Ultrafast imaging of free carriers: controlled excitation with chirped ultrafast laser Bessel beams

Abstract

Nonlinear propagation of intense ultrafast laser pulses inside transparent materials has a strong influence on the fabrication quality and accuracy for 3D laser-material processing. Due to their ability to maintain near-constant fluence profiles over an appreciable distance along the propagation direction in linear and nonlinear media, ultrafast Bessel beams are ideal sources for high aspect ratio sub-micron structuring applications. We report here on the interaction of transparent materials, especially fused silica, with ultrafast non-diffractive beams of moderate cone angle at various laser energies and pulse durations and define their impact on photoinscription regimes, i.e. formation of isotropic and non-isotropic (positive and negative) refractive index structures. The laser pulse duration was observed to be key in deciding the type of the structures via excitation efficiency. To understand the significant mechanisms for forming these different structures, the free carrier behavior as a function of laser pulse duration and energy was studied by capturing instantaneous excitation profiles using time-resolved microscopy. Time-resolved imaging and simulation studies reveal that low carrier densities are generated for ultrashort pulses leading to soft positive index alterations via presumably non-thermally induced structural transitions via defects. On the other hand, the high free carrier density generation in the case of longer pulse durations leads to a hydrodynamic expansion resulting in high aspect ratio sub-micron size wide voids. Delayed ionization, carrier defocusing and lower nonlinear effects are responsible for the confinement of energy, resulting in efficient energy deposition on-axis.