Encapsulated silicon nitride nanobeam cavity for nanophotonics using layered materials

Abstract: Most existing implementations of silicon nitride photonic crystal cavities rely on suspended membranes due to the low refractive index of silicon nitride. Such floating membranes are not mechanically robust, making them suboptimal for developing a hybrid optoelectronic platform where new materials, such as layered 2D materials, are transferred on a pre-existing optical cavity. To address this issue, we propose a silicon nitride nanobeam resonator design where the silicon nitride membrane is encapsulated by material with a refractive index of ~1.5, such as silicon dioxide or PMMA. The theoretically calculated quality factor of the cavities can be as large as 100,000 , with a mode-volume of 2.5 times the cubic wavelength. We fabricated the cavity, and measured the transmission spectrum with highest quality factor of 7000. We also successfully transferred monolayer tungsten diselenide on the encapsulated silicon nitride nanobeam, and demonstrated coupling of the cavity with the monolayer exciton and the defect emissions.