Improved Analytical Performance of Remote Laser-Induced Breakdown Spectroscopy via Concatenation of Filament-Driven Optical Waveguides Formed in Air

Abstract

Filamentation of ultrafast laser pulses in air, resulting from dynamic equilibrium of nonlinear self-focusing, consequent plasma formation and de-focusing, and diffraction, enables confined propagation over distances that greatly exceed the Rayleigh range. Extended delivery of high laser intensities supports multiple applications, including remote sensing via analytical spectroscopy methods that rely on atomic or molecular ionization or excitation. The structures formed in the medium in the wake of filamentation may also be used as optical waveguides. One such waveguiding mechanism involves shaping an annular structure with depressed refractive index caused by variations in the gas temperature and pressure. The tailored thermal gradient of air in the wake of the filament plasma resembles the equivalent of a waveguide core and cladding. In this work, we concatenate two such filament-driven waveguides and show that the longer, merged structure improves the signal collection when compared to its shorter segments, resulting in better analytical performance of laser-induced breakdown spectroscopy. The results improve the prospects for scaling of optical guiding structures in air to greater distances in remote sensing applications.