Boosting Second-Harmonic Generation in Monolayer Rhenium Disulfide by Reversible Laser Patterning

Active modification and control of transition metal dichalcogenides (TMDs) properties are highly desirable for next-generation optoelectronic applications. In particular, controlling one of the most important characteristics of TMDs-their crystal structure and symmetry-may open means for manipulating their optical nonlinearities and electrical transport properties. Here, we show that a monolayer ReS2, which does not have a broken inversion symmetry due to its stable 1T'-distorted phase and correspondingly shows only weak second-harmonic generation (SHG), can produce a significantly enhanced (similar to 2 orders of magnitude) SHG upon reversible laser patterning. This enhancement can be explained by the laser-induced transition from centrosymmetric 1T' to noncentrosymmetric 2H-phase. This hypothesis is confirmed by polarization-resolved SHG measurements, which reveal a gradual change from the 2-fold to 6-fold symmetry profiles upon laser patterning. Additionally, we found that laser patterning of the bilayer ReS2 samples, contrary to the monolayers, leads to a substantially reduced SHG signal. This result corroborates the 1T'-to-2H laser-induced phase transition. Finally, we show that the laser-induced patterning is reversible by heat. These results open a possibility to actively and reversibly control the crystal structure of mono- and few-layer ReS2 and thus its optical and electronic properties.