In the past, laser-induced breakdown spectroscopy (LIBS) signals have been reported to have a stability independent of the pulse length in solids. In this Letter, we perform the first stability study of femtosecond LIBS in gases (to our best knowledge) and show a significant improvement in signal stability over those achieved with longer pulses. Our study shows that ultrashort-pulse LIBS has an intrinsically higher stability in gas compared to nanosecond-pulse LIBS because of a deterministic ionization process at work in the femtosecond pulse. Relative standard deviations below 1% are demonstrated and are likely only limited by our laser output fluctuations. This enhanced emission stability may open up possibilities for a range of applications, from monitoring rapid gas dynamics to high-quality broadband light sources.

Both the single and multiple pulse ablation of dielectrics with high-intensity femtosecond lasers have been well studied. Additionally, the “pulse-shaping” regime, in which temporal peaks are separated by picoseconds, has also been investigated. In this paper, we show that a “prepulse configuration”—where one pulse is followed several nanoseconds later by a much stronger pulse—is qualitatively different from any of the previously investigated regimes. In particular, it leads to the recently discovered “concentric rings” feature in glass. We show that the damage pattern produced in glass is very different for two pulses separated by nanoseconds than for two pulses separated by either milliseconds or picoseconds. For nanosecond separations, the second pulse interacts with the shock wave created by the first, significantly altering the damage pattern. In addition to producing novel structures, this pump-probe technique provides a new way of studying ablation plume and shockwave dynamics.

The role of the Higher-Order Kerr Effects (HOKE) in intensity clamping is experimentally investigated. We fail to observe any evidence of HOKE-based intensity clamping in a tight geometrical focusing experiment. We introduce a polarization-based technique that can distinguish between spectral components from the leading and trailing edges of the pulse. The results of this time-resolved measurement support the ionization theory of intensity clamping.

The characteristics of a plasma formed by a focused ultrashort laser in atmospheric-pressure air are studied with linear and circular pulses. We show that the ionization threshold for circular pulses is 1.36 times higher than for that linear pulses. Using an intensified CCD camera, we study plasma emission over seven orders of magnitude in a dynamic range. In spite of possessing a lower total ion number, plasmas produced by circular pulses are found to be brighter visible-light emitters than those produced by linear pulses of the same energy. This indicates that circular pulses produce plasmas with more electron kinetic energy than linear pulses and that kinetic energy plays a role in the optical emission intensity. The presence of high energy electrons is verified by demonstrating that a high-ionization-potential gas can be made to radiate more brightly by the addition of a low-ionization-potential gas even though the second gas lowers the achievable focal intensity.

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