Fig. 4: Berry phase enhanced mode separations.

a–c frequency spacing of two classes of resonance modes (red for + mode, blue for – mode) at 0° twisting angle. d–f The frequency spacing of two resonance modes and their consecutive ones during the variation of twisting angle. g–i Frequency spacing of two classes of resonance modes at 360° twisting angle. The pump wavelength is (a, d, g): 1550.2607 nm, (b, e, h): 1550.6989 nm, and (c, f, i): 1551.0745 nm. a, b The initial spacing (0°) is narrowed when the pump wavelength is increased and (g, h) the final spacing (360°) is both broadened due to the non-zero Berry phase. A flipping of the resonances is observed by comparing (c) 0° and (i) 360° cases also due to the additional Berry phase.