It is shown that the continuous background of the laser-induced breakdown spectrum of Al produced by 800 nm femtosecond pulses is strongly polarized. Use of a polarizer to filter out the background significantly improves the signal/noise and signal/background ratios of the discrete line spectrum. The effects of the laser pulse energy, focal position, incidence and detection angles, and the polarization plane of the laser were investigated. Polarization resolved laser-induced breakdown spectroscopy (PRLIBS) is much less sensitive to these variables than conventional, ungated fs-LIBS, making this a much more versatile analytical tool. These measurements reveal qualitatively different mechanisms for the continuous and discrete parts of the spectrum.

It is well known that optical emission produced by femtosecond laser-induced breakdown on a surface may be enhanced by using a pair of laser pulses separated by a suitable delay. Here we elucidate the mechanism for this effect both experimentally and theoretically. Using a bilayer sample consisting of a thin film of Ag deposited on an Al substrate as the ablation target and measuring the breakdown spectrum as a function of fluence and pulse delay, it is shown experimentally that the enhanced signal is not caused by additional ablation initiated by the second pulse. Rather, particle-in-cell calculations show that the plasma produced by the first pulse shields the surface from the second pulse for delays up to 100 ps. These results indicate that the enhancement is the result of excitement of particles entrained in the plasma produced by the first pulse.

It is shown that the continuum emission produced in the ablation of an Al target with nanosecond laser pulses is much more strongly polarized than the discrete line emission. This effect may be utilized to improve the resolution of the laser-induced breakdown spectroscopy spectrum by using a polarizer to filter out the continuum background. The effects of laser fluence and focal position are also reported. It is further shown that the lifetime of the emission closely tracks the intensity spectrum.

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