Apparatus and methods for generating radiation via difference frequency generation (DFG). In one exemplary implementation, a quantum cascade laser (QCL) has a significant second-order nonlinear susceptibility (χ(2)) integrated in an active region of the QCL. The QCL is configured to generate first radiation at a first frequency ω1, second radiation at a second frequency ω2, and third radiation at a third frequency ω3=ω1−ω2 based on difference frequency generation (DFG) arising from the non-linear susceptibility. In one aspect, the QCL may be configured to generate appreciable THz radiation at room temperature.