  Periodically poled thin-film lithium niobate (TFLN) waveguides have emerged
as a leading platform for highly efficient frequency conversion in the
near-infrared. However, the commonly used silica bottom-cladding results in
high absorption loss at wavelengths beyond 2.5 $\mu$m. In this work, we
demonstrate efficient frequency conversion in a TFLN-on-sapphire platform,
which features high transparency up to 4.5 $\mu$m. In particular, we report
generating mid-infrared light up to 3.66 $\mu$m via difference-frequency
generation of a fixed 1-$\mu$m source and a tunable telecom source, with
normalized efficiencies up to 200%/W-cm$^2$. These results show
TFLN-on-sapphire to be a promising platform for integrated nonlinear
nanophotonics in the mid-infrared.
