Propagation Loss of Monolayer MoS₂ on Si₃N₄ Waveguides at Wavelengths from 1520 nm to 1620 nm

Monolayer molybdenum disulfide (1L MoS₂) is a two-dimensional (2D) semiconductor with a direct electronic bandgap of 1.88 eV and large nonlinear susceptibilities. Its second-order optical nonlinearity χ(2) leads to second-harmonic generation (SHG) and a Pockels electro-optic effect. Therefore, atomically thin 1L MoS₂ has been utilized in several on-chip integrated photonic devices. However, 1L MoS₂ must be sufficiently transparent under the wavelength of operation to be power efficient and not affect device performance. Previous literature reports 20 dB/cm to 43 dB/cm of excess propagation losses in silicon waveguides (WGs) covered with 1L MoS₂, which is too high for most applications in the infrared spectrum. Here, we experimentally investigate the infrared propagation loss in 1L MoS₂ monolithically integrated on silicon nitride (Si₃N₄) WGs and racetrack resonators (RTRs). We use linear regression to determine the propagation losses via RTR resonances. The excess propagation losses in wet-transferred 1L MoS₂ on Si₃N₄ WG encapsulated by polymethyl methacrylate are measured as less than 2 dB/cm from 1520 nm to 1620 nm. This was achieved with an evanescent field fraction of 0.03 % in the 1L MoS₂. We provide evidence that this upper boundary of the loss is not an intrinsic material property and is instead dominated by defects arising from transfer, i.e., further loss reductions via process optimization are possible. Introduction