Structure-Dependent Nonlinear Optical Effects in Spiral WS Nanosheets.

Spiral transition-metal dichalcogenides with broken crystal inversion symmetry and significant second-order nonlinear responses have shown great promise for further nonlinear optical applications. However, various spiral structures will be formed during their synthesis process, their second harmonic generation (SHG) varying with the layer thickness and which of them manifesting the most promising SHG response are still unresolved. Here, the layer-dependent SHG response is investigated for four representative spiral WS with different screw and twist angles, including aligned- and twisted-triangular spiral structures, aligned- and twisted-hexagonal spiral structures, respectively. Experimental results demonstrate that both aligned- and twisted-hexagonal spiral WS present weak SHG response. In contrast, the SHG signal of the aligned-triangular spiral WS almost quadratically increases with the lift of their thickness, which is two orders of magnitude stronger than hexagonal structures. Moreover, an oscillating layer-dependence SHG response for twisted-triangular spiral WS has been attributed to the restored inversion symmetry. The underlying mechanism has been explored by the evolution of their crystal symmetry. The results not only disclose that the nonlinear response of the spiral WS can be tailored on-demand through the novel structural designs, but also pave the way to scalable integrated photonics and lab-on-a-chip quantum devices based on spiral layered materials.