Single-shot visualization of evolving, light-speed refractive index structures
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An intense laser or charged particle pulse propagating through matter excites light-speed refractive index structures in its wake via Kerr effect, ionization, or displacement of electrons from background ions. Examples include plasma wakes used to accelerate charged particles and self-guided filaments used for atmospheric analysis and micromachining. Such applications constrain the shape, size and evolution of the index structure, yet often these are known in detail only through intensive computer simulations based on estimated initial conditions. Here we develop and demonstrate three methods for visualizing evolving light-speed structures directly in the laboratory in a single shot : (1) frequency-domain streak camera, (2) frequency-domain tomography, and (3) multi-object-plane phase-contrast imaging. All three methods are based on analyzing phase perturbations that an evolving object imprints on one or more probe laser pulses that cross its path obliquely. The methods are tailored to different propagation lengths, material densities, and dimensionality of imaging. Using these techniques, evolving laser-driven filaments in glass and air and plasma wakes in helium gas driven by laser pulses up to petawatt peak power are visualized in one shot, revealing underlying nonlinear laser-plasma interaction physics that is compared in detail to computer simulations.