  Achieving efficient terahertz (THz) generation using compact turn-key sources
operating at room temperature and modest power levels represents one of the
critical challeges that must be overcome to realize truly practical
applications based on THz. Up to now, the most efficient approaches to THz
generation at room temperature -- relying mainly on optical rectification
schemes -- require intricate phase-matching set-ups and powerful lasers. Here
we show how the unique light-confining properties of triply-resonant photonic
resonators can be tailored to enable dramatic enhancements of the conversion
efficiency of THz generation via nonlinear frequency down-conversion processes.
We predict that this approach can be used to reduce up to three orders of
magnitude the pump powers required to reach quantum-limited conversion
efficiency of THz generation in nonlinear optical material systems.
Furthermore, we propose a realistic design readily accesible experimentally,
both for fabrication and demonstration of optimal THz conversion efficiency at
sub-W power levels.
