Laser-fluence dependence of signal enhancement in femtosecond double-pulse laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) based on femtosecond laser pulses has widely been used for analyzing chemical elements in samples. In this research, we achieved the double-pulse operation with a two-polarizer configuration, and the signal enhancement factor of 4-15 was observed as compared with the single pulse LIBS scheme. It is found that with the change in interpulse delay time between 0 and 150 ps, the laser-fluence dependence of the LIBS signal enhancement can be classified into three types that are characterized by the different behavior of linear and quadratic intensity increase, followed by the intensity saturation. Such different responses are presumably ascribable to the mechanisms of electron-ion relaxation and plasma reheating. Also, different behavior is seen for the signal enhancement with different laser intensity and beam diameter while the laser fluence is kept constant. This indicates that generally, the use of a larger beam diameter is recommended for increasing the signal enhancement factor.