Direct evidence of laser-induced shock wave plasma from organic targets in low pressure He ambient gas, showing the effect of target hardness on its propagation speed and the resulted spectral performance

We report the experimental evidence of shock wave plasma generation by direct observation of the plasma propagation using 68 mJ 1064 nm Nd-doped yttrium aluminum garnet laser irradiation on a relatively soft organic sample in He ambient gas at 2 kPa. The density jump associated with the arrival of the plasma front at a certain position is detected by means of the highly sensitive interferometric technique, in conjunction with the observation of the first appearance of the plasma emission at the same position and time of the plasma front arrival. The result shows that the plasma front moves at the Sedov speed, which is typical of shock wave propagation, yielding an excellent emission spectrum for the harder sample. The shock front speed is further found to decrease in the case of a softer sample, resulting in lesser spectroscopic performance, as demonstrated by the emission spectra measured from mochi samples of different hardnesses. The result of this study is promising for paving the way to extending the laser-induced breakdown spectroscopy application to the much needed spectrochemical analysis of agricultural and food products. (c) 2017 Optical Society of America