Ablation and structural changes induced in InP surfaces by single 10 fs laser pulses in air

Ablation of single-crystalline (100) indium phosphide wafer surfaces with single 10 fs Ti:sapphire laser pulses in air has been studied by means of complementary cross-sectional transmission electron microscopy, scanning force microscopy, and optical microscopy. A local protrusion of ∼70 nm height is generated within the ablation craters for fluences exceeding 0.78 J/cm2. This morphological feature has been studied in detail, revealing the material structure of the laser-affected zone and its spatial extent. The resolidified layer (60–200 nm thick) consists of polycrystalline grains (5–15 nm diameter) and is covered by an ∼10 nm thick amorphous top layer. Interestingly, the sharp boundary of the solidified layer to the unaffected crystal underneath exhibits a Gaussian-like shape and does not follow the shape of the surface topography. Evidence is presented that the central crater protrusion is formed by near-surface optical breakdown, and that the absorption in the material transiently changes during the femtosecond-laser pulse.