Laser Induced Breakdown Spectroscopy (LIBS) by self-channeled femtosecond pulses is characterized for detection
of energetic materials. Different polymers are spin coated on silicon wafers to provide a thin organic layer with
controllable thickness ranging from 500 nm to 1 μm. Spectral analysis of atomic and molecular carbon emission
shows CN molecular signal from samples that do not contain nitrogen. This can be explained by possible molecular
recombination between native atomic carbon and atmospheric nitrogen. As a consequence, caution must be
exercised when using spectral signatures based on CN emission for explosive detection by filament-induced LIBS.
This study makes a comparison of LIBS emission from molecular species in plasmas produced from organic
residues on a non-metallic substrate by both a 5 ns Nd:YAG laser (1064 nm) and a 40 fs Ti:Sapphire laser (800 nm)
in air and argon atmospheres. The organic samples analyzed had varying amounts of carbon, nitrogen, hydrogen,
and oxygen in their molecular structure. The characterization was based on the atomic carbon, hydrogen, nitrogen,
and oxygen lines as well as the diatomic species CN (B2Σ+ - X2Σ+) and the C2 (d3Πg - a3Πu). Principal Component
Analysis (PCA) was used to identify similarities of the organic analyte via the emission spectra. The corresponding
Receiver Operating Characteristics (ROC) curves show the limitations of the PCA model for the nanosecond regime
As an alternative to focusing nanosecond pulses for stand-off LIBS detection of energetic materials, we use
self-channeled femtosecond pulses from a Ti:Sapphire laser to produce filaments at 12 meters and create a plasma on
copper, graphite and polyisobutylene film. We show the possibilities of this Laser-Induced Breakdown Spectroscopy
configuration for thin organic sample detection on a surface at a distance.
Laser interactions with bulk transparent media have long been investigated for material processing applications involving ablation and shock wave generation in both the nanosecond and femtosecond pulse width regimes1. Shock waves have been studied in fused silica and other optical glasses but previously have been characterized by the morphology of the concurrent ablation. We perform ablation at distances of 30 meters using the non-linear self-channeling effect. Using silicon wafers as targets because of their clearly defined ablation zones, we examine the effect that the filament has on the thin SiO2 layer coating the wafer's surface. It is observed that the surface layer experiences a shock wave resulting from the explosive forces produced by the plasma. The use of several laser pulses in burst mode operation leads to the observation of multiple shock fronts in the material, and the possibility of shock wave addition for higher damage. Optical interferometry will be used to characterize the shock wave dynamics, using both traditional means of focusing in the near field and at 30 meters using propagating self-channeled femtosecond pulses. The novelty of using self-channeling laser pulses for shock wave generation has many implications for military applications. These experiments are to be performed in our secure test range using intensities of 1014W/cm2 and higher incident on various transparent media. Interferometry is performed using a harmonic of the pump laser frequency. Experiments also include burst-mode operation, where a train of ultra-fast pulses, closely spaced in time, and novel new beam distributions, strike the sample.
We report on the use of a novel phase element to control the far-field intensity pattern generated by a high-peak-power, femtosecond laser. The pre-determined intensity pattern results in a well defined location of the filaments formed by the propagation of these beams through the atmosphere. This enhancement of the localization and repeatability of the intensity distribution can be extremely beneficial for laser induced breakdown spectroscopy (LIBS) of remote regions of interest.
The need for robust, versatile, and rapid analysis standoff detection systems has emerged in response to the increasing threat to homeland security. Laser Induced Breakdown Spectroscopy (LIBS) has emerged as a novel technique that not only resolves issues of versatility, and rapid analysis, but also allows detection in settings not currently possible with existing methods. Several studies have shown that femtosecond lasers may have advantages over nanosecond lasers for LIBS analysis in terms of SNR. Furthermore, since femtosecond pulses can travel through the atmosphere as a self-propagating transient waveguide, they may have advantages over conventional stand-off LIBS approaches1. Utilizing single and multiple femtosecond pulse laser regimes, we investigate the potential of femtosecond LIBS as a standoff detection technology. We examine the character of UV and visible LIBS from various targets of defense and homeland security interest created by channeled femtosecond laser beams over distances of 30m or more.