High-spatial-frequency nanoripples induced on a WS₂ film by femtosecond laser pulses for second harmonic generation...

Transition metal dichalcogenides (TMDCs) have attracted great interest due to their unique layer-related electronic and optical properties, which exhibit potential applications in optoelectronic devices. However, the fabrication of nanostructures on the surface of a TMDC film, which is necessary for their device applications, remains a big challenge. Direct laser writing has been widely employed to fabricate periodic structures on the surface of metallic and dielectric materials. Here, we demonstrate that high-spatial-frequency nanoripples with a period of only one-tenth of the laser wavelength could be produced on the surface of a WS₂ film by using femtosecond laser ablation. The period of nanoripples is controlled by adjusting the WS₂ film thickness and laser fluence, and their orientation is perpendicular to the writing laser polarization. It is revealed that the fabricated nanoripples exhibit polarization control for reflected light, second harmonic wave, and Raman signal. Interestingly, the second harmonic generation is enhanced (suppressed) when the polarization of the excitation light is parallel (perpendicular) to the orientation of nanoripples. In addition, the Raman signals are greatly enhanced in the presence of nanoripples. Based on numerical simulation, it is unveiled that the modification in the electric field distribution in the WS₂ film induced by nanoripples plays a crucial role in the second harmonic generation. At the same time, Raman signals are enhanced by the compressive strain formed in nanoripples. Our findings indicate the potential applications of the high-spatial-frequency nanoripples in controlling the optical properties of the TMDCs and pave the way for realizing polarization-sensitive optical storage and display based on TMDCs.