To account for interband absorption in our model, we considered the factorF=02Lexp(αPz)exp(αSFG(2Lz))dz2L=exp(2αPL)1exp(2(αSFGαP)L)2L(αSFGαP)(3)where αP and αSFG are the energy-dependent absorption coefficients of the pump and SFG, respectively, and L = 12 μm is the sample thickness. This factor takes into account the absorption of the pump (first term on the integrand) and the SFG (second term on the integrand) beams, in either forward (z from 0 to L) or backward (z from L to 2L) direction of the NIR excitation through the QCL, as shown schematically in Fig. 3B.