High aspect ratio submicron fabrication in bulk dielectrics with non-diffractive ultrafast Bessel beams; dynamics and interaction regimes
Résumé
Nonlinear propagation of intense ultrafast laser pulses inside transparent materials has a strong influence on the fabrication quality and accuracy in 3D laser-material processing. The ability to maintain near-constant intensity profiles over an appreciable distance along the propagation direction, sustaining nonlinear absorption, recommends ultrafast Bessel beams for high aspect ratio submicron structuring applications. We discuss here the characteristics of the interaction of transparent materials, especially fused silica, with ultrafast non-diffractive beams of moderate and high cone angle at various laser energies and pulse durations. We define their impact on photoinscription regimes, i.e. formation of isotropic and non-isotropic refractive index structures (Fig. 1) and the stability limits. The laser pulse duration was observed to be key in deciding the type and morphology of the structures. In particular, high aspect ratio void-like structures with submicron cores (down to 100 nm) over hundreds of microns are produced when using laser pulse of longer pulse duration, highlighting the important contribution of delayed ionization and light diffusion on excited carriers. On the contrary, for the same energy, smooth refractive index modified structures are produced for short laser pulses. To understand the formation mechanisms of these structures, we studied the ultrafast dynamics of excitation around the modification threshold using time-resolved microscopy and spectral imaging techniques. We reveal various relaxation mechanisms leading to either permanent refractive index changes accompanied by structural characteristic defect markers or to hydrodynamic phenomena and cavitation in hot states. We observe fast carrier trapping in structural matrix deformations for soft positive index changes accompanied by the formation of non-bridging oxygen centers and long living carriers characteristic of "phase" transition and hydrodynamic expansion for void-like domains. To illustrate the respective material changes we compare the ultrafast laser material interaction mechanisms in terms of energy deposition and relaxation characteristics in fused silica associated with Bessel beams [1]. Potential application in deep drilling and opto-fluidics are discussed.