Fig. 3. SHG spatial mapping of few-layer PdSe₂.

a, c The optical images of the PdSe₂ flakes for the SHG characterization. The red boxes indicate the selected area for the SHG measurements. The layer numbers are labeled by the white numbers. The region elected in a contain 1–5 L and 8 L PdSe_2, and the area selected in c includes 1–7 L PdSe₂, ensuring sufficient sampling. The PdSe₂ samples were excited by 800 nm wavelength using 50× objective with a NA value of 0.55. b, d The spatial mapping of the integrated SHG signals corresponding to the regions in a and c, respectively. The intensities of the SHG signals are different for different layers. The SHG signal is almost absent in the odd-layer PdSe₂, while it is strong in the even-layer PdSe₂. The color bars are shown on the right of the figure.