Experimental procedure: Non-diffractive beams

The laser source consists of an amplified ultrafast Ti:Sapphire system. The system delivers 60 fs laser pulses at 800 nm central wavelength and at 1 kHz repetition rate. The laser pulse duration can be tuned in the fs-ps range using additional second-order dispersion by adjusting the compressor inter-grating distance. High contrast single pulses can be extracted using opto-electronic and mechanical shutters. Several methods exists for generating non-diffractive beams by conical intersection (see ref.³⁶ and references therein). We opted here for axicon generated Bessel-Gauss beams (referred hereafter as Bessel beams). Zeroth-order Bessel beams³⁰ are generated using an axicon lens with an apex angle of 179°. The conical phase pattern of the axicon bends the incoming light to the axis. It thus determines a transverse to axial light transformation, where the incoming wavefronts create a stationary field encompassing a narrow intense central core axially elongated over a large distance (as compared to the Rayleigh range of an equivalent Gauss beam) and surrounded by low intensity rings following the Bessel transverse profile. The central core and the rings are the consequence of successive constructive interference relations of the incoming conical wavefront. The Bessel beam is then demagnified and imaged inside a water cuvette using a 4 f afocal imaging system³⁷ with a microscope objective as the end optical imaging/focusing element. Bessel beams with different cone angles can be obtained by employing objectives with various optical powers. Mostly moderate and tight focusing conditions (objective numerical aperture of NA = 0.28 and 0.42) are used inside the liquid, with half-cone angles θ = 9° and 16.5° resulting in FWHM core diameters of 1.5 and 0.7 μm respectively. The aspect ratio lies around 100. For observing the laser-excited regions in liquids, in-situ and in real-time, dynamic optical transmission (OTM) and positive phase-contrast (PCM) microscopy are employed. Several water (distilled) and water-ethylene glycol mixtures were used and in particular cases they were doped with gold nanoparticles.