Localization-enhanced second harmonic generation in twisted WS2 spirals

This is the authors' abstract. We don't add key points for this paper.

In moiré crystals formed by stacking twisted two-dimensional (2D) layered materials, surprisingly diverse correlated optical and electrical properties can be realized by a subtle change in twisted angle. In this work, we report the observation of localization-enhanced second harmonic generation (SHG) in twisted WS 2 spirals, thereby adding insight into moiré physics. The twisted WS 2 spirals with various twisted angles are synthesized on a Euclidean or non-Euclidean surface by a well-designed water-assisted chemical vapor deposition. Compared to WS 2 monolayer, the minimum and maximum SHG signals in different regions of the twisted WS 2 spirals are enhanced by a factor of 4–12 and 26–136 at similar thickness, respectively, as the twisted angle between adjacent layers decreases from 20° to ≈ 0°. The significant SHG enhancement is explained by the evolution of structural symmetry and moiré potentials in these WS 2 spirals at different twisted angles. Their layer-dependent photoluminescence and Raman spectra show significantly changed peak position and intensity, confirming the strong local confinement effect of moiré potentials in these spirals. These findings provide an efficient method for the rational growth of 2D moiré structures and the implementation of a localization-enhanced SHG, endowing them great potential for exploring strong coupling correlation physics and applications in twistronics.