Selective localised modifications of silicon crystal by ultrafast laser induced micro-explosion

Femtosecond (fs) laser pulses focused and confined inside the bulk of a material can deposit a volume energy density up to several MJ/cm³ in a sub-micron volume. This creates highly non-equilibrium, hot, dense and short-lived plasmas with conditions favorable for arrangement of atoms into unusual material phases. Singlecrystal silicon was exposed to strong shock waves induced by laser micro-explosion in confined geometry. The conditions of confinement were realized by focusing 170-fs pulses, with the energy up to 2.5 μJ, on a Si surface buried under a 10-μm thick SiO₂-layer formed by oxidation of a Si-wafer. The generated intensity was 10¹⁵ W/cm², well above the threshold for optical breakdown and plasma formation. The shock wave modified areas of the Si crystal were sectioned using a focused-ion beam and characterized with scanning electron microscopy. A void surrounded by a shock-wave-modified Si was observed at the Si/SiO₂ boundary. The results demonstrate that confined micro-explosion opens up new perspectives for studies of high-pressure materials at the laboratory table-top expanding the laser-induced micro-explosion capabilities into the domain of non-transparent materials.