Mechanism of Extreme Optical Nonlinearities in Spiral WS₂ above the Bandgap

Abstract

Layered two-dimensional transition-metal dichalcogenides (2D-TMDs) are promising building blocks for ultracompact optoelectronic applications. Recently, a strong second harmonic generation (SHG) was observed in spiral stacked TMD nanostructures which was explained by its low crystal symmetry. However, the relationship between the efficiency of SHG signals and the electronic band structure remains unclear. Here, we show that the SHG signal in spiral WS 2 nanostructures is strongly enhanced (∼100 fold increase) not only when the second harmonic signal is in resonance with the exciton states but also when the excitation energy is slightly above the electronic band gap, which we attribute to a large interband Berry connection associated with certain optical transitions in spiral WS 2 . The giant SHG enhancement observed and explained in this study could promote the understanding and utility of TMDs as highly efficient nonlinear optical materials and potentially lead to a new pathway to fabricate more efficient optical energy conversion devices.

Superior Nonlinear Optical Response in Non‐Centrosymmetric Stacking Edge‐Rich Spiral MoTe2 Nanopyramids

“…Such three maximum points are in good line with the absorption peaks of monolayer to trilayer MoTe 2 , [ 45 ] as previous research uncovered, corresponding to B′, C, D excitons, respectively. The reasons for the wide second‐order NLO response range are exciton resonances and the resonance of large interband Berry connections contributing to certain transitions within the high energy spectral regime, [ 12 ] and the redshift of local peaks in band nesting region (C‐ and D‐exciton quasi‐resonant level) is possibly related to positive strain caused by screw dislocations. [ 14 ] Then, the excitation wavelength is tuned to 930 nm (half for D‐exciton quasi‐resonant wavelength) for a tentative position‐dependent SHG output test (Figure 4c), and an obvious nonlinear enhancement of SHG from bottom to top is observed (Figure 4c inset).…”

Phase Transitions and Water Splitting Applications of 2D Transition Metal Dichalcogenides and Metal Phosphorous Trichalcogenides

“…[ 90 ] The piezoelectric effect of TMD refers to a complicated mixture of surface effects, electronic interactions and atomistic structure details that requires large scale self‐consistent numerical calculations. [ 88 ]…”

Growth and Properties of Dislocated Two-dimensional Layered Materials

“…There are three main strategies for the spiral growth of layered nanoplates: vapor, solution and epitaxy techniques. Vapor methods, such as chemical vapor deposition (CVD) 12,13,17,18,23,[28][29][30][31][32][33] and chemical vapor transport (CVT) 14 , are the most widely utilized owing to their high growth rates and good reproducibility. The growth of spiral structures using vapor methods is very sensitive to the local supersaturation of the adatoms near the dislocation ridge which depends on reaction temperature, the mass-transfer velocity and the precursor loss for the prior deposition (namely, the temperature distribution in the furnace) 23 .…”

“…3b, triangular TMDC plates are formed by 'AA' layer stacking, where the adjacent two layers show 0º rotation and break the inversion symmetry ( 1 3h D group). Thus, triangular TMDC spirals display strong SHG signals 12,13,23,33 . In contrast with monolayer TMDC, the SHG intensities from the TMDC spirals are substantially enhanced by the large constructive interference length 12,30,33 and the strong interband Berry connection 33 .…”

“…Thus, triangular TMDC spirals display strong SHG signals 12,13,23,33 . In contrast with monolayer TMDC, the SHG intensities from the TMDC spirals are substantially enhanced by the large constructive interference length 12,30,33 and the strong interband Berry connection 33 . On the contrary, hexagonal TMDC plates, which are formed by 'AB' layer stacking with an inversion center ( 1 3d D group), yield vanishing SHG (FIG.…”