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Optical nonlinear processes are indispensable in a wide range of applicationsincluding ultrafast laser sources, microscopy, metrology, and quantuminformation technologies. Combinations of the diverse nonlinear processesshould further lead to the development of unique functionalities, butsimultaneous use of second- and third-order nonlinear processes is generallydifficult. Second-order effects usually overwhelm the higher-order ones, exceptin centrosymmetric systems where the second-order susceptibility vanishes toallow the use of the third-order nonlinearity. Here we demonstrate a hybridphotonic platform whereby the balance between second- and third-ordersusceptibilities can be tuned flexibly. Ultrahigh-Q silica microcavitiescapable of generating third-order effects are functionalized by atomically thintungsten diselenide, and we observe cavity-enhanced second-harmonic generationand sum-frequency generation with continuous-wave excitation at a power levelof only a few hundred microwatts. Pump power dependence exhibits drasticincrease and saturation of the second-harmonic light, originating from thedynamic phase-matching process. We show that the coexistence of second- andthird-order nonlinearities in a single device can be achieved by carefullychoosing the size and the location of the two-dimensional material. Ourapproach can be generalized to other types of cavities, unlocking the potentialof hybrid systems with controlled nonlinear susceptibilities for novelapplications.

Van der Waals functionalization of ultrahigh-Q silica microcavities for $\chi^{(2)}$-$\chi^{(3)}$ hybrid nonlinear photonics