Surface ripples induced by N = 50 laser pulses of 150 fs delivering an energy of 30 μJ/pulse (averaged laser fluence of 0.77 J/cm2), on Ru, W, Ni, and Cu: (a) SEM images. The electric field direction is indicated by the arrow. (b) Gaussian fit profile of the laser beam intensity, as determined from the beam waist measurement.
Analysis of ripples morphology on the different materials employed: Ru (a,e), W(b,f), Ni(c,g), and Cu (d,h). AFM images on 10 μm × 10 μm areas (a,b,c,d) and typical profile lines from AFM images, showing the cross-section of the ripples topography at local fluence around 0.6 J/cm2 (e,f,g,h).
Height of the averaged cross-section of the surface ripples obtained for different materials with N = 50 pulses as a function of the local fluence in the spot. The dashed line indicates the fluence values corresponding to AFM measurements in Fig. 2. Solid lines guide the eye.
(Color online): Calculated electronic temperature dependence of the electron diffusion depth (Lc) in the various solid materials at the melting threshold. The window corresponding to the expected range of ripples formation is represented by the gray area.
Material properties for metals investigated (Refs. 29 and 30): absorptivity A, penetration depth δ, thermal conductivity κ, electron-phonon coupling strength γ, melting temperature Tm, and ion heat capacity Ci. γ values have been taken from Ref. 18 for W, Ni, Cu, and from Ref. 17 for Ru.
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