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•Laser ablation of Si was performed both in air and water environments at different fleunces and variable pulsewidths.•Only shallow spallation occurred in air for 10-ps laser pulses, ...comparing to spallation and phase explosion for shorter pulses.•Shallow laser ablation occurred in water at supercritical peak pulse powers due to laser beam self-focusing and filamentation.•Intense, phase explosion-like ablation takes place during wet ablation by 10-ps laser pulses at sub-critical peak pulse powers.
Comparative single-shot pulse study of ablation topographies and depths on Si surfaces excited by 1030-nm sub- (0.3 ps) and pico-picosecond (10 ps) laser pulses at different laser fluences in air and water environments was performed. Fast energy transport via 3D-diffusion of dense electron-hole plasma was revealed at picosecond timescale to extend transversely the ablation craters over the focal spot in air for laser pulse widths shorter, than electron–phonon thermalization time about 3 ps. In the given fluence range fluence-independent shallow ablation was observed in air for 10-ps pulses. Then, rather shallow laser ablation occurred in water at supercritical peak pulse powers, apparently, due to laser beam deterioration by self-focusing and filamentation. Finally, very intense, fluence-dependent material removal took place during wet ablation by 10-ps laser pulses at the sub-critical peak pulse powers. Electron-hole plasma and related energy transport on Si surface, 1030-nm laser pulse filamentation in water, pulsewidth and ambient medium effects were revealed during the surface ablation studies.
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•Ablation thresholds increase in case of fs ablation of Si in liquids.•The surrounding media has an impact on the ablated surface morphology.•Laser ablation in liquids results in the ...lesser extent of surface oxidation.•The lifetime of the ablative plume in liquid is three times shorter, than in air.
The experimental study of the single-shot 1030-nm femtosecond laser ablation of bulk silicon and silver in air and liquid media (deionized water, isopropyl alcohol) is reported. The main laser ablation mechanisms were studied via analysis of the resulting microcrater surface morphology and elemental composition in single- and multi-shot (number of pulses N = 10, 100) regimes by scanning electron microscopy with a built-in module for energy-dispersive X-ray spectroscopy. It was concluded, that multi-shot ablation in liquids leads to the lesser extent of the processed surface oxidation, than in dry conditions. The monitoring of the ablative plumes dynamics was implemented with the use of time-resolved emission spectroscopy in the non-filamentation regime, corresponding to the formation of nanoparticles without fragmentation of the ablation products. It was demonstrated, that the lifetime of the plume emission in liquid is significantly shorter, than in air: 5 and 15 ns, correspondingly, for silicon, and 10 and 30 ns for silver.
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•Ablation thresholds were measured for metals and Si at different laser pulsewidths.•Colloidal extinction coefficient was studied at variable laser pulsewidths.•NP morphology and size ...were studied at variable laser pulsewidths.•Ablated surfaces were studied by SEM at different laser pulsewidths.•MD-TTM modeling was performed for Au ablation in air by 0.3- and 4-ps laser pulses.
The effect of laser pulsewidth, tuned in the range τ = 0.3–10 ps, on infrared (IR) laser ablation (LA) thresholds of bulk gold, silver and silicon in air and in liquid environments (deionized water, isopropyl alcohol (IPA)) was studied in this work. The influence of laser pulsewidth on the morphology and yield of the ejected nanoparticles (NPs), as well as its impact on the efficiency of NP generation, was studied by UV–vis spectral measurement of the extinction coefficient spectra of NP colloids, fabricated by multi-pass ablation of bulk targets in both solvents at different pulse duration values, by analytical disc centrifuge and by high-resolution scanning electron microscopy. The ablation thresholds are raising with the increase of laser pulsewidth, exhibiting sublinear dependences. The extinction coefficient dependence on τ exhibited a non-monotonous character in case of water medium for all materials, and was gradually descending in case of LA in IPA. Additional MD-TTM (molecular dynamics- two-temperature model) modeling was performed for single-shot laser exposition of bulk Au target in air with τ = 0.3 ps and 4 ps, which allowed distinguishing the main mechanisms of the material removal, the formation of NPs, and the observed crater topography. The performed simulations supported the trend on the ablation threshold increase with the increase of laser pulse duration.
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.
