A broadly tunable THz source is realized via difference frequency generation,in which an enhancement to X(3) that is obtained via resonant excitation of Ⅲ-Ⅴ semiconductor quantum well excitons is utilized.The symmetry of the quantum wells (QWs) is broken by utilizing the built-in electric-field across a p-i-n junction to produce effective X(2)processes,which are derived from the high X(3).This X(2) media exhibits an onset of nonlinear processes at ～4Wcm 2,thereby enabling area (and,hence,power) scaling of the THz emitter.Phase matching is realized laterally through normal incidence excitation.Using two collimated 130 mW continuous wave (CW) semiconductor lasers with ～1-mm beam diameters,we realize monochromatic THz emission that is tunable from 0.75 to 3 THz and demonstrate the possibility that this may span 0.2-6 THz with linewidths of ～20 GHz and efficiencies of ～1 × 10-5,thereby realizing ～800 nW of THz power.Then,transmission spectroscopy of atmospheric features is demonstrated,thereby opening the way for compact,low-cost,swept-wavelength THz spectroscopy.