We demonstrate in this paper that laser ablation allows efficient analysis of organic and biological materials. Such analysis is based on laser-induced breakdown spectroscopy (LIBS) which consists in the detection of the optical emission from the plasma induced by a high intensity laser pulse focused on the sample surface. The optimization of the ablation regime in terms of laser parameters (pulse duration, wavelength, fluence) is important to generate a plasma suitable for the analysis. We first present the results of a study of laser ablation of organic samples with different laser parameters using time-resolved shadowgraph. We correlate the early stage expansion of the plasma to its optical emission properties, which allows us to choose suitable laser parameters for an efficient analysis of organic or biological samples by LIBS. As an illustration of the analytical ability of LIBS for biological materials, we show that the emission from CN molecules can be used to distinguish between biological and inorganic samples. Native CN molecular fragment directly ablated from a biological sample are identified using time-resolved LIBS. Those due to recombination with nitrogen contained in atmospheric air can be distinguished with their specific time evolution behavior.
Laser-Induced Breakdown Spectroscopy (LIBS) has been used since 40 years on typical samples such as metals, alloys,
rocks. Detection of organic hazards or analysis of biological compounds under atmospheric pressure with LIBS
represents a new challenge. For this purpose, we need better understandings of the physico-chemical properties of the
plasma in atmosphere and their influences on the LIBS signal.
As a model sample of organic materials, Nylon 6-6 has been studied under nanosecond ablation at different
wavelengths (1064 nm and 266 nm) and energies (from 1 to 5 mJ) in order to observe the influence of these parameters.
Shadowgraph technique is used to image the plasma at its early stage of expansion (0 to 40 ns). Time-resolved LIBS
signal is recorded for longer times (50 ns to 5 μs).
In the infrared regime, the expansion of the plume is faster along the laser axis, perpendicular to the sample
surface. On the contrary, for UV ablation, the expansion of the plume is quite isotropic. We can also observe different
regimes of expansion due to Laser-Supported Detonation Waves (LSDW) above 3 mJ in the UV regime.
In particular, these observations provide us ideas to understand the kinetics of the CN emission in the LIBS
signal. In the IR regime, a formation of CN due to carbon present in the sample and nitrogen in the air via the
reaction 2C + N2 → 2CN can be observed. In the UV regime, the direct ablation of CN bonds is clearly seen but other
effects like screening and recombination due to LSDW have also been observed.
The propagation of ultrashort, ultra-intense laser pulses gives rise to strongly nonlinear processes. In particular, filamentation is observed, yielding an ionized, conducting plasma channel where white-light supercontinuum due to self-phase modulation occurs. This supercontinuum, extending from the UV to the IR, is a suitable "white laser" source for atmospheric remote sensing, and especially Lidar (Light Detection and Ranging). Recent significant results in this regard are presented, as well as lightning control using ultrashort laser pulses. The application of ultrashort-pulse lidar to aerosol monitoring is also discussed.
High-power femtosecond laser pulses can lead to strong nonlinear interactions during the propagation through a medium. In air the well known self-guiding effect produces long intense and moderately ionized filaments, in which a broad white-light continuum from the near UV to the mid IR is generated. The forward directed white-light can be used to do range resolved broadband absorption measurements, which opens the way to a real multi-component lidar for the simultaneous detection of several trace gases. On the other hand, enhanced nonlinear scattering and characteristic emission from the filament region, as well as from the interaction of intense pulses with aerosols, can be observed. This opens perspectives towards a novel kind of analysis of atmospheric constituents, based upon nonlinear optics. Additionally, the conductivity of the filaments can be used for lightning control. Here we present the basic concepts of the femtosecond lidar, laboratory experiments and recent results of atmospheric measurements.
We present the results of a detailed lidar study on urban aerosols performed in summer 1996 in Lyon. Mie calculations have been performed to determine the optical backscattering and extinction coefficients of the real size distribution obtained by sampling, and then used for lidar measurements. The solid particles have been sampled using cut-off filters. Size distribution and composition, determined by scanning electronic microcopy (SEM) and x-ray microanalysis, reveal two main modes at 0.1 and 0.9 micrometer, the composition of which is soot for the first one and 60% soot - 40% silica for the second. The combined SEM and lidar techniques allowed to obtain the first quantitative lidar profiles of urban aerosols. Potential and limitations of the method are critically discussed.
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