Fig. 3: Observation of Berry phase-induced mode splitting.

a The twist angle denoted by \(\theta\) is varied to observe the transmission spectrum. b Theoretically calculated Berry phase (without the dynamical one) at a different twisting angle, the referencing dashed line marks the zero-phase level. c The experimental observation of the mode spacing of fast and slow axes at various twist angles and the frequency spacing between the two modes is visualized in the inset. d Theoretically results of the mode spacing by including both the dynamical and Berry phase.