Fig. 1. Diagram of phase (velocity) matching of tilted wavefront device and resulting THz wave. (a) Typical optical path; (b) tilted wavefront angle; (c) phase (velocity) matching

Fig. 2. Structure diagram of lithium niobate^[30]. (a) Above Curie temperature; (b) below Curie temperature. Red balls represent O^2-, white balls represent Li⁺, and green balls represent Nb⁵⁺

Fig. 3. Li₂O-Nb₂O₅ binary phase diagram^[47]

Fig. 4. 3 inch SLN crystal

Fig. 5. Large-aperture lithium niobate crystal. (a) z-axis 8-inch lithium niobate crystal (equal-diameter height 60 mm); (b) x-axis 8-inch lithium niobate crystal (equal-diameter height 65 mm)

Fig. 6. Phase matching, phase mismatching, and quasi-phase matching^[77]

Fig. 7. Schematic diagram of generating multi-period strong-field THz radiation based on PPLN quasi-phase matching

Fig. 8. Metasurface micro-nano-processing and THz enhancement effect of lithium niobate^[95]