Fig. 1. Interaction between femtosecond laser and material. (a) Timescale of a femtosecond laser exciting electron and lattice processes in solid^[28]; (b) ionization rate for fused silica with a gap of 9 eV from Keldysh’s theory (solid line) as function of laser intensity, and their comparison between multiphoton ionization (dotted line) and tunnel ionization (dash-dotted line)^[30]

Fig. 2. Forming mechanisms and applications of Type I structure. (a) Laser beam is used to modify the refractive index of the material to create the waveguide; (b) refractive index of Type Ⅰ material shows a uniform positive change accompanied by darkening^[49]; (c) element distribution caused by the heat transfer induced by temperature-gradient-driven diffusion^[52]; (d) the structure is formed after the laser with repetition of 25 MHz, pulse width of 30 fs, energy of 5 nJ is focused by an objective lens with a numerical aperture of 1.4^[56]; (e) X directional coupler formed by Type Ⅰ modification^[60]

Fig. 3. Ultrafast laser-induced Type Ⅲ/Ⅳ structures. (a) SEM images of the cross-section of nanopores in sapphires^[63]; (b) storing binary data in molten silicon dioxide^[73]; (c) storing images in molten silicon dioxide^[73]; (d) ultrashort laser pulses triggering space-limited micro-explosion to form high density phases^[67]; (e) optical preparation of three-dimensional photonic crystals in high refractive index LiNbO₃ crystals^[71]

Fig. 4. Formation mechanism of nano-grating. (a) SEM image of nano-grating^[19]; (b) incident light-polaron interference model^[19]; (c) anisotropic growth model of nano-plasma^[76]; (d) surface plasmon resonance model^[78]; (e) schematic diagram of electromagnetic formation mechanism of periodic nanostructures^[81]

Fig. 5. Five-dimensional (5D) optical storage based on nanograting. (a) Schematic diagram of 5D optical storage based on nano-grating^[85]; (b) birefringence model of nanograting; (c) pictures of time capsule samples^[21]

Fig. 6. Pulse width modulation effect^[88]. (a) Retardance (left) and transmission (right) images of birefringent structures written at different durations; (b) refractive index changes of modified regions written with different pulse widths; (c) retardance and transmittance of modified regions corresponding to different pulse widths; (d) SEM images of Type X and Type Ⅱ structures after polishing and etching

Fig. 7. Thermal modulation of nanograting^[90]. (a) Comparison of birefringence structures before and after modulating energy; (b) simulation of temperature evolution in the focus center; (c) simulation of laser intensity distribution around nanocrystals with different diameters; (d) birefringence photo of the structures induced by two seed pulses and eight subsequent pulses; (e) SEM images of nano-layered structures after polishing and KOH etching

Fig. 8. Solutions for the challenges which eternal optical storage is facing. (a) Simulations of the electric field intensity distribution^[41]; (b) slow-axis orientation map of the free-form written nanogrooves by OFIB technology (the retardance at a wavelength of 546 nm is 9 nm)^[41]; (c) schematic of picosecond laser space-time control optical path^[92]; (d) relationship between retardance and pulse number^[91]