Ultrafast laser-induced extrusion of mono-crystalline nano-pillars from sapphire

A single ultrafast laser pulse, shaped as a radially polarized first-order Bessel beam, induces the formation of high aspect ratio nano-pillars emerging from the bulk of sapphire in a laser-induced extrusion process. Three distinct regimes could be observed depending on laser fluence, ranging from straight nano-pillars to nano-structures shaped by capillary instability. Here, we report transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) structural analysis to reveal the underlying formation mechanisms of the nano-structures for all three regimes. We demonstrate, using crystalline orientation analysis, that they range from the translation of a crystalline rod to the resolidification of a hydrodynamic jet of molten sapphire. Remarkably, all nano-pillars are monocrystalline sapphire. The TEM structural analysis of the materials below the surface also provides new insights into the complex phenomena following the laser-matter interaction and the processes underlying the formation of these structures.