Ultrasmall silver nanopores fabricated by femtosecond laser pulses

Ultrasmall nanopores in silver thin films with a diameter of about 2 nm have been fabricated using femtosecond laser ablation in liquid. Ultrafast laser pulse ablation generates highly nonequilibrium excitated states, from which silver thin films emerge and progressively grow with the assistance of capping agent molecules. During this growth process, capping agent molecules are enclaved within the film, leaving individual ultrasmall pores in the thin film. Our first-principles calculations show that the pore size is critically determined by the dimension of the confined molecules. Our approach advances the capability of optical methods in making nanoscale structures with potential applications in areas such as near-field aperture probes, imaging masks, magnetic plasmonic resonances, and biosensing with individual nanopores.     

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.     

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.     

Differences in the evolution of surface-microstructured silicon fabricated by femtosecond laser pulses with different wavelength

We experimentally investigate the differences in the evolution of surface-microstructured silicon fabricated by femtosecond laser pulses with different wavelength as a function of irradiated laser energy. The results show that when laser energy absorbed by the silicon material is the same, laser pulses with a shorter wavelength can form the surface-microstructured silicon with less laser energy, while the corresponding spike height is much lower than that of laser pulses with a longer wavelength. This is because the penetration depth of the laser pulses increases exponentially at the increase of the laser wavelength. Additionally, for two laser pulses with the certain wavelength and the certain absorption efficiency of silicon, the proportional relations between their formed spike height and irradiated laser energy should be determined. In particular, the average spike height is 3 times with 8 times corresponding energy for 800 nm laser pulses than that of 400 nm. These results are a benefit for the fast and optimum-morphology preparation of microstructured silicon.     

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.     

Temperature distribution in dental tissue after interaction with femtosecond laser pulses

Algebraic and numerical solutions are presented of the temperature rise in dental tissue due to interaction with ultrashort optical radiation. Results of the studies with femtosecond laser pulses show agreement between theory and experiment. A temperature rise of typically 5 K is found for a 40 millisecond train of 7 nJ, 70 fs laser pulses at a repetition rate of 80 MHz. The peak irradiance in our experimental studies was limited to 3x10(6) W/cm(2). Applications include photoacoustic imaging and tomography of dental tissue.     

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.     

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.     

Fabrication and characterization of microstructures with optical quality surfaces in fused silica glass using femtosecond laser pulses and chemical etching

We present a study of the sidewall surface quality inside microchannels fabricated in fused silica glass by femtosecond laser pulses and chemical etching. Multiple combinations of laser exposure and etching solution parameters were examined. Results of scanning electron microscopy, atomic force microscopy, and optical reflection analyses of the surfaces are presented. The results obtained demonstrate the feasibility of optical quality surface fabrication, which in turn demonstrates the feasibility of fabricating complex integrated devices containing microfluidic channels and optical waveguides in the glass substrates.     

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.     

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.     

激光制备超疏水表面研究进展

由于超疏水表面在防腐、油水分离、流体减阻和液体转移方面的应用潜力,如何制作性能优异的超疏水表面成为研究热点。材料表面的形貌特征是决定其润湿性能的一个重要因素,因此,通常采用表面结构化来获得超疏水性能。在材料表面构织微纳结构方面,基于脉冲激光的微纳加工技术具有得天独厚的优势,尤其是在制作特定图案的复杂结构方面。本文根据激光器的脉冲宽度分类,通过刻蚀后材料表面形貌和润湿性特征对激光制作超疏水表面的基本理论和典型工艺方法进行介绍和总结,并对超疏水表面的发展前景作出展望。

     

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.     

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.     

Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses

Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply 'heats' the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.     

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.