Fig. 4: Control of quantum interference through twist angle in an excitonic three-level system in bilayer WSe₂ at 5 K.

Change of the A-exciton PL peak energy (red spheres), the HX PL peak energy (blue spheres), and the SHG quantum-interference dip energy (orange diamonds) with the twist angle. The lines serve as a guide to the eye. The effective excitonic three-level systems are illustrated for different twist angles. Since state \(\left| 3 \right\rangle\) shifts more strongly than state \(\left| 2 \right\rangle\), at angles approaching 0° or 60°, the \(\left| 1 \right\rangle \to \left| 2 \right\rangle\) transition energy E₁₂ is much larger than the \(\left| 2 \right\rangle \to \left| 3 \right\rangle\) transition energy E₂₃ so that quantum interference cannot occur within one excitation laser pulse (region not shaded in yellow).