Broken Symmetry Induced Strong Nonlinear Optical Effects in Spiral WS₂ Nanosheets

Abstract

Transition metal dichalcogenides (TMDs) have provided a fundamental stage to study light-matter interactions and optical applications at the atomic scale due to their ultrathin thickness and their appropriate band gap in the visible region. Here, we report the strong nonlinear optical effects, including second-harmonic generation (SHG) and third-harmonic generation (THG) in spiral WS structures. SHG intensity quadratically increases with layer numbers, other than diminishing the oscillation of 2H stacking TMDs. The contrary SHG behavior is attributed to the broken symmetry from twisted screw structures, revealed by aberration-corrected transmission electronic microscope observation. Furthermore, the twist angle of the screw structure (5 degrees) was obtained by high-resolution transmission microscope measurements and confirmed by polarization tests of SHG output. Moreover, we roughly estimate the effective second-order nonlinear susceptibility. The discovery and understanding of the accumulation of nonlinear susceptibility of spiral structures with increasing thickness will extend the nonlinear applications of TMDs.

2D Layered Materials: Synthesis, Nonlinear Optical Properties, and Device Applications

“…In addition, the two‐photon absorption response of WS 2 and WSe 2 films was also discovered, as shown in Figure d–f. Recently, more TMDCs including MS 2 , MSe 2 , MTe 2 , have been also deeply studied. These explorations lay a foundation for their applications in nonlinear photonics.…”

Nonlinear optics of two‐dimensional transition metal dichalcogenides

“…SHG of the bulk MoS 2 is very weak and χ (2) is around 10 −14 m/V; however, three groups simultaneously reported that SHG of monolayer MoS 2 , WS 2 , and WSe 2 increased dramatically . Later, numerous SHG studies were conducted on MoS 2 , WS 2 , WSe 2 , MoSe 2 , and MoTe 2 , and χ (2) was reported on the order of magnitude of 1 nm/V, which is much higher than the most used commercial nonlinear crystals (eg, χ (2) of BBO, KTP, and LiNbO 3 are on the order of magnitude of pm/V). The large nonlinear susceptibility of monolayer compared with bulk is ascribed to the inversion symmetry breaking.…”

“…By fitting the 6‐fold pattern with the cos 2 (3 θ + φ 0 ) or sin 2 (3 θ + φ 0 ) function, φ 0 can be resolved and the initial orientation of the crystal can be determined. This polarization dependence was widely utilized to probe the crystal orientation of h‐BN and monolayer TMDs . Especially, it is useful for the heterostructure stacking where the initial orientation need to be resolved accurately .…”

Chiral Coupling of Valley Excitons and Light through Photonic Spin–Orbit Interactions

“…With the development of CVD technology, valley polarization contrast in artificial structures of multilayers TMDCs has been attracting considerable attention. One remarkable structure fabricated using this technology is the spiral nanostructure of TMDCs which has a screw morphology as shown in Figure 3b 96–98. Interestingly, transmission electron microscopy analysis revealed that this spiral nanostructure has an AB stacking structure, as shown in Figure 3c.…”

“…Inset shows the zoomed‐in atomic force microscope image of a spiral WS 2 nanosheet. Reproduced with permission 96. Copyright 2017, American Chemical Society.…”