Fig. 1. Schematics of the interaction between cascaded optical waves and terahertz wave in cascaded difference frequency generation. m and M are integers, representing cascaded order. (a) Spectrum of two input laser beams; (b) terahertz wave ω_T1 generation by cascaded difference frequency, m－M＝1; (c) terahertz wave ω_T2 generation by cascaded difference frequency, taking m－M＝10 as an example

Fig. 2. Schematics of experimental structure of cascaded difference frequency generation. (a) Experimental structure diagram; (b) sectional drawing of slab waveguide, d₁ and d₂ denote thickness of waveguides Ⅰ and Ⅱ，respectively, r₁ and r₂ denote core thickness of waveguides Ⅰ and Ⅱ，respectively, L₁ and L₂ denote length of waveguides Ⅰ and Ⅱ，respectively; (c) relationship between effective refractive index of terahertz wave and cascaded order m when Δk_m is equal to 0, taking ω_T2＝5 THz as an example

Fig. 3. TE fundamental mode field distributions of terahertz wave in slab waveguide, terahertz wave frequency is 5 THz. (a) Slab waveguide Ⅰ；(b) slab waveguide Ⅱ；(c) sectional drawing of mode fields distributions

Fig. 4. Terahertz wave and cascaded optical spectra generated by cascaded difference frequency. (a) Cascaded optical spectra evolution in PPLN crystal, ω_T1＝0.5 THz; (b) cascaded optical waves distribution in PPLN crystal, ω_T1＝0.5 THz; (c) terahertz wave intensities and cascaded optical spectra versus slab waveguides length, ω_T2＝5 THz; (d) phase mismatch distributions for each order of different frequency in slab waveguides, ω_T2＝5 THz; (e) cascaded optical waves distributions in slab waveguides, ω_T2＝5 THz

Fig. 5. Terahertz wave and cascaded optical spectra generated by cascaded difference frequency in APPLN crystal, ω_T2＝5 THz. (a) Terahertz wave intensities and cascaded optical spectra versus crystal length; (b) phase mismatch distributions of cascaded difference frequency generation in APPLN crystal; (c) cascaded optical waves distributions in APPLN crystal