The laser ablation of a silicon target in distilled water is carried out by single femto- and picosecond pulses upon focusing with an objective lens with a numerical aperture NA = 0.25 as a function ...of the focus position relative to the target surface. A plasma channel in water is visualized for the given numerical aperture used. The ablation surface relief is characterized using scanning and probe electron microscopy. The maximum crater depths and volumes are detected and analyzed as functions of the position of the linear and extended nonlinear foci.
•Surface textures of austenite steel were produced by femtosecond laser.•One-dimensional nanograting and microrelief of various roughness have been received.•Ferrite (α-Fe) in the near-surface layers ...was formed in spallation regime.•Phases α-Fe, Fe3O4, γ-Fe2O3 in the near-surface layers were formed in the phase explosion regimes.•Heat-affected zone was observed in the high-fluence mode for phase explosion regime.
Topography, structure, and phase composition of surface layers of AISI 321 stainless steel textured by 1030-nm 320-fs-laser pulses were studied by scanning electron microscopy and X-ray diffraction analysis. Variation in single-pulse fluence and the number of pulses was found to change the laser-produced surface texture from one-dimensional quasi-periodic nanograting to microrelief of various roughness. It was shown that the nanograting formed in the spallation regime contained austenite, just as a non-irradiated steel surface, and ferrite. The redeposited oxide layers on the grating surface produced in the phase explosion regime with a thickness increasing versus single-pulse fluence were found to consist of austenite, ferrite, and Fe3O4. In case of high-fluence mode, the two-layer microporous oxide coating, consisting of Fe3O4 and Fe2O3 in addition to conventional phases, was detected. SEM studies showed that the submicrocrystalline structure of nano- and microsized ridges was preserved, except for the high-fluence mode.
Interaction of complex-shaped light fields with specially designed plasmonic nanostructures gives rise to various intriguing optical phenomena like nanofocusing of surface waves, enhanced nonlinear ...optical response and appearance of specific low-loss modes, which can not be excited with ordinary Gaussian-shaped beams. Related complex-shaped nanostructures are commonly fabricated using rather expensive and time-consuming electron- and ion-beam lithography techniques limiting real-life applicability of such an approach. In this respect, plasmonic nanostructures designed to benefit from their excitation with complex-shaped light fields, as well as high-performing techniques allowing inexpensive and flexible fabrication of such structures, are of great demand for various applications. Here, we demonstrate a simple direct maskless laser-based approach for fabrication of back-reflector-coupled plasmonic nanorings arrays. The approach is based on delicate ablation of an upper metal film of a metal-insulator-metal (MIM) sandwich with donut-shaped laser pulses followed by argon ion-beam polishing. After being excited with a radially polarized beam, the MIM configuration of the nanorings permitted to realize efficient nanofocusing of constructively interfering plasmonic waves excited in the gap area between the nanoring and back-reflector mirror. For optimized MIM geometry excited by radially polarized CVB, substantial enhancement of the electromagnetic near-fields at the center of the ring within a single focal spot with the size of 0.37λ
can be achieved, which is confirmed by Finite Difference Time Domain calculations, as well as by detection of 100-fold enhanced photoluminescent signal from adsorbed organic dye molecules. Simple large-scale and cost-efficient fabrication procedure offering also a freedom in the choice of materials to design MIM structures, along with remarkable optical and plasmonic characteristics of the produced structures make them promising for realization of various nanophotonic and biosensing platforms that utilize cylindrical vector beam as a pump source.
We report on high-quality infrared (IR)-resonant plasmonic nanoantenna arrays fabricated on a thin gold film by tightly focused femtosecond (fs) laser pulses coming at submegahertz repetition rates ...at a printing rate of 10 million elements per second. To achieve this, the laser pulses were spatially multiplexed by fused silica diffractive optical elements into 51 identical submicrometer-sized laser spots arranged into a linear array at periodicity down to 1 μm. The demonstrated high-throughput nanopatterning modality indicates fs laser maskless microablation as an emerging robust, flexible, and competitive lithographic tool for advanced fabrication of IR-range plasmonic sensors for environmental sensing, chemosensing, and biosensing.
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•Nanosecond-laser ablation of stainless steel and copper was studied in air and water.•Contact broad-band ultrasonics was used to acquire ablation parameters.•Ablation thresholds for ...these materials were almost the same in air and water.•Ablation pressures vary linearly vs intensity, being fivefold higher in water.•The sub-critical plasma pressure origin was derived.
Single-shot nanosecond laser ablation of stainless steel and copper surfaces in ambient air and liquid water environment was studied by contact broadband ultrasonics in the important laser intensity range of 6–450 GW/cm2, covering both regimes of surface phase explosion and formation of sub-critical ablative plasma via optical breakdown in the corresponding dense ablative plume. Similar dependences of ablative plasma pressure were obtained for the dry and wet ablation conditions, differing in their ultrasonic pulse amplitudes by the factor ∼5 and in their pulsewidths by the factor of 1.3 (copper) and 1.6 (steel) for the wet ablation because of the dynamic water-confinement effect. Excepting the confinement effect, these results indicate the similar conditions for pressure generation via sub-critical plasma formation both for the air and water environments, enlightening promising applications of laser ablation for liquid-assisted laser peening and generation of nanoparticles in liquids.
