Light is generally expected to travel through isotropic media independent ofits direction. This makes it challenging to develop non-reciprocal opticalelements like optical diodes or circulators, which currently rely onmagneto-optical effects and birefringent materials. Here we presentmeasurements of non-reciprocal transmission and spontaneous symmetry breakingbetween counter-propagating light in dielectric microresonators. The symmetrybreaking corresponds to a resonance frequency splitting that allows only one oftwo counter-propagating (but otherwise identical) light waves to circulate inthe resonator. Equivalently, the symmetry breaking can be seen as the collapseof standing waves and transition to travelling waves within the resonator. Wepresent theoretical calculations to show that the symmetry breaking is inducedby Kerr-nonlinearity-mediated interaction between the counter-propagatinglight. This effect is expected to take place in any dielectric ring-resonatorand might constitute one of the most fundamental ways to induce opticalnon-reciprocity. Our findings pave the way for a variety of applicationsincluding all optical switching, nonlinear-enhanced rotation sensing, opticallycontrollable circulators and isolators, optical flip-flops for photonicmemories as well as exceptionally sensitive power and refractive index sensors.
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