Second-harmonic generation in single-mode integrated waveguides through   mode-shape modulation
  A simple and flexible technique for achieving quasi-phase-matching in integrated photonic waveguides without periodic poling is proposed and experimentally demonstrated, referred to as mode-shape-modulation (MSM). It employs a periodic variation of waveguide width to modulate the intensity of the pump wave, effectively suppressing out-of-phase light generation. This technique is applied to the case of second-harmonic generation in thin-film lithium niobate ridge waveguides. MSM waveguides are fabricated and characterized with pulsed-pumping in the near-infrared, showing harmonic generation at a signal wavelength of 784 nm. 
Jeff Chiles, Seyfollah Toroghi, Ashutosh Rao, Marcin Malinowski,   Guillermo Fernando Camacho-Gonz\'alez, Sasan Fathpour
physics.optics