The article discusses the implementation of ultrafast broadband excitation of A-band luminescence of natural diamond by femtosecond-laser pulses with low-energy near-IR photons. Such regime provides ...A-band luminescence yield with the fourth-power nonlinearity at low laser intensities and second-power nonlinearity at higher laser intensities. These nonlinear dependences of A-band intensity on laser intensity can be related to quasi-resonant four-photon excitation at lower laser intensities and impact excitation of corresponding optical centers at higher intensities, or to similar nonlinear processes in electron–hole plasma relaxing to trapping donor-acceptor centers.
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Topography-dependent tuning of water wettability was achieved on a stainless steel surface textured by nanosecond-laser pulses at different laser fluences, with the minimal contribution of the ...surface chemical modification. Such differently-wet neighboring surface spots were demonstrated to drive an autonomous directional water flow. A series of elementary microfluidic devices based on the spatial wetting gradients were designed and tested as building blocks of "green", energy-saving autonomous microfluidic circuits.
•Multi-filamentation of high power femtosecond laser pulses in water.•Fabrication of sub-diffraction surface ripples on wet aluminum by high optical harmonics.
Relief ripples with sub-diffraction ...periods (≈λlas/3, λlas/4) were produced on a aluminum surface immersed in water and irradiated in a multi-filamentation regime by focused 744nm femtosecond laser pulses with highly supercritical, multi-GW peak powers. For the VUV (8.5eV) surface plasmon resonance on the wet aluminum surface, such small-scale surface nanogratings can be produced by high – second and third – optical harmonics, coming to the surface from the optical filaments in the water layer. Then, the sub-diffraction surface ripples may appear through interference of their transverse electric fields with the longitudinal electric fields of their counterparts, scattered on the surface roughness and appeared as the corresponding high-energy, high-wavenumber surface polaritons.
•Crowning of molten rims produced by laser-pulse ablation of noble-metal films was studied.•Crowning is governed by nanoscale Rayleigh-Plateau instability developing in molten rim.•Crowning period is ...proportional to the square root of the film thickness and nanocrown diameter.
Nanoscale hydrodynamic instability of ring-like molten rims around ablative microholes produced in nanometer-thick silver and gold films by tightly focused nanosecond (ns) laser pulses was experimentally explored in terms of laser pulse energy and film thickness. These parametric dependencies of basic instability characteristics - order and period of the resulting nanocrowns - were analyzed, revealing its apparently Rayleigh-Plateau character, as compared to much less consistent possible van der Waals and impact origins. Along with fundamental importance, these findings will put forward ns pulsed laser ablation as an alternative facile inexpensive table-top approach to study such hydrodynamic instabilities developing at ns temporal and nanometer spatial scales as well as to produce unique plasmon-active hierarchical surface morphologies applicable for chemo- and biosensing.
Tightly focused, highly spatially multiplexed femtosecond laser pulses, coming at sub-MHz repetition rates, were used to mask-less pattern thin plasmonic films film at ultrafast rates, approaching 25 ...million of microelements per second. For this purpose, the initial pulses were multiplexed by fused silica diffractive optical elements into linear arrays of 31, 51 and 101 circular light spot and then scanned over the films by a galvanometric scanner through a long-focus objective or high-NA aspherical lens. These optical scheme and 5-μJ pulse energy supported the only 31- and 51-beam micro-patterning, with the corresponding aperture, field-of-view and sub-100 nJ energetic limitations for the laser processing of the film. The resulting large (~105–106 holes per array) arrays of micro-holes of variable diameters and periods in thin films of different thickness and diverse plasmonic materials - Ag, Cu, Al and Au-Pd alloy (80%/20%) - were for the first time systematically characterized in the broad IR-range (1.5–25 μm) in terms of plasmonic effects in extraordinary optical transmission, indicating for the increasing wavenumber a smooth transition from the common Bethe-Bouwkamp transmission to its plasmon-enhanced analogue, ending up with common geometrical (wave-guide-like) transmission. Finally, promising label- and luminescence-free laboratory-scale, robust and high-sensitivity sampling of chemicals and biosamples via plasmonic and chemical contributions, uneven and structurally-sensitive regarding different functional groups of the model analyte molecules and band structure of the plasmonic metal, was demonstrated for the large IR-sensing arrays of micro-holes in plasmonic films, with the obvious perspectives for down-scaling of sensing elements for vis-IR surface-enhanced spectroscopies.
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•Fs-laser beam multiplexing by diffractive optical elements enabled high-throughput printing of plasmonic microholes.•Large-scale microhole arrays exhibit a transition from plasmon-enhanced IR transmission to wave-guide-like one.•Large-scale microhole arrays demonstrate surface-enhanced IR transmission and chemosensing.
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