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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 &#x03BB;1 = 8.39 &#x03BC;m and &#x03BB;2 = 9.38 &#x03BC;m. Simultaneously, the manifold of possible transitions within the active region is designed to provide a peak nonlinear susceptibility of |&#x03C7;(2)| = 29 nmV-1 at the conversion to 3.8 THz. The device emits up to 210-&#x03BC;W THz power at room temperature with a nonlinear conversion efficiency of &#x03B7; = 2.05 mWW-2.

Single Stack Active Region Nonlinear Quantum Cascade Lasers for Improved THz Emission