▎ 摘　　要

The reflectivities of monolayer two-dimensional crystals, immersed in polydimethylsiloxane, have been measured at a wavelength of 532 nm. The polydimethylsiloxane does not alter the physical properties of the atomic crystals. The only difference with respect to the study of a free-standing sample is that the two-dimensional crystals are embedded in a medium with a refractive index n = 1.414 rather than 1. In this particular experimental configuration, we observe a pseudo-Brewster angle for p polarization. This observation is direct evidence of a non null out-of-plane optical surface susceptibility for these crystals.