  Second harmonic generation (SHG), as one of the most significant \c{hi}(2)
nonlinear optical processes, plays crucial roles in a broad variety of optical
and photonic applications. Designing various delicate schemes to achieve highly
efficient SHG has become a long standing and challenging topic in field of
nonlinear optics. Despite numerous success on SHG based on birefringent phase
matching and quasi-phase matching, so far, modal phase matching (MPM) for SHG
in tightly light-confined structures has still in its infancy. Here, we propose
a new scheme to realize highly-efficient SHG via MPM by using a nanophotonic
LiNbO3 thin-film waveguide consists of two bonded layers with internally
reversed polarizations. In such a dual-layer ridge waveguide based on lithium
niobate on insulator, upon optical excitation at 1574.6 nm, we observe SHG at
787.3 nm with ultrahigh conversion efficiency of 5,540% /W/cm/cm
experimentally. This work advances our understanding on modal-phase-matched SHG
and other quadratic optical nonlinear process, offering additional strategies
for development of high-performance nonlinear photonic devices in on-chip
platforms.
