Tomi Smausz

Absorbing film assisted laser induced forward transfer of fungi (Trichoderma conidia)

We present an investigation on absorbing film assisted laser induced forward transfer (AFA-LIFT) of fungus (Trichoderma) conidia. A KrF excimer laser beam [λ=248nm,FWHM=30ns (FWHM, full width at half maximum)] was directed through a quartz plate and focused onto its silver coated surface where conidia of the Trichoderma strain were uniformly spread. The laser fluence was varied in the range of 0–2600mJ∕cm2 and each laser pulse transferred a pixel of target material. The average irradiated area was 8×10−2mm2. After the transfer procedure, the yeast extract medium covered glass slide and the transferred conidia patterns were incubated for 20 h and then observed using an optical microscope. The transferred conidia pixels were germinated and the areas of the culture medium surfaces covered by the pixels were evaluated as a function of laser fluence. As the laser fluence was increased from 0 to 355mJ∕cm2 the transferred and germinated pixel area increased from 0 to 0.25mm2. Further increase in fluence resulted...

Evaluation of laser-speckle contrast image analysis techniques in the cortical microcirculation of piglets

A new laser speckle-contrast analysis (LASCA) technique based on multi-exposure imaging was employed to simultaneously study pial arteriolar responses with cerebrocortical perfusion changes to various vasodilator (5-10% CO(2) ventilation, bradykinin (1-10 μM), N-methyl-D-aspartate (100 μM)) vasoconstrictor (10-100 μM noradrenaline, 1M KCl), or neutral (2.1% H(2) ventilation) stimuli as well as to asphyxia in the newborn piglet. Anesthetized, ventilated animals (n=20) were fitted with closed cranial windows. Multiple exposure laser-speckle image series (1-100 ms) were obtained using a near infrared diode laser (λ=808 nm). The autocorrelation decay time (τ) of speckle fluctuations was determined over pial arterioles and parenchymal areas to express 1/τ being proportional to blood flow velocity by two different LASCA techniques: our novel multi-exposure or a single exposure (2 and 20 ms) approach. 1/τ values yielded by different LASCA techniques were not significantly different at most points. LASCA easily detected both increases and decreases in cortical blood flow (CoBF). Cortical 1/τ changes to hypercapnia closely matched quantitative CoBF data determined previously, and were also in accordance with increases of pial arteriolar blood flow, calculated from arteriolar flow velocity and cross sectional area changes. In summary, LASCA emerges as an appealing method to simultaneously study microvascular reactivity and cortical perfusion changes in the piglet.

Pulsed laser deposition of compact high adhesion polytetrafluoroethylene thin films

Polytetrafluoroethylene (PTFE) thin films were prepared from pressed powder pellets via pulsed laser deposition by using ArF (193 nm) excimer laser. The applied laser fluences were in the 1.6-10 J cm-2 range, the substrate temperature was varied between 27°C and 250°C and post-annealing of the films was carried out in air at temperatures between 320°C and 500°C. Films deposited at 250°C substrate temperature were found to be stoichiometric while those prepared at lower temperatures were fluorine deficient. Morphological analyses proved that the film thickness did not significantly depend on the substrate temperature and the post annealing at 500°C resulted in a thickness reduction of approximately 50%. It was demonstrated that the films prepared at 8.2 J cm-2 fluence and annealed at 500°C followed by cooling at 1°C min-1 rate were compact, pinhole-free layers. The adherence of films to the substrates was determined by tensile strength measurements. Tensile strength values up to 2.4 MPa were obtained. These properties are of great significance when PTFE films are fabricated for the purpose of protecting coatings.

Real correlation time measurement in laser speckle contrast analysis using wide exposure time range images.

In laser speckle contrast analysis (LASCA) used for imaging of blood flow, besides the moving blood cells, the speckle pattern is also influenced by the imaging system and scattering properties of the laser-illuminated static surface. A latex microsphere (650 nm size) emulsion was covered with scattering semitransparent materials (Teflon foils, tracing paper). Speckle images were recorded with different exposure times (0.2 ms-500 ms), and correlation times were determined by parameterizing the theoretical contrast-exposure time function. The correlation times obtained for covered and uncovered microsphere emulsions were in good agreement. The possibility of obtaining comparable, setup-independent results in blood perfusion monitoring can contribute to better applicability of LASCA.

Femtosecond laser printing of living cells using absorbing film-assisted laser-induced forward transfer

The applicability of a femtosecond KrF laser in absorbing film-assisted, laser-induced forward transfer of living cells was studied. The absorbing materials were 50-nm-thick metal films and biomaterials (gelatine, Matrigel, each 50 μm thick, and polyhydroxybutyrate, 2 μm). The used cell types were human neuroblastoma, chronic myeloid leukemia, and osteogenic sarcoma cell lines, and primary astroglial rat cells. Pulses of a 500-fs KrF excimer laser focused onto the absorbing layer in a 250-μm diameter spot with 225  mJ/cm2 fluence were used to transfer the cells to the acceptor plate placed at 0.6 mm distance, which was a glass slide either pure or covered with biomaterials. While the low-absorptivity biomaterial absorbing layers proved to be ineffective in transfer of cells, when applied on the surface of acceptor plate, the wet gelatine and Matrigel layers successfully ameliorated the impact of the cells, which otherwise did not survive the arrival onto a hard surface. The best short- and long-term survival rate was between 65% and 70% for neuroblastoma and astroglial cells. The long-term survival of the transferred osteosarcoma cells was low, while the myeloid leukemia cells did not tolerate the procedure under the applied experimental conditions.

