We report the structural changes inside germania glass induced by femtosecond laser pulses. Inspection by polarization microscopy and secondary electron microscopy indicate that the periodic nanostructures consist of oxygen defects such as ODCs (oxygen deficient centers) and NBOHCs (nonbridging oxygen hole centers) for laser pulse energy less than 0.2 lJ. However, the glass network was dissociated and O 2 molecules were generated for laser pulse energy greater than 0.4 lJ. Two different structural changes, form-birefringence and dissociation, were induced in GeO 2 glass, depending on the laser pulse energy. The form-birefringence exhibited by the nanogratings in GeO 2 glass is larger than that in SiO 2 glass for pulse energy less than 0.2 lJ, as the density of nanovoids enclosed by ODCs in GeO 2 glass is higher than that in SiO 2 glass. Arrhenius plots of the phase retardation caused by the nanogratings in GeO 2 and SiO 2 indicate that the oxygen defects are relaxed at a temperature 100°C above the glass-transition temperature.

The spatial distributions of elements in a glass can be modulated by irradiation with high repetition rate femtosecond laser pulses. However, the shape of the distribution is restricted to being axially symmetric about the laser beam axis due to the isotropic diffusion of photo-thermal energy. In this study, we describe a method to control the shape of the elemental distribution more flexibly by simultaneous irradiation at multiple spots using a spatial light modulator. The accumulation of thermal energy was induced by focusing 250 kHz fs laser pulses at a single spot inside an alumino-borosilicate glass, and the transient temperature distribution was modulated by focusing 1 kHz laser pulses at four spots in the same glass. The resulting modification was square-shaped. A simulation of the mean diffusion length of molten glass demonstrated that the transient diffusion of elements under heat accumulation and repeated temperature elevation at multiple spots caused the square shape of the distribution.

High-density nitrogen-vacancy (NV) centers exceeding 1016/cm3 inside a diamond can be achieved by femtosecond laser irradiation. The number of pulses and pulse energy were adjusted considering the increasing trend of the NV concentration and damage generation. The NV concentration first grew as the number of laser pulses was increased, and then the concentration was temporarily saturated or decreased. By increasing the number of laser pulses more than 2.5–5 × 107, the concentration increased again. The Hahn-echo measurement revealed that electron spins of the substitutional nitrogen center (P1) were dominant source for decoherence and the number of paramagnetic defects induced by the laser irradiation was not significant for degradation of spin coherence of the laser-induced NV centers even without a post-annealing process. Such high-dense ensemble NV centers are crucial for a high-sensitive quantum sensor.

We investigated the effect of laser pulse duration on nitrogen-vacancy (NV) center generation inside a single crystal diamond. We compared pulse durations of 40 fs (femtosecond laser) and 1 ps (picosecond laser). We found that in both cases, ensemble NV centers could be generated inside the diamond. However, the maximum photoluminescence intensity of the NV center without graphitization for the 40 fs duration was higher than that for the 1 ps duration. This indicated that the femtosecond laser was harder to graphitize diamond and could generate more NV centers without graphitization. This difference may be due to the difference in the photo-absorption process and the resulting lattice dynamics.