Hidden State at C Exciton Observed by Second-Harmonic Generation Spectroscopy of Few-layer MoS2

Precise second-harmonic generation spectra of monolayer (ML)-MoS2, trilayer (3L)-MoS2 with a 3R structure, and 3L-MoS2 with a 2H structure are measured in the two-photon energy region from 2.4 to 3.2 eV. The spectra are resolved into two components, C1 and C2. As the number of layers increases from ML- to 3L-MoS2, the peak energy of the low-energy component, C1, is red-shifted and its peak width broadens. Peak intensity is also higher for 3L-MoS2 compared with ML-MoS2. This trend reveals that the C1 component originates from interband transitions in a ring-shaped region with bands nesting around the Γ point, similar to the C exciton observed in linear optical spectra. In contrast, for the high-energy C2 component, the variation of peak energy and peak width with layer numbers from ML- to 3L-MoS2 is negligible. Compared with ML-MoS2, the peak intensity increases for the 3L-MoS2 with the 3R structure, but decreases for the 3L-MoS2 with the 2H structure. These behaviors indicate that the C2 component is different from the C excitons in the linear optical spectrum, coming from excitation in a region where the band structure is less modified by interlayer coupling, revealing the presence of a transition to a hidden state, which is specific to the nonlinear optical spectrum.