner surface of biosynthetic vascular prostheses through perforations created by an excimer laser have shown that spontaneous endothelialization of the protheses can be achieved. Disadvantages of the excimer laser are a large and non-circular beam profile and an inconvenient handling of the UV-radiation for medical applications. Therefore, a new commercial laser system, consisting of a mirror-dispersion- controlled Ti:S oscillator and a femtosecond multipass amplifier, which avoids these disadvantages was sued to perforate grafts made of a new biomedical. The laser-tissue interaction is based ona different mechanism due to the short pulsewidth high peak intensity and near IR wavelength. The very short pulsewidth reduces thermal damage of the tissue. The basic mechanism of the laser tissue interaction is not yet fully understood and still subject of investigations. Several biografts with 1 mm wall thickness have been used. The hole diameter could be varied between 50 and 200 micrometers and the distance between the holes was 4 mm. At laser intensities of approximately 20-30 TW/cm2 and a repetition rate of 1 kHz it took less than a second to drill a single hole. Subsequently to the perforation procedure, one perforated and one untreated biograft were implanted into the carotid arteries of several test sheep. It can be concluded that very intense near IR laser radiation is an ideal means for structuring biological tissue.
In this contribution we will discuss recent results obtained from Molecular Dynamics (MD) calculations of early stages of thin film deposition for metal atoms and clusters on metal substrates. As a first example we will discuss the deposition of Cu atoms on a Pb single crystal surface. In this case our MD calculations could verify an experimentally observed new growth mode in deposition, which was named subsurface island growth. As a second example we will discuss the possibilities and processes occurring during thin film deposition, if instead of atoms clusters (consisting of up to a few hundred atoms) are used. As an example we present the deposition of Al cluster of different size (ranging from 60 atoms up to 1000 atoms per cluster) on a Cu surface.
Excimer Laser Corneal Shaping using an 193 nm Excimer Laser (ArF) provides a possibility for the fabrication of corneal transplants of various forms for various clinical applications such as (epi-)keratoplasty. Another area of application envisioned is the production of 'living contact lenses' for epikeratophakia. A device for lathing and perforating corneal donor tissue with a scanning laser beam is presented. A new ablation algorithm (Optimized Scanning Laser Ablation) was recently developed and increased the quality of lenticules and donor buttons considerably.
This study was undertaken to investigate the feasibility of transmural capillary ingrowth into the inner surface of biosynthetic vascular prostheses (OmniflowTM) through perforations created by an excimer-laser, thus inducing an endothelial cell coverage. The biosynthetic vascular prostheses (10 cm length, 6 mm (phi) ) were perforated with an excimer laser ((phi) of the holes 50 - 100 micrometer, distance 4 mm) and implanted into the carotid arteries of 8 sheep. The laser tissue interaction process of 193 nm radiation ensures minimal thermal damage to the prostheses. They were compared to untreated OmniflowTM prostheses implanted at the contralateral side. Three months after implantation the prostheses were explanted and evaluated by gross morphology, histological examination and scanning electron microscopy. Scanning electron microscopy showed endothelial cells in the midgraft portion of all perforated prostheses, whereas collagen fibers, fibrin meshwork and activated platelets formed the inner layer in 6 out of 8 untreated OmniflowTM prostheses. It can be concluded, that spontaneous endothelialization of biosynthetic vascular prostheses can be achieved by transmural capillary ingrowth through perforations in the wall of the prostheses in an experimental sheep model.
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