The Kerr effect is a third order nonlinear effect in optics that affects the refractive index with respect to the optical intensity of a light wave travelling through a nonlinear medium. Ultra-high-Q microresonators have the ability to induce significant third order nonlinearities, at low optical input powers. Power-dependent resonance shifts of more than 30 linewidths were measured, induced by the Kerr effect, for continuous-wave lasers in both co- and counter-propagating directions. The frequency shifts are different for lasers of different intensity, due to the nonequal contributions of self- and cross-phase modulation. This results to an effective splitting of the optical mode. A further effective splitting occurs from unequal Kerr contributions in the two counterpropagating circulating directions of the cavity, due to four-wave mixing only occurring in the co-propagating direction. Two otherwise identical counterpropagating light
fields would therefore undergo different resonance shifts. The induced nonreciprocity finds applications in optical switching, memories, and logic gates, while the splitting itself affects
optomechanical coupling to mechanical degrees of freedom. Controlling the pump laser’s power
and detuning determines how the amplitudes and phases of the mechanical oscillation behave,
measured through the generated Stokes and anti-Stokes sidebands. Better understanding of
the optically-induced mechanical motion benefits research done on phonon lasers, mechanically induced rf combs, and mechanical solitons, as well as applications in which the optical field mediates phonon transfer between different mechanical modes.