The Origin of Linearly-Polarized Photoluminescence in WS2/WSe2 Moiré superlattices

Abstract: Reliable optical control of valley degrees of freedom in moire excitons requires that the emitted polarization faithfully reflect the underlying valley state. Here, we show that linearly polarized photoluminescence from WS2/WSe2 moiré excitons is largely insensitive to the excitation polarization and therefore is not primarily governed by valley-contrast selection rules. Automated polarization-resolved photoluminescence and Raman mapping at cryogenic temperatures reveal that the degree of linear polarization correlates strongly with local Raman shifts and moiré-exciton observables, identifying strain as the dominant experimental correlate. Exhaustive linear-regression analysis further shows that strain-related descriptors provide the best prediction of the observed polarization. Guided by theory, we attribute this behavior to strain-amplified breaking of C3 symmetry in the moire potential: weak uniaxial strain produces only partial cancellation of locally elliptical emission, yielding a finite far-field degree of linear polarization. These results establish strain as a key control parameter for reliable optical readout in TMD moire superlattices.

• The paper demonstrates that linearly polarized photoluminescence in WS2/WSe2 moiré superlattices originates from strain-induced C3 symmetry breaking rather than valley-coherent exciton recombination.
• Spatially resolved polarization and Raman spectroscopy reveal that even 0.1% uniaxial strain in WS2 can cause 2.3% moiré distortion, emphasizing strain’s critical role in optical behavior.
• The findings imply that precise strain control is essential for effective valley manipulation in TMD devices, impacting future quantum optoelectronic and valleytronic applications.

Strain-Induced Linear Polarization in WS$_2$/WSe$_2$ Moiré Superlattices

Introduction

The study investigates the physical origin of linearly polarized photoluminescence (PL) in WS$_2$/WSe$_2$ moiré superlattices, focusing on the interplay between strain, exciton wavefunctions, and the valley degree of freedom (VDF). Traditional understanding in monolayer transition metal dichalcogenides (TMDs) attributes strong coupling between VDFs and light polarization to symmetry-protected optical selection rules, enabling deterministic valley manipulation for valleytronics. The emergence of moiré patterns in vertically stacked TMDs further modulates the electronic structure, leading to interlayer excitons with distinct optical and quantum behaviors. However, the fidelity of optical VDF readout in these systems, especially under realistic conditions involving strain and disorder, remains poorly understood.

Experimental Approach

Precisely stacked WS$_2$/WSe$_2$ moiré heterostructures were fabricated via sequential CVD growth on hBN substrates, ensuring atomically flat interfaces and minimal twist-angle disorder. Polarization-resolved PL and Raman spectroscopy—fully automated and spatially resolved at 3.5 K—enabled concurrent mapping of the PL DLP, local vibrational modes, and moiré exciton characteristics across the same micron-scale regions. Spectral drift and focus correction algorithms ensured map registration at sub-micron accuracy throughout mapping sessions lasting multiple days.

Photoluminescence Polarization Behavior

In sharp contrast to monolayer WSe$_2$, where the PL polarization directly aligns with the linear polarization of the excitation owing to robust valley optical selection rules, WS$_2$/WSe$_2$ moiré superlattices exhibit PL whose polarization axis is essentially insensitive to the excitation polarization. The measured DLP in the moiré system was finite (3–7%) yet exhibited negligible rotation upon changing excitation polarization—demonstrating that the PL polarization is not governed by valley-coherent exciton recombination.

Strain as the Principal Determinant of PL DLP

Comprehensive spatial correlations unveiled a strong association between local DLP and Raman shifts corresponding to both WS$_2$ and WSe$_2$0 A$_2$1$_2$2 modes, implicating strain as a critical factor. The DLP shows negligible correlation with carrier density indicators (trion intensities), ruling out doping-related effects as the main origin. Pearson’s coefficients and linear regression feature selection further reinforced that strain descriptors (Raman shifts), moiré exciton energy shifts, and polarization axes collectively offer optimal predictive power for the DLP spatial profile.

Microscopic analysis revealed that even minuscule uniaxial strain (0.1% in WS$_2$3) leads to especially large (2.3%) distortions in the moiré period due to geometric amplification—driven by the near lattice parameter commensurability of WS$_2$4 and WSe$_2$5. These distortions break the local C$_2$6 rotational symmetry of the moiré potential, impeding the symmetry-imposed cancellation of locally elliptical emissions, and resulting in nonzero far-field linear PL polarization.

Theoretical Account of Strain-Amplified Symmetry Breaking

Electronic structure calculations clarified that the strain-induced deformation of the moiré potential molds the excitonic envelope functions from symmetric to ellipsoidal. While the Bloch part ($_2$7) determines the polarization selection through the optical transition matrix element, symmetry breaking from strain ceases the perfect destructive interference of linearly polarized emission components at different moiré registry sites. This results in a finite residual DLP, reproducible even with minimal strain due to geometric amplification. The result is fundamentally distinct from pristine monolayer TMDs, where valley selection rules yield circular polarization, or linear valley coherence when excited by linear polarization.

Implications for Valleytronics and Device Engineering

The central implication is that linear polarization of PL in TMD moiré superlattices is not a reliable indicator of underlying valley coherent states in the presence of non-negligible strain. This breaks the traditional one-to-one correspondence between excitation and emission polarization leveraged for VDF control in valleytronic protocols. Even weak, unintentional strain in heterostructures introduces significant symmetry breaking, challenging the deterministic optical manipulation of valley degrees of freedom.

Practically, these findings impose stringent requirements on the control and characterization of local strain environments in moiré-engineered TMD devices. Strain engineering, while representing an additional degree of freedom for device control, also acts as a source of decoherence and selection rule breakdown unless rigorously managed. These considerations are relevant for all quantum optoelectronic applications relying on robust valley addressability, including optically-addressed spin–valley qubits.

Future Prospects

Results in this study suggest that the feasibility of all-optical VDF control in moiré TMDs may ultimately depend as much on strain minimization and local symmetry protection as on electronic or excitonic quantum engineering. Further theoretical treatment is warranted to quantitatively connect microscopic strain tensor fields to DLP and to elucidate limits of valleytronic operability under realistic, inhomogeneous device conditions. Advanced local probe techniques (such as near-field PL or strain mapping via transmission electron microscopy) could offer finer spatial correlation between nanoscale strain and valley optical response, guiding the design of strain-robust moiré systems.

Conclusion

The study rigorously demonstrates that the principal origin of linearly-polarized PL in WS$_2$8/WSe$_2$9 moiré superlattices is strain-induced C$_2$0 symmetry breaking, not valley-contrast optical selection rules. The findings directly challenge the prevailing assumption that the PL polarization in such systems reliably encodes the valley state, and underline the necessity of detailed strain control for the advancement of valleytronic and quantum photonics applications in 2D moiré heterostructures (2604.16934).