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We analyzed the formation of mid-infrared conical emission patterns possessing spiral and half-ring shaped wavelength contours from a beam of a few optical filaments. The complex patterns were generated and modified experimentally by adaptive wavefront shaping of the femtosecond laser pulse. Mutual interactions between co-propagating filaments can induce curvature in their paths, and the spiral and half-ring emissions were shown to be a direct consequence of this angular deflection. Based on our experimental and computational results, the spirals form in the far-field due to self-interference of conical emission from a helically moving filament. The presented findings will advance the tailoring of spatial conical emission patterns potentially beneficial for spectroscopic applications and terahertz generation.

We report the influence of water content, droplet displacement and laser fluence on the laser-induced breakdown spectroscopy (LIBS) signal of precisely controlled single droplets. For the first time in single particle LIBS scheme, the degree of evaporation of an additive-free droplet was followed and the position of the residual particle was adjusted at micrometer resolution using electrodynamic trapping. The results show signal intensification throughout the 6 s period of the complete evaporation of the droplet into a dry residual particle. The analyte line emission remained stable when the particle was moved within the focal spot area and almost tenfold compared with situation where the particle lies 15 μm outside the laser beam path. Combination of low, about 6 mJ, excitation laser pulse energy and short, about 1 μs detection delay time was found to be the optimal in the detection of most metals. The presented findings will pave the way for more sensitive and reproducible single particle elemental analysis exploited in the real-time monitoring of water, atmospheric aerosols or industrial emissions.