Materials with strong secondorder (
Thinfilm lithiumniobateoninsulator (LNOI) has emerged as a superior integratedphotonics platform for linear, nonlinear, and electrooptics. Here we combine quasiphasematching, dispersion engineering, and tight mode confinement to realize nonlinear parametric processes with both high efficiency and wide wavelength tunability. On a millimeterlong, Zcut LNOI waveguide, we demonstrate efficient (
 Award ID(s):
 1842680
 NSFPAR ID:
 10166472
 Publisher / Repository:
 Optical Society of America
 Date Published:
 Journal Name:
 Optics Letters
 Volume:
 45
 Issue:
 13
 ISSN:
 01469592; OPLEDP
 Page Range / eLocation ID:
 Article No. 3789
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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${\mathrm{\chi <\#comment/>}}^{\phantom{\rule{negativethinmathspace}{0ex}}(2)}$ ) optical nonlinearity, especially lithium niobate, play a critical role in building optical parametric oscillators (OPOs). However, chipscale integration of lowloss${\mathrm{\chi <\#comment/>}}^{\phantom{\rule{negativethinmathspace}{0ex}}(2)}$ materials remains challenging and limits the threshold power of onchip${\mathrm{\chi <\#comment/>}}^{\phantom{\rule{negativethinmathspace}{0ex}}(2)}$ OPO. Here we report an onchip lithium niobate optical parametric oscillator at the telecom wavelengths using a quasiphasematched, highquality microring resonator, whose threshold power ($\sim <\#comment/>30\phantom{\rule{thickmathspace}{0ex}}\text{\xb5<\#comment/>}\mathrm{W}$ ) is 400 times lower than that in previous${\mathrm{\chi <\#comment/>}}^{\phantom{\rule{negativethinmathspace}{0ex}}(2)}$ integrated photonics platforms. An onchip power conversion efficiency of 11% is obtained from pump to signal and idler fields at a pump power of 93 µW. The OPO wavelength tuning is achieved by varying the pump frequency and chip temperature. With the lowest power threshold among all onchip OPOs demonstrated so far, as well as advantages including high conversion efficiency, flexibility in quasiphasematching, and device scalability, the thinfilm lithium niobate OPO opens new opportunities for chipbased tunable classical and quantum light sources and provides a potential platform for realizing photonic neural networks. 
Quasiphasematched interactions in waveguides with quadratic nonlinearities enable highly efficient nonlinear frequency conversion. In this paper, we demonstrate the first generation of devices that combine the dispersion engineering available in nanophotonic waveguides with quasiphasematched nonlinear interactions available in periodically poled lithium niobate (PPLN). This combination enables quasistatic interactions of femtosecond pulses, reducing the pulse energy requirements by several orders of magnitude compared to conventional devices, from picojoules to femtojoules. We experimentally demonstrate two effects associated with second harmonic generation (SHG). First, we observe efficient quasiphasematched SHG with
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