Optical nonreciprocity — light behaving differently in forward and backward directions — usually requires magnetic materials, which are lossy and difficult to integrate into photonic circuits. The search for magnetic-free nonreciprocity has explored time modulation, nonlinear effects, and optomechanical coupling. Each introduces its own constraints.
Spinning dielectric cylinders arranged in a two-dimensional photonic crystal produce strong nonreciprocity through mechanical rotation alone (arXiv:2603.11839). The rotating cylinders support chiral modes — hybridized multipole resonances and symmetry-protected bound states in the continuum — that carry intrinsic spin angular momentum. The rotation breaks time-reversal symmetry mechanically rather than magnetically.
When circularly polarized light strikes the crystal at an angle, the handedness of the light interacts with the spin angular momentum of the rotating modes. Forward and backward propagation experience different effective indices because the rotation adds or subtracts angular momentum depending on propagation direction. The nonreciprocity is enhanced by the high quality factors of quasi-bound states in the continuum, which create sharp spectral features where the transmission difference between forward and backward directions is maximized.
The mechanism is Doppler-like but enhanced by resonance. A single spinning cylinder produces weak nonreciprocity through rotational Doppler shift. The photonic crystal amplifies this through collective resonance — the same enhancement that makes photonic band gaps from weak individual scatterers. The crystal multiplies the effect of rotation the same way it multiplies the effect of refractive index contrast.
Rotation replaces magnetism. The gate opens in one direction because it is spinning.