LIBS signal enhancement by laser surface microstructuring of copper

This paper describes the optimization of femtosecond laser surface microstructuring of copper to obtain LIBS signal enhancement. Microstructures in the form of periodic rows are created on a copper sample with depths between 2 μm and 24 μm by tuning the femtosecond laser power, frequency, as well as the translation stage speed. A darkening effect is observed in the zones with microstructures, corresponding to a better light trapping effect than on a polished surface. The LIBS signal enhancement of the Cu I line at 521.8 nm is used as an indicator to select the type of microstructures which induces the maximum enhancement of the plasma emission. Characterizations of the microstructures through confocal microscopy and Time of Flight Secondary Ion Spectrometry are used to explain the change in signal enhancement between different microstructures. More specifically, deeper and darker microstructures are shown to provide less signal enhancement due to the higher content of copper oxide within the typical LIBS crater depth. A signal enhancement as high as 11 is obtained on the optimized microstructure. This enhancement is the result of a 70 % increase in electron density, 17 % increase in temperature and two times higher plasma lifetime. Finally, the resulting microstructured copper sample is used as a substrate for the LIBS analysis of a liquid. In this study, mineral water is dropped and dried on the substrate, and the LIBS signal of the Ca I lines intensities at 526.2 nm, 526.5 nm and 527.0 nm is found to be roughly four times higher than on the copper substrate without microstructures. • A difference in the amount of oxide present within the typical LIBS crater was found for microstructures with different depths, using Time of Flight Secondary Ion Mass Spectrometry. The deeper the microstructures, the higher the oxide concentration which causes lower enhancement for microstructures deeper than 4.5 μm. • A signal enhancement of 4 was found for the Ca I line at 527.0 nm, Ca being contained in the mineral water dropped and dried onto the optimized microstructured copper substrate. • A signal enhancement lower than 2 was found for the Mg I line at 518.3 nm, indicating that, for a given substrate, the signal enhancement is not constant.