Spatial confinement of laser-induced plasma by laser-induced and obstacle-reflected shock wave and its effect on optical emission of laser-induced plasma

The laser-induced plasma (LIP) and the shock wave generated by pulsed laser ablation of a graphite target in air and reflected by a flat obstacle were examined by optical emission spectroscopy and probe beam deflection measurements. The interaction between the LIP and the shock wave and its effects on the expansion of the LIP as well as on the optical emission of carbon atoms were studied. The carbon atomic emission can be enhanced or reduced in the situation with a flat obstacle standing in the propagation path of the shock wave. The enhancement or reduction of the carbon atomic emission has a close connection with the shock wave generated by graphite ablation and reflected by the obstacle. The reflected shock wave confines the expansion of the LIP and impedes the travelling of the plasma species. The enhancement was observed at the detection position close to the target and with a short block-target distance. The shock wave thus reflected encounters the luminous LIP at its early expanding stage and confines the expansion of the LIP, resulting in the enhancement in the optical emission of carbon atoms. But at the detection position far from the target and with a longer block-target distance, a reduction in the optical emission due to spatial confinement was observed. The possible mechanisms responsible for the effects of spatial confinement on the optical emission were discussed.