The Key Role of the Semiconductor Property of Water in Femtosecond Laser-Induced Plasma Spectroscopy

We investigate femtosecond laser-induced plasma spectroscopy (fs-LIPS) of water, and find that at different input laser energies the measured characteristic fs-LIPS emissions from trace metal elements in water exhibit different dependences on the chirped pulse duration. As the pulse is temporally stretched from 50 fs to longer durations up to 500 fs, the emission intensity from Al I on the 3s(2)4s S-2(1/2) -> 3s(2)3p S-2(1/2), (3/2) transitions experiences three typical variation regions, i.e., the increasing, plateau, and decreasing regions, respectively, but their temporal range varies as a function of the input laser energy. By comparing with the optical emissions of fs-LIPS of different materials including ambient air, solid metal Aluminum, and the two semiconductor materials with different bandgaps, i.e., silicon and sapphire, we ascribe the observed dependence of fs-LIPS emission of trace Al element in water on the pulse duration to the tradeoff between the strong field ionization (SFI) and the subsequent avalanche ionization (AI), both of which are determined by the wide bandgap semiconductor property of water.