Fig. 1. State of big data storage and eternal time capsules fabricated by femtosecond laser direct writing. (a) Annual data increasing of the global datasphere^[5]; (b) proportion of different methods for data storage^[3]; (c) global data center compute instances^[21]; (d) time capsules of eternal data storage fabricated by laser direct writing^[36]

Fig. 2. Mechanism of femtosecond laser interaction with matter. (a) Process of multi-photonic ionization and avalanche ionization; (b) graph of pulse energy versus pulse duration defining three regimes of material modification using an NA＝0.65 objective lens^[52]

Fig. 3. Development of femtosecond laser-induced bulk damage for data storage. (a) Laser-induced refractive index changes^[61]; (b) binary data stored in fused silica^[66]; (c) plane of photonic lattice in Ge-doped silica^[67]; (d) optical image of bits written inside fused silica^[70]; (e) pattern of voids produced in sapphire^[68]; (f) 100-point simultaneous multi-bit recording, reading, and signal enhancement^[71]; (g) fused silica prints with images and information for the next 300 million years^[72]

Fig. 4. Development of laser-induced nanogratings. (a) Four different polarization directions focused inside the sample^[73]; (b) damage trajectories inside fused silica by unpolarized light transmission (left) and by orthogonal polarization light transmission (right), and arrows indicate the two thresholds for type Ⅰ and type Ⅱ damages^[75]; (c) backscattering electron images^[76]; (d) Auger spectra and corresponding line scanning results of oxygen and silicon on same silica glass^[76]; (e) sideview of nanogratings induced inside fused silica^[56]; (f) schematic of form birefringence introduced by the self-organized nanograting^[77]

Fig. 5. Characteristics of nanogratings. (a)(d) Rewriting of nanograting voxels^[80]; (e) 5D optical storage readout (left) and Arrhenius plot of the nanogratings decayrate (right)^[19]; (f) a pseudo-color map of the world made with femtosecond laser-induced nanogratings^[84]

Fig. 6. Mechanism of nanograting formation. (a)(d) Nanoplanes evolved by nanoplasmas^[86]; (e) asymmetric enhancement effect of nanoplasmons^[52]; (f) formation and relaxation process of STEs^[82]

Fig. 7. High transmittance through Type X structure^[99]. (a) Photo of quartz glass plate modified with Type Ⅱ (left) and Type X (right); (b) transmission spectra of birefringent structures of Type Ⅱ (red dashed line) and Type X (blue solid line); (c) retardance (blue) and transmission (red) images of birefringent structures written at different pulse densities; (d) retardance (blue) and transmission (red) images of birefringent structures written at different pulse durations

Fig. 8. High-speed writing of birefringent structure through pulse energy modulation^[101]. (a) Slow axis azimuth images of the writing structure at different repetition rates (1 MHz, 5 MHz, and 10 MHz); (b) simulation of light intensity distribution around nanobodies with different diameters; (c) contrast of writing birefringent structure before and after modulating energy; (d) simulation of temperature evolution of focal center

Fig. 9. Readout schematic for permanent optical storage by femtosecond laser direct writing. (a) Diagram of the setup of a birefringent microscope^[103]; (b) optical storage encoding method, data readout image，and decoding method^[37]

Fig. 10. Diagram of O-FIB^[108]. (a) Theoretical and experimental verification; (b) self-regulation of O-FIB; (c) curvature and separation control of O-FIB