Photoinduced Structural Change in MgO Single Crystal

The photoexcited structural changes by the femtosecond laser pulses is one of the basic principles for laser processing techniques such as cutting and slicing. The femtosecond laser pulses photoexcite various structural changes of defects and nanovoids in MgO single crystals, and then such deficiencies develop into cracks. However, the precise roles of laser polarization during this process are unknown. Here we show that the photoinduced defects and nanovoids were formed along the <100> direction corresponding to the slip plane. Remarkably, the distribution of defects and nanovoids in [100] and [010] directions changed according to the polarization direction, resulting in polarization-dependent birefringence. Furthermore, depending on the pulse energy, the laser damage along the scanning direction was observed in the lower part of the focal region due to multiple filamentations.