Fig. 1. Schematics of CCDFG generating THz wave. (a) Schematic of photon interactions, where laser fill ratio indicates intensity of laser, red arrow indicates red-shifted process of cascaded optical difference frequency, and blue arrow indicates blue-shifted process of cascade optical difference frequency; (b) evolution of wave vectors; (c) schematic of experiment, where M₁ and M₂ can realize total reflection to dual idler waves, M₃ can realize total reflection to dual idler waves and full transmission to dual signal wave, half wave plate changes dual idler waves from o-wave to e-wave, polarizer separates orthogonally polarized signal wave from idler wave, PE is polyethylene filter plate to filter out residual cascaded optical waves, and beam filter is used to remove residual pump waves

Fig. 2. At 300 K temperature, power density of pump wave, dual signal waves, and dual idle waves generated by coupled optical parametric amplification in AFB-KTP crystal (pump wavelength λp=532 nm, pump power density I_p=4000 MW/cm², power density of seed ωs1z and ωs2z is 1/10¹⁹ of pump power density)

Fig. 3. At 100 K temperature, CCDFG generates THz wave and cascaded optical waves, where power density of ωs1z and ωs2z is 1022.77 MW/cm² and 1027.10 MW/cm², respectively, and power density of ωi1z and ωi2z is 972.78 MW/cm² and 983.85 MW/cm², respectively. (a) Poling period versus crystal length; (b) phase mismatch distributions of each CDFG stimulated by dual signal waves; (c) phase mismatch distributions of each CDFG stimulated by dual idler waves; (d) evolution of THz wave, ωs,mz and ωi,mz generated by stimulating CCDFG with dual signal waves and dual idler waves, where I_c-s and I_c-i indicate the power density of ωs,mz and ωi,mz; (e) distribution of cascaded optical waves ωs,mz at crystal lengths x'= 0, 3.02, 5.00, 6.79, 7.95 mm; (f) distribution of cascaded optical waves ωi,mz at crystal lengths x'= 0, 3.02, 5.00, 6.79, 7.95 mm

Fig. 4. At 100 K, evolution of THz wave and cascaded optical waves generated in CCDFG and CDFG, where I_s1=1022.77 MW/cm²,I_s2=1027.10 MW/cm², I_i1=972.78 MW/cm², I_i2 =983.85 MW/cm². (a) Evolution of ωs,mz in CCDFG and CDFG, where I_c-s denotes power density of ωs,mz generated by stimulating CCDFG with dual signal waves and dual idler waves, I_s denotes power density of ωs,mz generated by CDFG; (b) evolution of ωi,mz in CCDFG and CDFG, where I_c-i denotes power density of ωi,mz generated by stimulating CCDFG with dual signal waves and dual idler waves, and I_i denotes power density of ωi,mz generated by CDFG; (c) THz wave power density versus crystal length, where I_T,c-s denotes power density of THz wave generated by CCDFG, I_T,c-i denotes power density of THz wave generated by CCDFG, I_T,s and I_T,i denote power density of THz wave generated by stimulating CDFG with dual signal waves and dual idler waves, respectively

Fig. 5. At 300 K temperature, CCDFG generates THz wave and cascaded optical waves, where power density of ωs1z and ωs2z is 1022.77 MW/cm² and 1027.10 MW/cm², respectively, and power density of ωi1z and ωi2z is 972.78 MW/cm² and 983.85 MW/cm². (a) Variation of poled period with crystal length; (b) phase mismatch distribution of each CDFG stimulated by dual signal waves; (c) phase mismatch distribution of each CDFG stimulated by dual idler waves; (d) evolution of THz wave, ωs,mz and ωi,mz generated by stimulating CCDFG with dual signal waves and dual idler waves; (e) distribution of cascaded optical waves ωs,mz at crystal length x'= 0, 2.28, 4.98, 5.56, 5.95 mm; (f) distribution of cascaded optical waves ωi,mz at crystal length x'= 0, 2.28, 4.98, 5.56, 5.95 mm

Fig. 6. At 300 K, evolution of THz wave and cascaded optical waves in CCDFG and CDFG, where I_s1=1022.77 MW/cm²,I_s2=1027.10 MW/cm²,I_i1=972.78 MW/cm²,I_i2=983.85 MW/cm². (a) Evolution of ωs,mz in CCDFG and CDFG, where I_c-s denotes power density of ωs,mz generated by stimulating CCDFG with dual signal waves and dual idler waves, and I_s denotes power density of ωs,mz generated by CDFG; (b) evolution of ωi,mz in CCDFG and CDFG, where I_c-i denotes power density of ωi,mz generated by stimulating CCDFG with dual signal waves and dual idler waves, I_i denotes power density of ωi,mz generated by CDFG; (c) THz wave power density versus crystal length

Fig. 7. THz wave power density (I_T,c-i) and energy conversion efficiency (η) generated by stimulating CCDFG at different pump power densities, where λp=532 nm, power density of seeds ωs1z and ωs2z is 1/10¹⁹ of pump wave power density