Time integrated detection of femtosecond laser pulses scattered by small droplets

Scattering of femtosecond laser pulses by small droplets has been measured and compared with predictions, yielding some interesting new applications for time integrated detection of the scattered field. The scattering intensity of integrated detection becomes monotonic with droplet size over large regions of scattering angle and morphology dependent resonances are surpressed, opening the way for particle sizing using the scattered intensity. Furthermore, the ripple structure no longer appears in the rainbow region of scattering, simplifying rainbow refractometry significantly. These scattering proporties of femtosecond laser pulses have been demonstrated in the laboratory using a novel Paul trap for levitating single droplets.     

Time-resolved explosion dynamics of H2O droplets induced by femtosecond laser pulses

Series of time-resolved still images of the explosion dynamics of micrometer-sized water droplets after femtosecond laser-pulse irradiation were obtained for different laser-pulse intensities. Amplified pulses centered around a wavelength of 805 nm with 1-mJ energy and 60-fs duration were focused onto the droplet to initiate the dynamics. Several effects, such as forward and backward plumes, jets, water films, and shock waves, were investigated. Additionally, the influence of different pulse durations produced by chirping the laser pulses was observed.     

Structural changes accompanying color center formation in lithium fluoride exposed to femtosecond laser pulses

This paper examines structural changes produced in crystalline LiF by high-power femtosecond laser pulses and provides some insight into the mechanisms of structural damage to the material. In the region of the onset of multiple femtosecond laser beam filamentation and in the central part of the filaments, we observed partial melting of the material, cracking, and the formation of structural nanodefects that showed up as step pyramids and nanogratings in atomic force microscopy images.     

Three-dimensional nanostructuring with femtosecond laser pulses

The development of simple laser-based technologies for the fabrication of complicated three-dimensional (3-D) microstructures with a structure size down to 100 nm is reported. These technologies are based on nonlinear multiphoton laser-matter interaction processes allowing to overcome the diffraction limit and to fabricate 3-D structures inside transparent materials. Examples on nanostructuring of metals, dielectrics, and polymers are presented.     

Application of femtosecond laser pulses in biomedical cell technologies

The results are presented of the works in the field of development of equipment, investigation techniques, and technologies for biology and medicine performed in the Joint Institute for High Temperatures of the Russian Academy of Scienses (JIHT RAS). On the base of the new generation infrared femtosecond lasers, the experimental models are developed and manufactured of laser tweezers, scalpel, and the “tweezers-scalpel” combined system. The results are presented of the experimental studies on the noncontact mammalian cell fusion (blastomeres of mouse embryos on day 1.5 of development) by means of the femtosecond laser pulses.     

Noncrystalline micromachining of amorphous alloys using femtosecond laser pulses

Femtosecond laser micromachining of a Zr-based amorphous alloy in air, including measuring the ablation threshold, micro-drilling and trenching, was investigated. The threshold of ablating this amorphous alloy was determined by experiment. Laser-induced ablation and associated damage were examined by means of optical microscope, scanning electron microscopy, transmission electron microscopy and electron diffraction diagram. The results show that conventional processing method induced defects in the vicinity of machined area, such as crystallization, molten trace and spatter, were absent in femtosecond laser ablation area with selected parameters. This indicates that femtosecond laser ablation is a promising method for micromachining amorphous alloys without crystallization.     

Micromachining of hardened Portland cement pastes using femtosecond laser pulses

Femtosecond laser pulses (30 fs in length) of various energies were utilised for production of single and multiple overlapping ablation sites on flat polished surfaces of hardened Portland cement pastes. In order to assess the sizes of the ablation sites and possible subsurface laser-induced damage, the ablation sites were investigated using environmental scanning electron microscopy (ESEM) – both from normal top–down view and in cross-sections. Furthermore, approximately 10-μm wide notches were produced using femtosecond pulses on cylindrical microspecimens (150 μm in diameter) of hardened Portland cement pastes. In addition to electron microscopy observations, several microspecimens were investigated using synchrotron-based X-ray computed microtomography (SRμCT). The results suggest that production of “damage-free” samples for micromechanical testing of hardened Portland cements pastes is possible.

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Résumé Des impulsions laser (de durée 30 fs) et d’énergie variable ont été utilisées pour produire une ablation ponctuelle ou linéique à la surface d’un ciment durci de type Portland et préalablement polie. Pour déterminer la taille des impacts et d’éventuels dommages causés par le laser sous la surface, les points d’impact ont été visualisés à l’aide de la technique de l’ESEM (microscope électronique à balayage environnemental). De plus, des piliers de 10 micromètres de diamètre environ ont été réalisés par ablation femtoseconde sur des échantillons cylindriques de 150 microns de diamètre. Pour compléter les observations faites au microscope électronique, certains échantillons ont été observés grace à la technique SRμCT (synchrotron-based X-ray computed microtomography). Les résultats montrent que la production d’échantillons non-endommagés de ciment de type Portland pour des tests micro-mécaniques ultérieurs est possible.

     

Fine-pitched microgratings encoded by interference of UV femtosecond laser pulses

Fine-pitched microgratings are encoded on fused silica surfaces by a two-beam laser interference technique employing UV femtosecond pulses from the third harmonics of a Ti:sapphire laser. A pump and prove method utilizing a laser-induced optical Kerr effect or transient optical absorption change has been developed to achieve the time coincidence of the two pulses. Use of the UV pulses makes it possible to narrow the grating pitches to an opening as small as 290 nm, and the groove width of the gratings is of nanoscale size. The present technique provides a novel opportunity for the fabrication of periodic nanoscale structures in various materials.     

