Patent ID: 7974325

Claim:
An apparatus, comprising: a quantum cascade laser (QCL) having a significant second-order nonlinear susceptibility (X (2) ) integrated in an active region of the QCL, the QCL configured to generate first radiation at a first frequency ω 1 , second radiation at a second frequency ω 2 , and third radiation at a third frequency ω 3=ω 1 −ω 2 based on difference frequency generation (DFG) arising from the nonlinear susceptibility; wherein the QCL comprises: a first quantum cascade (QC) structure configured to generate the first radiation; and a second QC structure configured to generate the second radiation, wherein the nonlinear susceptibility is integrated in at least one of the first QC structure and the second QC structure; wherein the first QC structure includes at least one two-phonon structure; and the second QC structure includes at least one bound-to-continuum structure.