Fig. 2: Flake thickness optimization for second harmonic generation (SHG).

a Sketch of the SHG measurement of the 3R-MoS₂ flakes exfoliated on glass substrate. The height of each flake was measured with atomic force microscopy (AFM). b Standard study steps of the flake analysis: (i) Optical image of the flake of interest. Scale bar is 10 μm; (ii) SHG mapping of the intensity at a given pump wavelength; (iii) and (iv) AFM analysis of the flake height. The white dashed line in (i) corresponds to the AFM area of analysis in (iv). c Experimental SHG intensity of 3R-MoS₂ flakes for various pump wavelengths and flake thicknesses. The flake thicknesses, h, are measured by AFM. Dashed lines correspond to cross-sections in (d). d SHG intensity cross-sections at given pump wavelengths. Markers represent experimental points. Solid lines connect neighboring points for visual perception. The error bars in y direction are associated with the APD sensitivity. The error bars in x direction are associated with the AFM precision. Both of them are less than the marker size. e Theoretically calculated SHG intensity of the 3R-MoS₂ flakes using the transfer matrix method. Dashed lines correspond to cross-sections shown in (f). f SHG intensity from flake thickness at given pump wavelengths.