Figure 1

(a) Optical microscope image of the MoS${}_2$ thin film. (b) AFM image of the same MoS${}_2$ thin film made within the dashed triangle shown in part a, scale bar 1 $\mu$m. By measuring the height relative to the substrate, the number of MoS${}_2$ layers are determined as 1L for monolayer, 2L for bilayer, and 3L for trilayer MoS${}_2$. (c) Second-harmonic image collected from the same MoS${}_2$ thin film, pump-laser wavelength at 800 nm, 1.55 eV. Brighter colors mean stronger SHG intensity, and the scale bar is 5 $\mu$m. (d) Log vs log plot of the power dependence of SHG from the X marked position in part c. The circles are the measured data, and the red line is a linear fit of the data with an extracted slope of $2.00\pm 0.02$. (e) Intensity profile of the SHG image from left to right at the yellow line shown in part c. (f) Atomic structure of monolayer (left) and bilayer (right) MoS${}_2$.Reuse & Permissions

Figure 2

(a) Polar plot of the second-harmonic intensity from monolayer MoS${}_2$ as a function of the sample angle. (b) Top view of the MoS${}_2$ crystallographic orientation with respect to the incident laser polarization (${\widehat{e}}_{\omega }$). (c) SH image of the same sample as in Fig. 1c showing the crystallographic direction of the monolayer and trilayer MoS${}_2$ sample determined by our polarization measurements.Reuse & Permissions

Figure 3

(a) Optical image of the few-layer MoS${}_2$ sample on transparent quartz substrate. The number of layers is indicated as measured by AFM. Parts b–d show the SH image for the same sample with pump-laser wavelength at (b) 860 nm and 1.44 eV, (c) 900 nm and 1.38 eV, and (d) 960 nm and 1.29 eV. (e) Second-order susceptibility, left scale, of a sheet of MoS${}_2$ for monolayer (blue circles) and trilayer (green circles) samples as a function of pump-laser energy, with measured linear absorption (right scale) spectra of monolayer (blue line) and trilayer (green line) with the labeled optical transitions. Inset: Diagram of the second-harmonic enhancement in monolayer MoS${}_2$ where the two-photon energy is in resonance with the C absorption peak.Reuse & Permissions