  Generating widely tunable, continuous wave light at long wavelengths via
difference frequency generation (DFG) remains challenging due to high
absorption and dispersion. The relatively new platform of thin film lithium
niobate enables high-confinement nonlinear waveguides, which could improve
efficiency. We simulated DFG in thin film lithium niobate waveguides that are
periodically poled for surface emission at 30 THz. Maximum efficiency for a 1
cm device is 9.16 $\times$ 10$^{-6}$ W$^{-1}$ assuming $d_{33}$ = 30 pm/V. The
tuning range within 50$\%$ of efficiency at 30 THz is as wide as 25 THz, from
25 THz to 50 THz.
