Polarization-dependent periodic nanostructures inside various materials are successfully photoinduced by ultrafast laser pulses. These periodic nanostructures in various materials can be empirically classified into the following three types: (1) structural deficiency, (2) strained structure, (3) partial phase separation. Such self-assembled nanostructures exhibit not only optical anisotropy but also intriguing electric, thermal, and magnetic properties. We demonstrate new possibilities for functionalization of common materials ranging from an eternal 5D optical storage, a polarization imaging, to a thermoelectric conversion, based on the indicated phenomena.
We have successfully observed self-assembled periodic nanostructures inside Si single crystal and GaP crystal, by the femtosecond double-pulse irradiation. These results experimentally indicate that the self-assembly of the periodic nanostructures inside semiconductors triggered by ultrashort pulses irradiation are possibly associated with a direct or an indirect band gap. More recently we have also empirically classified the photoinduced bulk nanogratings into the following three types: (1) structural deficiency, (2) compressed structure, (3) partial crystallization. We have still a big question about what material properties are involved in the bulk nanograting structure formation. In this study, to expand the selectivity of the material for bulk nanograting formation, we have employed β-Ga<sub>2</sub>O<sub>3</sub> crystals (indirect bandgap Eg ~ 4.8 eV) as a sample for femtosecond laser irradiation. The nanograting structure inside β-Ga<sub>2</sub>O<sub>3</sub> crystal was aligned perpendicular to the laser polarization direction. Such phenomenon is similar to the nanograting in SiO<sub>2</sub> glass (Eg ~ 9 eV). Moreover, to clarify the band structure, we have also investigate the photoinduced structure in Sn doped β-Ga<sub>2</sub>O<sub>3</sub> crystals, which exhibit direct bandgap according to the first principle calculation.