Thermal training of functional surfaces fabricated with femtosecond laser pulses

The effect of thermal training of a functional surface fabricated by means of femtosecond laser surface processing on a crystalline silicon (c-Si) surface is found for the first time. Hydrodynamic and thermal properties of the c-Si surface are revealed to be considerably modified by femtosecond laser processing and follow-up thermal training. The carried out experimental studies of wetting, evaporation, and boiling processes on the thermal trained laser processed surface open up ample opportunities in creation of tailored functional surfaces for micro/optoelectronic devices and power engineering applications.     

Formation of the condensed phase of metals exposed to submicrosecond laser pulses

Using the method of laser probing, the laws governing the formation of the liquid-droplet phase of a number of metals (Ni, Zn, Pb, Ag, and Cu) on exposure of metal targets to intense submicrosecond pulses have been determined. It has been established that condensation from the vapor of the erosive laser jet is the main mechanism of formation of the liquid-droplet phase of a metal under given conditions.     

Surface damage of extreme-ultraviolet gratings exposed to high-energy 20-fs laser pulses

The damage fluences of gratings for diffraction of ultraviolet radiation, which are used in high-order harmonic generation experiments, have been measured with respect to the fundamental laser beam radiation. We have tested gold and platinum coatings of 40- and 50-nm thickness, respectively, deposited onto fused-silica substrates, after irradiation of high-energy, spatially filtered, 20-fs laser pulses at 780 nm. The damage appears at a fluence of ~0.3 J cm(-2) for gold and at a fluence of ~0.4 J cm(-2) for platinum. Scanning electron microscopy of the irradiated regions revealed different damaging mechanisms for the two coatings.     

Two-photon fluorescence excitation spectroscopy by pulse shaping ultrabroad-bandwidth femtosecond laser pulses

A fast and automated approach to measuring two-photon fluorescence excitation (TPE) spectra of fluorophores with high resolution (~2 nm) by pulse shaping ultrabroad-bandwidth femtosecond laser pulses is demonstrated. Selective excitation in the range of 675-990 nm was achieved by imposing a series of specially designed phase and amplitude masks on the excitation pulses using a pulse shaper. The method eliminates the need for laser tuning and is, thus, suitable for non-laser-expert use. The TPE spectrum of Fluorescein was compared with independent measurements and the spectra of the pH-sensitive dye 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) in acidic and basic environments were measured for the first time using this approach.     

Filamentation of femtosecond laser pulses with amplitude modulation of a diffraction-free beam

Filamentation with the amplitude modulation of Mathieu beam in air is investigated numerically. Non-diffracting characteristic of Mathieu beam is partly maintained in the intense femtosecond laser area even though it is only an amplitude modulation. The amplitude modulation of Mathieu beam delays filament onset and the high intensity will be regenerated in the on-axis area when intensity clamping disappears. During the evolution process, the bunch of filaments merges into a single filament eventually only by the property of non-diffraction. Using an amplitude modulation of an invariable beam as an initial condition of intense femtosecond laser pulse offers a new way to control filaments’ onset, distribution and prolongation, which would be useful in their potential applications.     

Dual-pulse laser-induced breakdown spectroscopy with combinations of femtosecond and nanosecond laser pulses

Nanosecond and femtosecond laser pulses were combined in an orthogonal preablation spark dual-pulse laser-induced breakdown spectroscopy (LIBS) configuration. Even without full optimization of interpulse alignment, ablation focus, large signal, signal-to-noise ratio, and signal-to-background ratio enhancements were observed for both copper and aluminum targets. Despite the preliminary nature of this study, these results have significant implications in the attempt to explain the sources of dual-pulse LIBS enhancements.     

Spherical aberration in spatial and temporal transforming lenses of femtosecond laser pulses

We study the influence of third-order spherical aberration on the group velocity dispersion and on the propagation time delay of a plane pulse that is focused by a thin lens. Applications in refractive-diffractive propagation time delay compensation systems and in Gaussian temporal-shaped pulses are analyzed.     

Optical properties of waveguides fabricated in fused silica by femtosecond laser pulses

With tightly focused femtosecond laser pulses, waveguides are fabricated in fused silica. The guiding and attenuation properties of these waveguides at wavelengths of 514 nm and 1.5 microm are studied. We demonstrate that by changing only the writing speed, waveguides with a controllable mode number can be produced.     

Raman spectroscopy and luminescent properties of ZnO nanostructures fabricated by femtosecond laser pulses

In this paper, we report the laser-induced periodic structure with different spatial characteristics on the surface of polished ZnO single-crystalline by high repetition rate femtosecond laser pulses. This study demonstrates that, using different laser parameters and irradiation conditions, ZnO nanoripples and nanorods were successfully prepared. We have investigated the surface by means of scanning electron microscope (SEM), Raman scattering and photoluminescence (PL). We propose that second-order harmonic has a strong influence on the formation of nanostructures.     

Laser desorption and imaging of proteins from ice via UV femtosecond laser pulses

We have employed 200-fs, 400-nm laser pulses to desorb intact protein molecular ions directly from a frozen aqueous matrix. The resulting spectra obtained using a variety of proteins varying in molecular weight from 1060 (bradykinin) to 5778 Da (insulin) are compatible with those obtained with traditional matrix-assisted laser desorption/ionization experiments. High-quality spectra could be generated using a fluence of 4.0-9.0 J/cm2 to desorb proteins from an aqueous solution frozen onto metal substrates with a sensitivity in the femtomole range. Although the mechanism behind this effect is still not clear, we speculate that it involves explosive boiling of the ice layer due to rapid heating of the substrate. Imaging experiments conducted on the ice layer suggest that the yield of protein is approximately independent of the film thickness and is very reproducible from shot to shot. The results are particularly significant since they open the possibility of examining a range of biomaterials directly from the in vivo aqueous environment.