Fig. 1. Schematic of femtosecond laser direct writing inside Li₂O-Nb₂O₅-SiO₂ glass setup. Laser propagation direction (along Z direction) is perpendicular to glass surface. Glass is fixed on translation stages to obtain line writing in XY plane

Fig. 2. Domain of femtosecond laser induced crystallization in Li₂O-Nb₂O₅-SiO₂ glasses with various repetition rates（f）and pulse energies（e）at different writing speeds and focus depths^[2,23]. Dashed markers illustrate regime 1 (Ⅰ), open markers for regime 2 (Ⅱ) and solid markers for regime 3 (Ⅲ). Dotted line marks boundary between regime 1 (Ⅰ) and regime 2 (Ⅱ). Dashed line marks boundary between regime 2 (Ⅱ) and regime 3 (Ⅲ). Solid black line marks laser modification at higher laser power^[23]. Insets are SEM images of laser modified area (in plane perpendicular to laser writing direction). Laser propagation direction is from top to bottom, illustrated by white down arrow. Laser parameters: 1030 nm, 300 fs, NA=0.6

Fig. 3. Femtosecond laser polarization controlled nanocrystal orientation and periodic structure orientation at the same time at moderate laser power. (a), (b) SEM images and (c), (d) corresponding inverse pole images (IPFs); (e), (f) 0001- pole figures (PFs), respectively. IPF is based on LiNbO₃ (R3c), coding the crystal axis along laser polarization direction. (a), (c), (e) Writing direction is parallel to laser polarization; (b), (d), (f) writing direction is perpendicular to laser polarization. Laser polarization directions are illustrated by two-headed arrows. Other parameters: 1030 nm, 250 fs, 250 kHz, 0.5 µJ/pulse, 1.5 µm/s, NA=0. 6, focal depth 320 µm, 33Li₂O-33Nb₂O₅-34SiO₂

Fig. 4. Phase shift interferometry images of femtosecond laser modified area (a) before and (b) after HF treatment. The sample is polished along XY plane (perpendicular to laser propagation direction). Other parameters: 1030 nm, 300 fs, 250 kHz, 0.8 µJ/pulse, 5 µm/s, NA = 0.6, focal depth 300 µm in air, 32.5Li₂O-27.5Nb₂O₅-40SiO₂. A broken edge [part illustrated by a lowermost arrow in Fig. 4(a)] is used to locate laser modified area

Fig. 5. Femtosecond laser induced crystallization and periodic structure inside glass with relation of writing mode at high laser power. (a)~(d) Writing direction is parallel to laser polarization (E, X direction); (e)~(h) writing direction (X direction) is perpendicular to laser polarization (Y direction, E, illustrated by two-headed arrows); (a), (e) SEM images of femtosecond laser induced four regions (A-D) and (b), (f) corresponding IPFs, respectively. IPF is based on LiNbO₃ (R3c), coding crystal axis along laser polarization direction; (c), (g) magnification of (a), (e) marked with solid rectangles, and (d), (h) schematic of laser-induced periodic structures. Other parameters: 1030 nm, 300 fs, 500 kHz, 1.0 µJ/pulse, 5 µm/s, NA=0. 6, focal depth 300 µm in air; 32.5Li₂O-27.5Nb₂O₅-40SiO₂

Fig. 6. femtosecond laser induced periodic structure inside glass at high laser power.(a) SEM image of laser track in XY plane (marked C and D regions);(b) magnified SEM image of D region, and (c) IPF. IPF is based on LiNbO₃ (R3c), coding crystal axis along laser propagation direction. Other parameters: 1030 nm, 300 fs, 500 kHz, 1.4 µJ/pulse, 5 µm/s, NA=0. 6, focal depth 300 µm in air, 32.5Li₂O-27.5Nb₂O₅-40SiO₂