Fig. 1. Fundamental modes of ultrafast laser induced refractive index change in LN^[31-32]. (a) Single-line structure based on Type-I modification; (b) double-line structure based on Type-Ⅱ modification; (c) depressed-cladding structure; (d) optical-lattice cladding structure; (e) 1×3 splitter made of optical-lattice cladding structures

Fig. 2. Ultrafast laser-induced void structures in LN^[44-45]. (a) Optical image of void structures; (b) SEM image of a FIB-slice of a void structure; (c) bright-field TEM image of a void structure

Fig. 3. Ultrafast laser-induced amorphous structures in LN^{[17, 55]}. (a) SEM image of etched amorphous region; (b) SEM image of amorphous voxel array; (c) photonic crystal structure made of amorphous voxels; (d) schematic of single-pulse ultrafast laser-induced LN amorphization mechanism; (e) TEM image and selected area electron diffraction pattern of amorphous region

Fig. 4. Mechanism of ultrafast laser-induced domain inversion/erasure^[57]. (a) Simulated thermoelectric field excited by a Gaussian laser beam; (b) threshold electric field for LN domain inversion; (c) Z-component of the excited thermoelectric field; (d) when the laser scans along +Z direction, the domain inversion induced by E₁ will be erased by E₂, and thus no domain structure will be induced; (e) when the laser scans along the -Z direction, domain structure will be induced; (f) when the laser scans along the +Y/-Y direction, domain structure can be partially induced; (g) erasing the induced domain structure by a laser scanning along +Z direction

Fig. 5. Ultrafast laser-induced nanoscale domain structures in LN^[57]. (a) Domain structures with different widths; (b) a domain tip of 30 nm width fabricated by applying laser erasure

Fig. 6. Hybrid photonic devices constructed from different modifications^[73]. (a) Direct integration of waveguide and embedded Bragg gratings in LN; (b) extraordinary polarized mode in the waveguide; (c) top view of Bragg grating with a period of 704 nm; (d) cross section of a Bragg grating waveguide with integrated electrodes

Fig. 7. Ultrafast laser fabrication of depressed cladding waveguides in PPLN^[74]. (a) Schematic of processing scheme; (b)‒(d) written depressed cladding waveguides with different cross-section sizes

Fig. 8. Ultrafast laser prepared NPC in LN^[29-30]. (a) Schematic of ultrafast laser direct writing ferroelectric domains in an LN waveguide; (b) optical micrograph of two-dimensional patterns of optically poled domain; (c) three-dimensional profile of inverted domains obtained by Cherenkov second-harmonic microscopy; (d) Cherenkov second-harmonic microscopic image of a three-dimensional NPC; (e) second harmonic image of NPC in the XY plane; (f) distribution of second harmonic intensities along the black line in (e), the intensity of the second harmonic in the modified region is much lower than that in the non-modified region, confirming the significant reduction of the nonlinear coefficient caused by laser irradiation

Fig. 9. Fabrication of nonlinear beam shaping devices in LN^[80]. (a) Two-dimensional fork-grating nonlinear NPC for nonlinear beam shaping; (b) three-dimensional fork-grating nonlinear NPC applied to greatly improve conversion efficiency of nonlinear beam shaping

Fig. 10. Ultrafast laser-induced amorphization for multi-dimensional optical data storage^[17]. (a) Three-dimensional pixelated structural colors written inside LN by ultrafast laser; (b) five-dimensional data encoding by multiplexing color and intensity signals; (c) five-dimensional data readout by image recognition