Spatio-temporal dynamics in nondiffractive Bessel ultrafast laser nanoscale volume structuring

Nondiffractive ultrafast optical beams with quasi-stationary characteristics enable new regimes and scales in light-matter interactions. We discuss the action of ultrashort Bessel laser beams in bulk fused silica, emphasizing excitation dynamics with energy localization beyond diffraction limit. We shed light on relaxation channels leading to one-dimensional structures with nanoscale sections and morphologies ranging from densified matter to nanosized cavities. Space-and time-resolved absorption and phase-contrast microscopy reveals two main carrier relaxation paths. Fast exciton trapping in self-induced matrix deformations results in positive index contrast driven by swift accumulation of non-bridging oxygen hole centers and defect-driven structural rearrangements. High excitation densities determine thermomechanical paths, with onset of phase transitions and the release of pressure waves. High-aspect-ratio nanosized channels are thus created via rarefaction and liquid cavitation, accompanied by molecular decomposition and generation of oxygen deficiency. The characteristic electronic relaxation identifies the nature of structural transitions up to the onset of phase transformation. Temporal pulse dispersion regulation allows driving unique carrier dynamics with precise control over energy deposition down to the 100 nm scale. Extreme high-aspect-ratio uniform void structures can thus be fabricated in conditions of sub-micron transverse light confinement.