Figure 1.

(a) and (e): optical images of WS₂ (a) and WSe₂ (e) slabs on Si substrates with 300 nm SiO₂ cap-layer; (b) and (f): photoluminescence images at direct gap transition energy of the corresponding WS₂ (b) and WSe₂ (f) slabs excited at 2.41 eV. Only monolayers are visible at the present contrast; (c) and (g): the corresponding SHG under a 800 nm excitation at normal incidence (150 fs, 80 MHZ) on WS₂ (c) and WSe₂ (g) respectively. The highest intensity labeled in red arises from monolayers. The SHG signal from the silicon/silicon oxide surface is negligible compared with those from multilayers; (d) and (h): The relative intensity of SHG as a function of the film thickness in WS₂ (d) and WSe₂ (h). The SHG shows an even-odd oscillation dependence on the layer number.