  Periodically poled lithium niobate (PPLN) waveguide is a powerful platform
for efficient wavelength conversion. Conventional PPLN converters however
typically require long device lengths and high pump powers due to the limited
nonlinear interaction strength. Here we use a nanostructured PPLN waveguides to
demonstrate an ultrahigh normalized efficiency of 2600%/W-cm$^2$ for
second-harmonic generation of 1.5-$\mu$m radiation, more than 20 times higher
than that in state-of-the-art diffused waveguides. This is achieved by a
combination of sub-wavelength optical confinement and high-fidelity periodic
poling at a first-order poling period of 4 $\mu$m. Our highly integrated PPLN
waveguides are promising for future chip-scale integration of classical and
quantum photonic systems.
