Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling

We present experimental and modeling studies of UV nanosecond pulsed laser desorption and ablation of (111) bulk silicon. The results involve a new approach to the analysis of plume formation dynamics under high-energy photon irradiation of the semiconductor surface. Non-thermal, photo-induced desorption has been observed at low laser fluence, well below the melting threshold. Under ablation conditions, the non-thermal ions have also a high concentration. The origin of these ions is discussed on the basis of electronic excitation of Si surface states associated with the Coulomb explosion mechanism. We present a model describing dynamics of silicon target excitation, heating and harge-carrier transport

High-speed manufacturing of highly regular femtosecond laser-induced periodic surface structures: Physical origin of regularity

Highly regular laser-induced periodic surface structures (HR-LIPSS) have been fabricated on surfaces of Mo, steel alloy and Ti at a record processing speed on large areas and with a record regularity in the obtained sub-wavelength structures. The physical mechanisms governing LIPSS regularity are identified and linked with the decay length (i.e. the mean free path) of the excited surface electromagnetic waves (SEWs). The dispersion of the LIPSS orientation angle well correlates with the SEWs decay length: the shorter this length, the more regular are the LIPSS. A material dependent criterion for obtaining HR-LIPSS is proposed for a large variety of metallic materials. It has been found that decreasing the spot size close to the SEW decay length is a key for covering several cm2 of material surface by HR-LIPSS in a few seconds. Theoretical predictions suggest that reducing the laser wavelength can provide the possibility of HR-LIPSS production on principally any metal. This new achievement in the unprecedented level of control over the laser-induced periodic structure formation makes this laser-writing technology to be flexible, robust and, hence, highly competitive for advanced industrial applications based on surface nanostructuring

Selective Delamination upon Femtosecond Laser Ablation of Ceramic Surfaces

We report on the experimental observation of selective delamination of semi-transparent materials on the example of yttria-stabilized zirconia ceramics upon femtosecond laser processing of its surface with low numerical aperture lens. The delamination of a ceramic layer of dozens of micrometers takes place as a by-side effect of surface processing and is observed above the surface ablation threshold. The onset of delamination (delamination threshold) depends on the degree of overlap of the irradiation spots from consecutive laser pulses upon beam scanning over material surface. Analysis of the delaminated layer indicates that the material undergoes melting on its both surfaces. The mechanism of delamination is identified as a complex interplay between the optical response of laser-generated free-electron plasma and nonlinear effects upon laser beam propagation in semi-transparent ceramics. The discovered effect enables controllable laser microslicing of brittle ceramic materials

Environmentally Friendly Improvement of Plasmonic Nanostructure Functionality towards Magnetic Resonance Applications

Plasmonic nanostructures have attracted a broad research interest due to their application perspectives in various fields such as biosensing, catalysis, photovoltaics, and biomedicine. Their synthesis by pulsed laser ablation in pure water enables eliminating various side effects originating from chemical contamination. Another advantage of pulsed laser ablation in liquids (PLAL) is the possibility to controllably produce plasmonic nanoparticles (NPs) in combination with other plasmonic or magnetic materials, thus enhancing their functionality. However, the PLAL technique is still challenging in respect of merging metallic and semiconductor specific features in nanosized objects that could significantly broaden application areas of plasmonic nanostructures. In this work, we performed synthesis of hybrid AuSi NPs with novel modalities by ultrashort laser ablation of bulk gold in water containing silicon NPs. The Au/Si atomic ratio in the nanohybrids was finely varied from 0.5 to 3.5 when changing the initial Si NPs concentration in water from 70 µg/mL to 10 µg/mL, respectively, without requiring any complex chemical procedures. It has been found that the laser-fluence-insensitive silicon content depends on the mass of nanohybrids. A high concentration of paramagnetic defects (2.2·× 1018 spin/g) in polycrystalline plasmonic NPs has been achieved. Our findings can open further prospects for plasmonic nanostructures as contrast agents in optical and magnetic resonance imaging techniques, biosensing, and cancer theranostics

Volumetric glass modification with Gaussian and doughnut-shaped pulses: From localized laser energy absorption to absorption delocalization

Volumetric modification of glass materials by ultrashort laser pulses is a powerful technique enabling direct writing of three-dimensional structures for fabrication of optical, photonic, and microfluidic devices. The level of modification is determined by the locally absorbed energy density, which depends on numerous factors. In this work, the effect of the spatial pulse shape on the ultrashort laser excitation of fused silica was investigated experimentally and theoretically for the volumetric modification regimes. We focused on two shapes of laser pulses, Gaussian and doughnut-shaped (DS) ones. It was found that, at relatively low pulse energies, in the range of 1-5 microjoules, the DS pulses are more efficient in volumetric structural changes than Gaussian pulses. It is explained by the intensity clamping effect for the Gaussian pulses, which leads to the delocalization of the laser energy absorption. In the DS case, this effect is overcome due to the geometry of the focused beam propagation, accompanied by the electron plasma formation, which scatters light toward the beam axis. The thermoelastoplastic modeling performed for the DS pulses revealed intriguing dynamics of the shock waves generated because of tubular-like energy absorption. It is anticipated that such a double shock wave structure can induce the formation of high-pressure polymorphs of transparent materials that can be used for investigations of nonequilibrium thermodynamics of warm dense matter. The DS laser pulses of low energies of the order of 100 nanojoules which generate a gentle tubular-like modification can be perspective for a miniature waveguide writing in glass.Comment: 7 pages, 4 figure

Dual-wavelength femtosecond laser-induced low-fluence single-shot damage and ablation of silicon

A study of damage and ablation of silicon induced by two individual femtosecond laser pulses of different wavelengths, 1030 and 515 nm, is performed to address the physical mechanisms of dual-wavelength ablation and reveal possibilities for increasing the ablation efficiency. The produced ablation craters and damaged areas are analyzed as a function of time separation between the pulses and are compared with monochromatic pulses of the same total energy. Particular attention is given to low-fluence irradiation regimes when the energy densities in each pulse are below the ablation threshold and thus no shielding of the subsequent pulse by the ablation products occurs. The sequence order of pulses is demonstrated to be essential in bi-color ablation with higher material removal rates when a shorter-wavelength pulse arrives first at the surface. At long delays of 30-100 ps, the dual-wavelength ablation is found to be particularly strong with the formation of deep smooth craters. This is explained by the expansion of a hot liquid layer produced by the first pulse with a drastic decrease in the surface reflectivity at this timescale. The results provide insight into the processes of dual-wavelength laser ablation offering a better control of the energy deposition into material.Comment: 8 pages, 4 figure

Fundamentals of ultrafast laser processing

When laser light of visible, near-IR or UV spectral range hits condensed matter, it interacts with the valance and/or conduction electrons of the system under action. Depending on laser intensity and irradiation geometry, the interaction can have different far-reaching consequences such as melting, ablation, changing of optical properties, mechanical and chemical transformations. Among existing laser systems, ultrafast lasers have become an extraordinary tool for processing of any kind of materials. With proper choosing the irradiation conditions, laser action allows either inducing highly-localized gentle modifications or obtaining strongly damaged material sites with desired or deleterious structures such as voids, periodic nanocracks, periodic surface structures or craters of various shapes and dimensions. This chapter presents a review on tremendous efforts of researchers in order to achieve clearer insights into laser-matter interactions in ultrashort irradiation regimes. The review does not pretend to completeness and aims to outline main ideas, achievements, and most intriguing findings still waiting for explanations and theoretical treatments