Irradiation of crystalline silicon with femtosecond laser pulses produces a variety of quasi-periodic surface structures, among which sub-wavelength ripples creation is largely studied. Here we report an experimental investigation and a theoretical interpretation focusing on the seldom considered issue of quasi-periodic, micron spaced grooves formation. We characterize the morphological evolution of the grooves generation and experimentally single out the variation of the threshold fluence for their formation with the number of pulses N, while typical ripples simultaneously produced in the irradiated area are always considered for comparison. Our experimental findings evidence a power law dependence of the threshold fluence on the number of pulses both for ripples and grooves formation, typical of an incubation behavior. The incubation factor and single pulse threshold are (0.76 ± 0.04) and (0.20 ± 0.04) J/cm² for ripples and (0.84 ± 0.03) and (0.54 ± 0.08) J/cm² for grooves, respectively. Surface-scattered wave theory, which allows modeling irradiation with a single pulse on a rough surface, is exploited to interpret the observed structural modification of the surface textures. A simple, empirical scaling approach is proposed associating the surface structures generated in multiple-pulse experiments with the predictions of the surface-scattered wave theory, at laser fluencies around the grooves formation threshold. This, in turn, allows proposing a physical mechanism interpreting the grooves generation as well as the coexistence and relative prominence of grooves and ripples in the irradiated area.

The dynamics of ions in ultrafast laser ablation of metals is studied over fluences ranging from the ablation threshold up to approximate to 75 J/cm(2) by means of three well-established diagnostic techniques. Langmuir probe, Faraday cup, and spectrally resolved intensified charge coupled device imaging simultaneously monitored the ions produced during ultrafast laser ablation of a pure copper target with 800 nm, approximate to 50 fs, Ti: Sapphire laser pulses. The fluence dependence of ion yield is analyzed, resulting in the observance of three different regimes. The specific ion yield shows a maximum at about 4-5 J/cm(2), followed by a gradual reduction and a transition to a high-fluence regime above approximate to 50 J/cm(2). The fluence dependence of the copper ions angular distribution is also analyzed, observing a gradual increase in forward-peaking of Cu ions for fluences up to approximate to 10 J/cm(2). A broader ion component is observed at larger angles for fluences larger than approximate to 10 J/cm(2). Finally, an experimental characterization of the ionic angular distribution for several metallic targets (Mg, Al, Cr, Fe, Cu, and W) is carried out at a relatively high fluence of approximate to 66 J/cm(2). Interestingly, the ion emission from the volatile metals shows a narrow, forward-peaked distribution, and a high peak ion yield compared to the refractory metals. Moreover, the width of ionic angular distributions presents a striking correlation with the peak ion yield

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Lockdown measures applied in the aftermath of the COVID-19 pandemic spread to Italy in the period March 13th-May 4th strongly limited the social and industrial activities with consequent effects on the air pollution. Here we report a study on the influence of the lockdown measures on the air quality in the city of Naples (Italy). The comparison of the levels of various gaseous pollutants (C6H6, CO, NO2 and SO2) and particulate matter (PM10, PM2.5, PM1) at ground level as well as of atmospheric aerosol properties registered by remote sensing techniques during the lockdown period with the values observed in the earlier months and during the same period of the previous year is used to gain interesting information on the environmental impact of the human activities. Our findings show a rather significant reduction of the pollution due to NO2 (49-62 %) in urban as well as in green suburban area, while CO and SO2 showed a more important reduction in urban or industrial districts of the city (50-58 % and 70 %, respectively). Particulate matter at ground level is also affected but to a more limited extent (29-49 %). Nevertheless, characterization of atmospheric aerosol columnar properties suggests an interesting variation of its composition. The observed features have been associated to the strong meteorological interference from Saharan Dust in the Mediterranean area also affecting the city of Naples.

Extensive research work has been carried out on the generation and application of laser-induced periodic surface structures (LIPSS). LIPSS with a sub-wavelength period generated by femtosecond laser irradiation, generally indicated as ripples, have been extensively investigated. Instead, the other ordered surface structures characterized by a supra-wavelength period, indicated as grooves, have been much less studied. Grooves typically form at larger irradiance levels or for higher number of laser pulses. Here, we report a comprehensive overview of recent investigations on the supra-wavelength grooves formed on crystalline silicon irradiated by femtosecond laser pulses. The authors’ recent experimental work is mainly addressed giving an explicit picture of the grooves generation process, namely illustrating the influence of the various experimental parameters, including, e.g., polarization, wavelength, fluence and repetition rate of the laser beam as well as number of laser pulses hitting the surface of the material. The effect of irradiation of a static or moving target and of the environmental conditions (e.g., vacuum or air ambient) will also be discussed. Finally, possible mechanisms envisaged to explain grooves formation and still open issues are briefly discussed.