  Second-order optical nonlinear effects (second-harmonic and sum-frequency
generation) are demonstrated in the telecommunication band by periodic poling
of thin films of lithium niobate wafer-bonded on silicon substrates and
rib-loaded with silicon nitride channels to attain ridge waveguide with
cross-sections of ~ 2 {\mu}m2. The compactness of the waveguides results in
efficient second-order nonlinear devices. A nonlinear conversion of 8% is
obtained with a pulsed input in 4 mm long waveguides. The choice of silicon
substrate makes the platform potentially compatible with silicon photonics, and
therefore may pave the path towards on-chip nonlinear and quantum-optic
applications.
