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Lithium Niobate (LN) is an important nonlinear optical material. Here we demonstrate LN microdisk resonators that feature optical quality factor ~ 10 5 , realized using robust and scalable fabrication techniques, that operate over a wide wavelength range spanning visible and near infrared. Using our resonators, and leveraging LN's large second order optical nonlinearity, we demonstrate on-chip second harmonic generation with a conversion efficiency of 0.109 W -1 .

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Integrated thin-film lithium niobate platform has recently emerged as a promising candidate for next-generation, high-efficiency wavelength conversion systems that allow dense packaging and mass-production. Here we demonstrate efficient, phase-matched second harmonic generation in lithographically-defined thin-film lithium niobate waveguides with sub-micron dimensions. Both modal phase matching in fixed-width waveguides and quasi-phase matching in periodically grooved waveguides are theoretically proposed and experimentally demonstrated. Our low-loss (~3.0 dB/cm) nanowaveguides possess normalized conversion efficiencies as high as 41% W^-1cm^-2.

Synthetic single-crystal diamond has recently emerged as a promising platform for Raman lasers at exotic wavelengths due to its giant Raman shift, large transparency window and excellent thermal properties yielding a greatly enhanced figure-of-merit compared to conventional materials [1, 2, 3]. To date, diamond Raman lasers have been realized using bulk plates placed inside macroscopic cavities [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13], requiring careful alignment and resulting in high threshold powers (~W-kW). Here we demonstrate an on-chip Raman laser based on fully-integrated, high quality-factor, diamond racetrack microresonators embedded in silica. Pumping at telecom wavelengths, we show Stokes output discretely tunable over a ~100nm bandwidth around 2-μm with output powers >250 μW, extending the functionality of diamond Raman lasers to an interesting wavelength range at the edge of the mid-infrared spectrum [14]. Continuous-wave operation with only ~85 mW pump threshold power in the feeding waveguide is demonstrated along with continuous, mode-hop-free tuning over ~7.5 GHz in a compact, integrated-optics platform.Diamond serves as a compelling material platform for Raman lasers operating over a wide spectrum due to its superlative Raman frequency shift (~40 THz), large Raman gain (~10 cm/GW @ ~1-μm wavelength) and ultra-wide transparency window (from UV (>220nm) all the way to THz, except for a slightly lossy window from ~2.6 -6 μm due to multiphonon-induced absorption) [1,15]. Furthermore, the excellent thermal properties afforded by diamond (giant thermal conductivity of ~1800 W/m/K @ 300K and low thermo-optic coefficient of ~10 -5 /K) [1,2] along with negligible birefringence [3,15] make it an ideal material for high-power Raman lasing with greatly reduced thermal lensing effects [1,3].The availability of CVD-grown, high-quality polished, single-crystal diamond plates has enabled the development of bulk Raman lasers using macroscopic optical cavities across the UV [6], visible [4, 5], near-infrared [7, 8, 9, 10, 11, 12] and even mid-infrared [13] regions of the optical spectrum. Although showing great performance with large output powers (many Watts) [12] and near quantum-limited conversion efficiencies [5,9], most operate in pulsed mode in order to attain the very high pump powers required to exceed the Raman lasing threshold [5,6,11,12]. Demonstration of continuous-wave diamond Raman lasing has been challenging, with very few reports [3,7,8]. Bulk cavity systems also require precise alignment and maintenance of optical components for the laser to function robustly.