A system and method that may enable the generation of highly-efficient, high-power, narrow-linewidth, and tunable light sources from microwave frequencies to mid-infrared wavelengths is disclosed. The light source comprises a nonlinear medium coupled to a multi-modal cavity and a high-energy pump source. The nonlinear medium provides three-wave mixing between modes present in the cavity to generate, for example, terahertz waves. The broadband cavity enables cascading nonlinear processes. By engineering the Q-factors of the cavity's many modes, red-shifted (stokes) cascaded nonlinear processes strongly dominate over their blue-shifted (anti-stokes) counterparts, resulting in a quasi-complete depletion of the pump energy into the THz mode.