Destructive interference of second harmonic generation in AA stacked MoTe$_2$/WSe$_2$

TL;DR

This study uncovers an unconventional destructive second-harmonic generation interference in AA-stacked MoTe2/WSe2 heterobilayers, revealing how exciton resonances and twist angles influence nonlinear optical responses.

Contribution

It demonstrates the origin of destructive SHG interference due to phase differences in exciton resonances and explores polarization control via twist angles in moiré heterobilayers.

Findings

Unconventional destructive SHG interference observed in AA-stacked heterobilayers.

Two-photon resonances cause a phase difference leading to destructive interference.

SHG polarization can be tuned to nearly circular with twist angle and phase interplay.

Abstract

The stacking configuration of two-dimensional materials critically governs their optical and electronic responses. Monolayer transition-metal dichalcogenides (TMDC) lack inversion symmetry and exhibit exciton-enhanced second-harmonic generation (SHG). In TMDC bilayers, 60{\deg} (0{\deg}) stacking is conventionally expected to suppress (enhance) SHG owing to destructive (constructive) interference of the layer-resolved nonlinear polarizations. Here, we report an unconventional destructive SHG interference in nearly 0{\deg}-stacked (AA-stacked) MoTe2/WSe2 heterobilayers using two independent probes: atomic-resolution imaging and stacking-sensitive exciton hybridization measurements. Supported by ab initio GW and Bethe-Salpeter equation calculations, we show that distinct two-photon resonances associated with the WSe2 C exciton and the MoTe2 D exciton generate a nearly $\pi$ phase difference ($\Delta\phi$) in their second-order nonlinear susceptibilities $\chi^{(2)}$, leading to the anomalous destructive interference. We further demonstrate that in small-angle twisted MoTe2/WSe2, the SHG polarization state is governed by the interplay between twist angle $\alpha$ and phase difference $\Delta\phi$, and can be mapped onto trajectories on the Poincar\'e sphere. At excitation energies satisfying $\Delta\phi$ + 3$\alpha$ = 180{\deg}, the SHG output becomes nearly circularly polarized (ellipticity ~ 0.91) and undergoes an abrupt 90{\deg} azimuthal rotation, corresponding to a geometric polarization singularity in the parameter space. Our findings open new routes for exciton-resonance engineered nonlinear photonics and stacking-resolved optical functionality in moir\'e materials.