Emerging second-order nonlinearity in two-dimensional material functionalized silica microcavities*

One fundamental challenge for nonlinear optics is the simultaneous use of second- and third-order nonlinear processes since the nonlinear susceptibility is generally intrinsic to the material. An essential requirement for second-order nonlinear processes to occur is inversion symmetry breaking, but state-of-the-art ultrahigh-Q devices are made of centrosymmetric crystals and amorphous materials such as silica and silicon, and only third-order processes can be utilized in these platforms. We herein demonstrate a second-order nonlinear photonic platform by functionalizing ultrahigh-Q silica microspheres with few-layer tungsten diselenide (WSe₂) two-dimensional material. The presence of the manolayer flake drastically changes the nonlinear susceptibilities of the silica microcavity, leading to cavity-enhanced second-harmonic (SH) generation and sum-frequency generation by continuous-wave (CW) excitation at a power level of only a few hundred microwatts. This result relies on the giant second-order nonlinearity of the monolayer WSe₂ [1,2]. The layer number dependence of SH light confirms that the second-order nonlinearity originates from the integrated WSe₂, not from intrinsic surface symmetry breaking of the cavity material [3]. We reveal the mechanism of the dynamic phase-matching process, which governs the conversion efficiency of SH light by measuring the pump power dependence. We also demonstrate the coexistence of second- and third-order nonlinearities and its flexible control in a single device, unlocking the fundamental limitation of optical nonlinearities intrinsic to the cavity material.

[1] Y. Li, et al., Nano Lett. 13, 3329 (2013).

[2] N. Kumar, et al., Phys. Rev. B 87, 161403 (2013).

[3] X. Zhang, et al., Nat. Photon. 13, 21 445 (2019).