Ultra-short pulsed laser deposition of thin silver films for surface enhanced Raman scattering

Nanostructured thin films have been obtained by ultra-short pulsed laser deposition (PLD) of silver performed in vacuum. The ablation source used in these experiments was a frequency doubled (? = 527 nm) Nd:glass laser with a pulse duration of 250 fs. The films have been characterized by UV-vis spectroscopy, atomic force microscopy and transmission electron microscopy. The surface enhanced Raman scattering activity results, investigated by Raman scattering of rhodamine B adsorbed onto rough silver substrates, are strongly related to the effective surface area (ESA) of the silver surfaces. Surprisingly, silver films obtained by ultra-short PLD are very similar to those obtained by short PLD. With the aim of explaining this likeness, the dynamics of plasmas induced by short and ultra-short laser pulses have been studied by optical emission spectroscopy and fast imaging. Composition, velocity, excitation temperature and density of short and ultra-short plasmas are quite similar, showing that film properties are strongly related to the plasma features.