Functional nanostructures inside materials induced by femtosecond laser pulses

Self-assembled periodic nanostructure in bulk material can be successfully photoinduced by femtosecond laser pulses. Such nanograting structure inside material can be empirically classified into the following three types: (1) structural deficiency, (2) strained crystal structure, (3) partial phase separation. The formation mechanisms of the periodic nanostructure was interpreted in terms of the interaction between incident light field and the generated electron plasma. Furthermore, the fact that the periodic nanostructures could be formed empirically only if it is indirect bandgap semiconductor materials indicates the stress-dependence of bandgap structure and/or the recombination of the excited electrons are also involved to the nanostructure formation. In the case of indirect bandgap semiconductor crystal such as Si, GaP, β−Ga₂O₃, the electronic stress induced by the deformation potential of electronic states is considered to be one of important key for nanostructure formation. While, we have recently observed that the partial phase separation in nanoscale also occurred in Al₂O₃−Dy₂O₃ binary glass with a low glass formation ability. In this case, we assumed that the crystallization were periodically photoinduced in the subwavelength nanoplanes based on the periodic modulation of the photoinduced electron plasma density. Particularly, we have also observed that the garnet (Dy₃Al₅O₁₂) and the perovskite phase (DyAlO₃) can be selectively precipitated according to the laser condition. Based on the intriguing phenomena ranging from optical, electric, thermal, to magnetic properties caused by the nanograting structure, we demonstrate new possibilities for functionalization of material ranging from an eternal 5D optical storage, a polarization imaging, to a thermoelectric conversion.