Single shot depth sensitivity using femtosecond Laser Induced Breakdown Spectroscopy

•Depth dependence of LIBS emission within a single 800 nm femtosecond laser shot.•LIBS emission spectra dominated by only a thin surface layer.•Depth sensitivity, a new concept, is ~3 nm when actual crater depth ~100’s of nm.•Qualitative agreement with TTM model of lattice temperature versus depth and time.

Depth profiling measurement using multiple pulse Laser Induced Breakdown Spectroscopy (LIBS) can be used to determine the characteristics of buried layers. However for femtosecond pulses the emission spectra does not necessarily reflect the average depth composition for the single shot case and instead has much higher sensitivity for the surface layer of the ablation region. We introduce a concept of “depth sensitivity” to characterize this behavior for single shot LIBS depth profiling. Experiments were carried out using 800 nm femtosecond laser pulses irradiating layered targets while monitoring the plasma emission with a spectrometer system. Laser induced plasma formed at the surface layer exhibits a strong LIBS signature while deeper ablated region contributes very little to the emission spectrum. The sensitive depth region, the source of the major part of the emission, is much less than the ablation crater depth and is shown to be of the order of 3 nm in the case of actual crater depths of 100's of nm. A two temperature model has been used to determine the lattice temperature profile versus depth which qualitatively predicts the observed behavior.