Terahertz (THz) waves are in demand for applications in high-speed wireless communications, nondestructive material testing, macromolecular spectral analysis, and medical detection of biological tissues. However, the lack of high-power, miniaturization, and affordable THz radiation sources has severely limited the application of THz waves in the above-mentioned fields. Cascaded difference frequency generation (CDFG), in which a pump photon can continuously generate multiple THz photons simultaneously through nonlinear optical effects, breaks the limits of the Manley-Rowe relationship and substantially improves the energy conversion efficiency of THz waves. In this study, we propose a method for efficient THz wave generation by coupled cascaded difference frequency generation (CCDFG), which uses two sets of CDFG to jointly generate and amplify THz waves. The THz wave generated by this method can provide positive feedback to the CCDFG, further driving it to expand to higher-order Stokes differential frequencies, thus significantly improving the THz wave energy conversion efficiency. We hope that this new scheme will facilitate the generation of high-power efficient THz waves.

A pump wave with a wavelength of 532 nm stimulated an adhesive-free-bonded KTiOPO₄ (AFB-KTP) crystal generating dual signal waves and dual idler waves using a coupled optical parametric process. The dual signal waves and dual idler waves stimulated two CDFG in an aperiodic periodically poled lithium niobate (APPLN) crystal, generating two sets of cascaded optical waves and THz waves simultaneously. The above two CDFG were intensely coupled by THz waves with identical frequency, polarization direction, and propagation direction. The CCDFG generated and amplified the THz wave, and the amplified THz wave enhanced the CCDFG simultaneously, resulting in the further frequency transformation from dual signal waves and dual idler waves to high-order Stokes waves. Moreover, the cascaded optical waves were further transformed to high-order Stokes waves by depressing the phase mismatches of cascaded Stokes processes and enlarging the phase mismatches of cascaded anti-Stokes processes simultaneously, thus substantially improving the THz wave energy conversion efficiency.

With a pump intensity of 4000 MW/cm² and a poling period of 6104.9864 μm of AFB-KTP, two signal waves with wavelengths of 289.0173 THz and 288.0173 THz respectively, and two idler waves with wavelengths of 274.8924 THz and 275.8924 THz are generated. The power densities of the two signal waves are 1022.77 MW/cm² and 1027.10 MW/cm² respectively, and the power densities of the two idler waves are 972.78 MW/cm² and 983.85 MW/cm² respectively (Fig. 2). At 100 K, a THz wave with a power density of 1483.6 MW/cm² is realized, corresponding to energy conversion efficiencies of 37%. The frequencies of the cascaded optical waves are converted from 280 THz to about 70 THz, indicating that one signal or idler wave photon can produce 210 THz photons (Fig. 3). The majority of signal wave photons and idler wave photons in a single set of CDFG under the same conditions are transferred to approximately 120 THz (Fig. 4). At 300 K, the absorption coefficient of the APPLN crystal for THz waves is significantly large, limiting the transfer of CCDFG to higher-order Stokes waves. The generated THz wave, therefore, has a power density of 183.7 MW/cm², corresponding to energy conversion efficiencies of.4.6% (Fig. 5). The power densities of THz waves generated by CCDFG at the same temperature are 177.9 MW/cm² and 182.8 MW/cm², and the power densities of THz waves generated by a single set of CDFG are 58.8 MW/cm² and 53.1 MW/cm², respectively (Fig. 6). At both 100 K and 300 K, the THz wave power densities generated by the CCDFG is considerably higher than the sum of the THz wave power densities generated by the two sets of CDFG excited by dual signal waves and dual idler waves.

In this paper, a scheme to efficiently generate THz waves by CCDFG is proposed that substantially improves the energy conversion efficiency of THz waves. Unlike the reported scheme using two near-infrared (NIR) laser beams for cascaded difference frequency generation of THz waves, we use one high-energy laser beam and two NIR seed beams to generate two signal waves and two idle frequency waves in an AFB-KTP crystal. The two signal waves and two idler waves are generated in the same APPLN crystal by exciting a set of CDFG processes to generate THz waves simultaneously. The THz waves can effectively couple two sets of CDFG together to amplify the THz waves, and the amplified THz waves can provide positive feedback to the CCDFG to further drive the pump photon to higher-order Stokes light transfer, which substantially improves the THz wave energy conversion efficiency.