| Summary: | We present a single stack active region design for terahertz emission by difference frequency generation in quantum cascade lasers. The active region contains a single design, which is based on multiple optical transitions within one period. This results in both a giant nonlinearity and an ultra-broad optical gain. The provided optical gain spectrum is broad enough to support two distinct mid-infrared modes with a significant spectral separation. For dual-emission, the waveguide contains a buried, index-coupled distributed feedback grating. This grating is based on a sampled approach to provide selective feedback for two midinfrared modes at λ<sub>1</sub> = 8.39 μm and λ<sub>2</sub> = 9.38 μm. Simultaneously, the manifold of possible transitions within the active region is designed to provide a peak nonlinear susceptibility of |χ<sup>(2)</sup>| = 29 nmV<sup>-1</sup> at the conversion to 3.8 THz. The device emits up to 210-μW THz power at room temperature with a nonlinear conversion efficiency of η = 2.05 mWW<sup>-2</sup>.
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