Integrated GaN photonic circuits on silicon (100) for second harmonic   generation
  We demonstrate second order optical nonlinearity in a silicon architecture through heterogeneous integration of single-crystalline gallium nitride (GaN) on silicon (100) substrates. By engineering GaN microrings for dual resonance around 1560 nm and 780 nm, we achieve efficient, tunable second harmonic generation at 780 nm. The \{chi}(2) nonlinear susceptibility is measured to be as high as 16 plus minus 7 pm/V. Because GaN has a wideband transparency window covering ultraviolet, visible and infrared wavelengths, our platform provides a viable route for the on-chip generation of optical wavelengths in both the far infrared and near-UV through a combination of \{chi}(2) enabled sum-/difference-frequency processes. 
Chi Xiong, Wolfram Pernice, Kevin K. Ryu, Carsten Schuck, King Y.   Fong, Tomas Palacios and Hong X. Tang
physics.optics cond-mat.mtrl-sci