Optimization of Remote Filamentation Induced Breakdown Spectroscopy (R-FIBS)

Laser-induced breakdown spectroscopy (LIBS) is a promising method for detection and characterization of materials at a range of standoff distances. LIBS involves the collection of light from a micro-plasma formed when a laser pulse ablates a material surface. The light emitted from the plasma can be spectrally analyzed to yield both elemental and isotopic composition. With the advent of ultrafast terawatt-class pulsed lasers, it is now possible to achieve light intensities where substantial non-linear effects occur even in air, and which lead to production of long plasma filaments. This nonlinear self-focusing effect enables the laser pulse to be delivered in a tight focal spot over long distances. The use of laser filaments in LIBS has been previously investigated and is referred to as Remote Filament Induced Breakdown Spectroscopy (RFIBS). Standoff distances of up to 180 m have been demonstrated in the use of RFIBS. The position at which the laser filament strikes the target has an impact on background and signal intensity. The primary contributions to background are the scattered continuum emission from the long filament, ranging in wavelength from 200 nm to 4 µm, and the inversebremsstrahlung background from the plasma itself. By optimizing the distance from the target at which the filament terminates (i.e. the energy remaining in the pulse at target surface not used for filament formation), the contrast between the atomic line emission (signal) and filament background (continuum) can be increased. We report a 400% increase in the intensity of the RFIBS signal in a natural uranium sample through the optimization of the laser beam focusing condition.