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Light from thin semiconductor layers can be polarized: its waves oscillate in a single plane. Previously, it was thought that the direction of oscillation was set by how the sample was illuminated. But new experiments with two layers of different materials (WSe2 and WS2) showed otherwise: when cooled to cryogenic temperatures, the polarization of the emitted light barely changes, no matter how we illuminate the sample.

The key is strain in the crystal lattice. Even a weak stretch or compression breaks the symmetry of the structure, and light from different parts of the sample becomes elliptically polarized: its waves trace out ellipses. Away from the sample, these ellipses partially cancel each other out, but not completely: a noticeable linear polarization remains. Automated mapping with polarization and Raman spectral resolution revealed a strong link between the degree of linear polarization and local frequency shifts caused by strain.

Linear regression confirmed that strain parameters best predict the observed polarization. So for reliable optical readout in moiré superlattices, it is strain that needs to be controlled, not the polarization of the excitation light. The discovery offers a simple and practical way to control light in future optoelectronic devices based on two-dimensional materials.

Original abstract

as written by the authors arXiv:2604.16934

Reliable optical control of valley degrees of freedom in moir\'e excitons requires that the emitted polarization faithfully reflect the underlying valley state. Here, we show that linearly polarized photoluminescence from WSe2/WS2 moir\'e excitons is largely insensitive to the excitation polarization and therefore does not arise from valley coherence. Automated polarization-resolved photoluminescence and Raman mapping at cryogenic temperature reveals that the degree of linear polarization correlates strongly with local Raman shifts and moir\'e-exciton observables, identifying strain as the dominant experimental correlate. 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 moir\'e 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 moir\'e superlattices.