Second Harmonic Generation from Artificially Stacked Transition Metal Dichalcogenide Twisted Bilayers

TLDR

Optical second harmonic generation (SHG) is a sensitive probe of crystalline symmetry in few‑layer transition metal dichalcogenides, with layer‑number dependent and polarization‑resolved signals observed in Bernal‑stacked structures, but its behavior in non‑ideal or twisted stacks remains largely unexplored. This work reports SHG from homo‑ and heterostructural TMD bilayers fabricated by artificial stacking at arbitrary angles. SHG from twisted bilayers arises as a coherent superposition of the SH fields from each layer, with a phase difference governed by the stacking angle, and the authors demonstrate that this interference can probe domain boundaries and crystal polarity in mirror twins. The study shows that SHG provides an efficient, sensitive, and nondestructive characterization of stacking orientation, crystal polarity, and domain boundaries in van der Waals heterostructures made of noncentrosymmetric layered materials.

Abstract

Optical second harmonic generation (SHG) is known as a sensitive probe to the crystalline symmetry of few-layer transition metal dichalcogenides (TMDs). Layer-number dependent and polarization resolved SHG have been observed for the special case of Bernal stacked few-layer TMDs, but it remains largely unexplored for structures deviated from this ideal stacking order. Here we report on the SHG from homo- and heterostructural TMD bilayers formed by artificial stacking with an arbitrary stacking angle. The SHG from the twisted bilayers is a coherent superposition of the SH fields from the individual layers, with a phase difference depending on the stacking angle. Such an interference effect is insensitive to the constituent layered materials and thus applicable to hetero-stacked bilayers. A proof-of-concept demonstration of using the SHG to probe the domain boundary and crystal polarity of mirror twins formed in chemically grown TMDs is also presented. We show here that the SHG is an efficient, sensitive, and nondestructive characterization for the stacking orientation, crystal polarity, and domain boundary of van der Waals heterostructures made of noncentrosymmetric layered materials.