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Silicon photonics, based on silicon-on-insulator waveguides, has been successful in establishing itself as a reliable solution for datacom optical transceivers. However, due to the centrosymmetry of silicon’s crystalline structure and the omnipresence of nonlinear absorptions, conventional silicon photonics has not been as successful for second- and third-order nonlinear integrated-optic applications, respectively. Traditionally, second-order nonlinear integrated optics, X(2) effects, have been pursed in waveguides out of materials such as lithium niobate. However, these devices have their own downsides, such as low optical confinement and bulkiness. A solution is heterogeneously integrating ultrathin layers of such noncentrosymmetric materials on silicon substrates, in order to simultaneously achieve efficient X(2) effects and high optical confinement/compactness, as well as partial or full compatibility with silicon foundry processing. Similarly, researchers have for the most part migrated to heterogeneous integration of materials with high third-order nonlinearity, X(3) effects, but negligible nonlinear loss. Recent efforts typically focus on ultracompactness, reduced loss and dispersion engineering for fairly sophisticated integrated chips and advanced nonlinear functionalities, such as supercontinuum generation and optical comb generation.
Progress in second-order nonlinear integrated photonic devices heterogeneously on silicon are reviewed. Demonstrations of second-harmonic generation and spontaneous parametric down-conversion, based on ultracompact lithium niobate photonics on silicon, are presented. Also reviewed are heterogeneous third-order nonlinear devices on silicon, and more interestingly, approaches for integration of second- and third-order devices on the same chip. Example demonstrations will be octave-spanning supercontinuum generation on chalcogenide waveguides on silicon and their seamless integration with lithium niobate devices.
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