In this work the formation of laser-induced periodic surface structures (LIPSS) on a titanium surface upon irradiation by linearly polarized femtosecond (fs) laser pulses with a repetition rate of 1 kHz in air environment was studied experimentally. In particular, the dependence of high-spatial-frequency-LIPSS (HSFL) characteristics on various laser parameters: fluence, pulse number, wavelength (800 nm and 400 nm), pulse duration (10 fs - 550 fs), and polarization was studied in detail. In comparison with low-spatial-frequency-LIPSS (LSFL), the HSFL emerge at a much lower fluence with orientation perpendicular to the ridges of the LSFL. It was observed that these two types of LIPSS demonstrate different fluence, shot number and wavelength dependencies, which suggest their origin is different. Therefore, the HSFL formation mechanism cannot be described by the widely accepted interference model developed for describing LSFL formation.
In this paper, the influence of the pulse duration on the ablation threshold and the incubation coefficient was investigated for three different types of materials: metal (copper), semiconductor (silicon) and biopolymer (gelatin). Ablation threshold values and the incubation coefficients have been measured for multiple Ti:sapphire laser pulses (3 to 1000 pulses) and for four different pulse durations (10, 30, 250 and 550 fs). The ablation threshold fluence was determined by extrapolation of curves from squared crater diameter versus fluence plots. For copper and silicon, the experiments were conducted in vacuum and for gelatin in air. For all materials, the ablation threshold fluence increases with the pulse duration. For copper, the threshold increases as s 0.05 , for silicon as s 0.12 and for gelatin as s 0.22 . By extrapolating the curves of the threshold fluence versus number of pulses, the single-shot threshold fluence was determined for each sample. For 30 fs pulses, the singleshot threshold fluences were found to be 0.79, 0.35, and 0.99 J/cm 2 and the incubation coefficients were found to be 0.75, 0.83 and 0.68 for copper, silicon and gelatin, respectively.
We have demonstrated for the first time that an array of nanoantennas (central nanotips inside sub-micron pits) on an aluminum surface, fabricated using a specific double-pulse femtosecond laser irradiation scheme, results in a 28-fold enhancement of the non-linear (three-photon) electron photoemission yield, driven by a third intense IR femtosecond laser pulse. The supporting numerical electrodynamic modeling indicates that the electron emission is increased not owing to a larger effective aluminum surface, but due to instant local electromagnetic field enhancement near the nanoantenna, contributed by both the tip's "lightning rod" effect and the focusing effect of the pit as a microreflector and annular edge as a plasmonic lens.Strong-field plasmonics, involving excitation of plasmons (collective free-electrons oscillations) in different nanoobjects by intense femtosecond (fs) laser pulses, is of high interest for basic and applied research. Surface-plasmonenhanced multi-photon photoelectric emission One of the most popular plasmonic elements is a metallic nanotip, providing strong optical field enhancement via the "lightning rod effect". The nanometer-long decay length of the evanescent field corresponds to its strong gradients, which can be used for nanoscale acceleration of photo-emitted electrons in different regimes (multiphoton [7], above-threshold [8] or optical-field [9] regimes). Interestingly, that the strong gradient of localized evanescent field can suppress the quiver motion of the electrons in the oscillating laser electric field [10]. Such a strong-field steering of electrons in the vicinity of nanostructures with large local field enhancement and steep field gradients leads to emission of highly-directed, confined coherent electron wavepackets [7,[9][10][11]. Generally, such a pulsed electron nanoemitter, triggered by femtosecond laser irradiation, could serve as an efficient source for time-resolved nanoscale imaging. For instance, ultrashort electron pulses were employed for time-domain visualization of metal melting [12] and ionization dynamics of H 2 [13].Fabrication of plasmonic nanotips usually faces problems of long fabrication cycle, chemical treatment and production costs. To provide more efficient fabrication ways, tight focusing of single nanosecond [14] and femtosecond [15] laser pulses into diffraction-limited spots was tested to produce one nanotip per shot. However, femtosecond laser irradiation makes it possible and realistic to easily fabricate huge arrays of nanostructures (down to the sub-100-nm scale) via intense surface plasmons polaritons (SPPs) excitation, where only weak laser beam focusing on the surface is required [16]. Such a method for surface nanograting formation was i.e. successfully used for surface-plasmonenhanced photoelectron emission [1]. In the same manner, also an array of nanotips can be easily fabricated by means of fs-laser beam weak focusing on a metallic surface [17].In this Letter, we report a simple, double-pulse fs-laser fabrication scheme to produ...
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