Fig. 1. Comparison of terahertz output and pump evolution in LiNbO₃ and 4H-SiC crystals without pre-chirping. (a) Conversion efficiencies of terahertz generation versus propagation distance L along the z direction. (b) Terahertz spectra |A_THz|² in the linear scale at the optimal L in (a), multiplied by 1, 10^–2 and 10^–1, respectively. (c), (d) The corresponding pump evolution processes influenced by cascading and SPM effects, including depletion, widening and frequency shift compared with the input pump. In (d), low- and high-intensity input pumps are normalized to distinguish their difference more clearly.

Fig. 2. TPF OR conversion efficiency (color scale) along the propagation distance L for various GDD values at six TPF angles from 31° to 38°. The white curves represent the maximum efficiency (η_max) obtained by optimizing the GDD at each L. Areas I, II and III indicate different OR efficiencies affected by GDD, defined as excessive, insufficient and suitable pre-chirping, respectively.

Fig. 3. Terahertz spectrum intensity |A_THz|² (color scale) as a function of GDD at six TPF angles from 31° (a) to 38° (f), when L = 4 mm. The white curves indicate the conversion efficiency regulated by GDD.

Fig. 4. Spectrum tuning of TPF OR in 4H-SiC. (a) Terahertz spectrum intensity |A_THz|² at six TPF angles under optimal GDD when L = 4 mm. (b) Shift of peak frequency versus TPF angle. The black line is calculated by the PM condition cosγ = n_g/n_T, while the red line is simulated by the 1D model.

Fig. 5. The evolution of terahertz spectrum intensity |A_THz|² (color scale) along the propagation distance L at six TPF angles, under optimal GDD and at L = 4 mm. The white curve indicates the conversion efficiency as a function of L.

Fig. 6. The evolution of the terahertz spectrum ((a) and (c)) and temporal waveform ((b) and (d)) with L at two different conditions: (a), (b) γ = 31.5°, GDD = 4200 fs²; (c), (d) γ = 33°, GDD = 5050 fs². The former forms only one continuous terahertz pulse throughout the crystal, while the latter forms two independent pulses.

Fig. 7. Modulation of the terahertz spectra (a) and temporal waveform (b) by varying pre-chirping when γ = 32° and L = 4 mm. Different terahertz spectra (flat-top broadband, efficient and concentrated, split and widened) and temporal waveforms (single- to multi-cycle pulses) can be flexibly tuned by varying the GDD.

Fig. 8. Terahertz spectrum ((a) and (c)) and temporal waveform ((b) and (d)) at different TPF angles (γ = 31° and γ = 38°) and pre-chirping (GDD = 800, 4000 and 8000 fs²) at the corresponding optimal L. The insets in (d) show the initially established terahertz signal when the pre-chirped pump pulse enters the crystal.