Influence of the oxide thickness of a SiO2/Si(001) substrate on the optical second harmonic intensity of few-layer MoSe2

Abstract

The nonlinear optical properties of few-layer MoSe2 on a SiO2/Si substrate were investigated with our optical second harmonic generation (SHG) microscope. Few-layer flakes were mechanically exfoliated from a single crystal onto a 90- or 270-nm-thick SiO2-coated Si(001) substrate. The polar plot of the second-harmonic (SH) intensity from a mono- or trilayer MoSe2 flake as a function of the rotation angle of incident polarization shows a threefold symmetry, indicating that the isolated few-layer flakes retain their single crystallographic orientation. SHG spectra were found to depend strongly on the oxide thickness of the substrate (90 or 270 nm), which was interpreted using the interference among the multiply reflected SH light beams in the system. By taking this interference into account, a resonant peak may be identified at a two-photon energy of equal to or less than 2.9 eV in an SHG spectrum. The spatial resolution of the SHG microscope was estimated as 0.53 µm.