  Thin-film lithium niobate (TFLN) is an emerging platform for compact,
low-power nonlinear-optical devices, and has been used extensively for
near-infrared frequency conversion. Recent work has extended these devices to
mid-infrared wavelengths, where broadly tunable sources may be used for
chemical sensing. To this end, we demonstrate efficient and broadband
difference frequency generation between a fixed 1-micron pump and a tunable
telecom source in uniformly-poled TFLN-on-sapphire by harnessing the
dispersion-engineering available in tightly-confining waveguides. We show a
simultaneous 1-2 order-of-magnitude improvement in conversion efficiency and
~5-fold enhancement of operating bandwidth for mid-infrared generation when
compared to conventional lithium niobate waveguides. We also examine the
effects of mid-infrared loss from surface-adsorbed water on the performance of
these devices.