Crystallization of Ge2Sb2Te5 thin films by nano- and femtosecond single laser pulse irradiation

The amorphous to crystalline phase transformation of Ge2Sb2Te5 (GST) films by UV nanosecond (ns) and femtosecond (fs) single laser pulse irradiation at the same wavelength is compared. Detailed structural information about the phase transformation is collected by x-ray diffraction and high resolution transmission electron microscopy (TEM). The threshold fluences to induce crystallization are determined for both pulse lengths. A large difference between ns and fs pulse irradiation was found regarding the grain size distribution and morphology of the crystallized films. For fs single pulse irradiated GST thin films, columnar grains with a diameter of 20 to 60 nm were obtained as evidenced by cross-sectional TEM analysis. The local atomic arrangement was investigated by high-resolution Cs-corrected scanning TEM. Neither tetrahedral nor off-octahedral positions of Ge-atoms could be observed in the largely defect-free grains. A high optical reflectivity contrast (~25%) between amorphous and completely crystallized GST films was achieved by fs laser irradiation induced at fluences between 13 and 16 mJ/cm2 and by ns laser irradiation induced at fluences between 67 and 130 mJ/cm2. Finally, the fluence dependent increase of the reflectivity is discussed in terms of each photon involved into the crystallization process for ns and fs pulses, respectively.

Immobilization of urease by laser techniques: Synthesis and application to urea biosensors

Urease thin films have been immobilized using matrix-assisted pulsed laser evaporation for biosensor applications in clinical diagnostics. The targets exposed to laser radiation were made of frozen composites that had been manufactured by dissolving urease in distilled water. An UV KrF* (lambda = 248 nm, tauFWHM congruent with 30 ns, nu = 10 Hz) excimer source was used for the multipulse laser irradiation of the targets that were cooled down to solidification using Peltier elements. The incident laser fluence was set at 0.4 J/cm2. The surface morphology and chemical bonding states of the laser immobilized urease thin films were investigated by atomic force microscopy and Fourier transform infrared spectroscopy. The enzymatic activity and kinetics of the immobilized urease were assayed by the Worthington method, which monitors urea hydrolysis by coupling ammonia production to a glutamate dehydrogenase reaction. Decreased absorbance was found at 340 nm and correlated with the enzymatic activity of urease.

Excimer laser ablation of molten metals as followed by ultrafast photography

Abstract Molten Sn and Bi are ablated in vacuum by an ArF excimer laser. Pictures of the surface and the ablated material are taken by ultrafast photography, with temporal resolution of 1 ns using delayed dye laser pulses. The series of snapshots covering the 0 ns–200 μs time domain contain information on the ablated plume, the development of waves on the target surface, and the initial phase of droplet formation. The velocity of the front of the ablated plume is approximately 6 km/s for both Sn and Bi at 5.5 J/cm2. While on the molten Sn surface only wave generation is observed with practically no droplet emission, the Bi surface emits a remarkable amount of material in the form of droplets originating from liquid jets. The speed of these droplets is two orders of magnitude smaller than that of the plume front. The relaxation of the whole perturbed melt pool lasts second(s) after ablation. By decreasing the fluence below 2.5 J/cm2 the Bi droplet formation can also be suppressed.

Laser-induced backside dry etching : wavelength dependence

The laser-induced backside dry etching (LIBDE) method has been developed by analogy with the well-known LIBWE technique for micromachining of transparent materials. In our experiments tin thin films were applied as absorbing layers and the dependence of the etch depth on the applied laser wavelength was investigated. Multipulse irradiation experiments at different wavelengths have also been carried out. A fused silica plate coated by a tin layer was used as a target. The metal film–transparent material interface was irradiated through the fused silica plate by the most frequently used nanosecond excimer laser beams: ArF (193 nm), KrF (248 nm) and XeCl (308 nm). It was found that the etch depth increased linearly with decreasing wavelength in investigated range. Multipulse investigations proved that the energy of the absorbed photons has a strong influence on the mechanism of LIBDE. The etch depth–pulse number curves showed saturation at 1500 mJ cm−2 applied laser fluence at around 2, 3 and 30 pulses for 308 nm, 248 nm and 193 nm wavelengths, respectively. This indicates that the effectiveness of multipulse LIBDE increases when the wavelength of the applied laser source is decreased in the UV range. XPS and spectroscopic ellipsometric analyses showed that formation and embedding of tin-oxides into the upper surface layer can be responsible for the shift of the saturation in the case of ArF as compared with the other studied wavelengths.

Solid state and liquid ablation of polyethylene-glycol 1000 : temperature dependence

Abstract Time-resolved investigations of solid and liquid phase ablation on the same sample (polyethylene-glycol (PEG) 1000) is presented in this paper. Because this polymer has a relatively low melting point (35°C) we could study the ablation mechanism in both solid and liquid states of matter by varying the sample temperature in the 20–80°C range. The target was irradiated by an ArF excimer laser ( λ =193 nm, FWHM=20 ns) at 1.95 J/cm 2 fluence. Pictures of the surface and the material ejection processes were taken by fast photography, with a temporal resolution of 1 ns using electronically delayed dye laser exposing pulses. It was demonstrated that plasma development and expansion (in the 0–50 ns time range), propagation parameters of shock waves and contact fronts did not depend on sample temperature in the investigated temperature range and state of matter. In contrast with this the significant material ejection (between 1–100 μs) showed a strong temperature dependence. Below the melting point (solid samples) material ejection took place in the form of dense material clouds, and in the form of squish for liquid (molten) samples. The velocity of the ejected jets depended significantly on the temperature of the molten polymer sample. This might be due to the different viscosity of PEG 1000 in the investigated temperature range.