Ultrafast intense photoexcitation of a silicon surface is complementarily studied experimentally and theoretically, with its prompt optical dielectric function obtained by means of time-resolved ...optical reflection microscopy and the underlying electron-hole plasma dynamics modeled numerically, using a quantum kinetic approach. The corresponding transient surface plasmon-polariton (SPP) dispersion curves of the photo-excited material were simulated as a function of the electron-hole plasma density, using the derived optical dielectric function model, and directly mapped at several laser photon energies, measuring spatial periods of the corresponding SPP-mediated surface relief nanogratings. The unusual spectral dynamics of the surface plasmon resonance, initially increasing with the increase in the electron-hole plasma density but damped at high interband absorption losses induced by the high-density electron-hole plasma through instantaneous bandgap renormalization, was envisioned through the multi-color mapping.
Energy-dispersive X-ray microspectroscopy is used for the first time to quantitatively study the spatial displacement of the material of a 100-nm silver film irradiated by a single femtosecond laser ...pulse focused on a small spot in the diffraction limit. The silver mass distribution over radial cross sections is determined and matter balance is analyzed for the resulting radially symmetric submicron structures of a microcone with a nanospike with various heights and a through hole. Hydrodynamic processes and phase transitions inducing the melting of the film, motion of the melt, and its recrystallization within a focal spot are studied.
A 50-nm-thick alloyed gold-palladium film on a silica substrate was dewetted by 1060-nm/ns laser pulses and characterized by scanning electron microscopy. Optical transmission spectroscopy of the ...millimeter-sized spot of the dewetted film demonstrates pronounced Fano resonances, typical for large regular arrays of oligomers of plasmonic nanoparticles. The oligomer configuration and almost pure gold composition of the nanoparticles, underlying the spectral Fano resonances, were revealed via numerical simulations with the input electron microscopy visualization and hydrogen absorption tests. The promising application of Fano resonance for simple optical characterization of ordering or packaging parameters in large-scale arrays of deeply sub-wavelength plasmonic and/or dielectric nano-oligomers was proposed.
Donut-shaped visible (515 nm) femtosecond laser pulses with linear, radial and azimuthal polarizations after tight focusing (numerical aperture NA = 0.65) have been used for spectrally adjusted ...excitation of plasmonenhanced rhodamine 6G dye luminescence on the surface of a nanocrystalline 45-nm thick gold film with its plasmon resonance near the laser line. Significant luminescence output increments—1.7-fold for radial polarization compared to the azimuthal one, and almost 3-fold—compared to the linear one, have been observed. This effect is related to significant transformation of the transverse electrical field component into longitudinal one for radial polarization (up to 33% in intensity, according to our calculations), and corresponding increase in excitation efficiency of plasmons in the smaller nanocrystallites throughout the film.
Single-shot thresholds of surface ablation of aluminum and silicon via spallative ablation by infrared (IR) and visible ultrashort laser pulses of variable width τ
las
(0.2–12 ps) have been measured ...by optical microscopy. For increasing laser pulse width τ
las
< 3 ps, a drastic (threefold) drop of the ablation threshold of aluminum has been observed for visible pulses compared to an almost negligible threshold variation for IR pulses. In contrast, the ablation threshold in silicon increases threefold with increasing τ
las
for IR pulses, while the corresponding thresholds for visible pulses remained almost constant. In aluminum, such a width-dependent decrease in ablation thresholds has been related to strongly diminished temperature gradients for pulse widths exceeding the characteristic electron-phonon thermalization time. In silicon, the observed increase in ablation thresholds has been ascribed to two-photon IR excitation, while in the visible range linear absorption of the material results in almost constant thresholds.
Nano- and microscale holes, as well as related sub-ablative nanospikes and sub-micron bumps, were produced in a 30-nm thick silver film on a silica substrate by single femtosecond laser pulses with ...variable pulse energies, focused by different strong focusing optics. Characteristic laser energy deposition dimensions exceed the expected focal spots by nearly 2 microns, indicating the considerable lateral thermal transport in the film, while the effective hole formation thresholds decrease versus increasing numerical aperture of focusing optics. Morphologies of the sub-ablative solidified surface nanostructures and numerical estimates of deposited volume energy density undermine blowing-off the molten film due to subsurface boiling and near-critical phase explosion at lower and higher sub-threshold fluences, respectively.
