Optical detection of the susceptibility tensor in two-dimensional crystals

The out-of-plane optical constants of monolayer two-dimensional materials have proven to be experimentally elusive. Owing to their reduced dimensionality, optical measurements have limited sensitivity to these properties which are hidden by the optical response of the substrate. Therefore, there remains an absence of scientific consensus on how to correctly model these crystals. Here we perform an experiment on the optical response of a single-layer two-dimensional crystal that addresses these problems. We successfully remove the substrate contribution to its optical response by a step deposition of a monolayer crystal inside a thick polydimethylsiloxane prism. This allows for a reliable determination of both the in-plane and the out-of-plane components of its surface susceptibility tensor. Our results prescribe one clear theoretical model for these crystals. This precise characterization of their optical properties will be relevant to future progresses in photonics and optoelectronics with two-dimensional materials. Experimental testing of the expected linear optical anisotropy of 2D crystals, arising from their finite thickness, have so far been complicated by substrate contributions. Here, immersion of monolayer samples in a low-refractive-index polymer allows more accurate determination of their optical constants.