Morphology change from nanocrack into periodic pore array formed by femtosecond laser pulses

Defects inside single crystals are an important concern because they directly affect the physical or chemical properties of the material, especially in sapphire used as substrates for semiconductors.We have investigated the thermally activated transformations of nanometer-scale cracks and phase transitions inside sapphire by femtosecond laser irradiation and successive heat treatments. The nanocracks transformed into periodic arrays of pores and dislocations that aligned along the [11̄02] planes after heat treatments above 1300 °C. The amorphous phase at the focal point recovered into the initial single crystalline phase after the heat treatments. Our study provides useful information on the recovery behavior of nanometer-scale defects in a single crystal.