Laser ablation-induced spectral plasma characteristics in optical far- and near fields

The aim of this work is to elucidate the ablation-induced plasma physics for chemical species analysis by laser-induced breakdown spectroscopy at higher spatial resolution. To accomplish this, the effect of the laser spot size on the laser ablation-induced plasma characteristics is experimentally investigated, both in optical far-field and near-field ablation configurations, utilizing a Cr thin film of ∼200 nm thickness on quartz substrate as a target. The far-field ablation is affected under tight focusing conditions, wherein nanosecond laser pulses of 532 nm wavelength are focused to laser focal spot diameters of ∼7 and 1.5 μm. The measured results show that the ablation-induced plasma from sub- 10 μm ablation craters exhibits complex three-dimensional behavior, leading to greatly reduced laser-plasma interaction and an order of magnitude shorter plasma lifetime. Nanosecond laser pulses of 532 nm wavelength are also coupled to a pulled fiber based near-field scanning optical microscopy probe. Due to the sharp tip presence in close proximity of the ablation craters, entirely different plasma evolution behavior is observed, highlighted by orders of magnitude shorter plasma lifetime and strongly directional material ejection. The ablation-induced plasma from reduced lateral crater dimensions both in far- and near-field shows improved contrast of atomic transition signals with respect to the wide-spectrum background, hence confirming the potential for laser-induced breakdown spectroscopy with high spatial resolution.