Hybrid integration of two-dimensional MoSe2 on a silicon waveguide for second-order nonlinear optics

摘要

Summary form only given. Two-dimensional (2D) transition-metal di-chalcogenides (TMDCs) with intrinsically-broken crystalline inversion symmetry have shown great promise for future nonlinear light sources. However, the sub-nanometer thickness of such active materials limits their overall nonlinear conversion efficiency. In this work, we demonstrate, for the first time to our knowledge, enhanced second-harmonic generation (SHG) from 2D MoSe₂ through integrating it with Si waveguide. Light from free space is coupled into a planar silicon waveguide by the inscribed grating coupler on the right. A MoSe₂ monolayer crystal is placed on top of the waveguide and is strongly bond to the surface by Van der Waals interactions. The evanescent field of the guided mode at the fundamental frequency (FF) of ~ 1550 nm interacts with the MoSe₂ material on top to generate second harmonic (SH). This structure increases the light-matter interaction length dramatically compared to free-space excitation, as well as allows for exact phase matching second order nonlinear processes through engineering of the waveguide dispersion. As a result, our MoSe₂-Si platform could dramatically boost the SHG. Under the excitation by a focused FF beam at around 0.8 eV (resonant with the excitonic transitions in the 2D materials), we observed approximately 5 times enhancement of the SH signal at 1.6 eV, when exciting the monolayer by the evanescently-coupled waveguide mode, as compared to pumping the MoSe₂ piece directly from free space. The observed six-fold rotational pattern reflects the three-fold rotational symmetry of the crystal, and identifies the measured signal as SHG. The scheme we demonstrate here is Si based and ready for integration with silicon photonics platform. Especially, our work shows the light-matter interaction length of 2D TMDCs limited by monolayer thickness could be overcome by integration with a waveguide, which paves the way for many other nonlinear optical applications for 2D materials, including parametric amplification as well as generation of entangled photons.