Nonlinear Optics has been a source of surprises for physicists for almost a century
comprising both fundamental physics and real-world applications. To access optical
nonlinearities, significant light intensities are required; thus nonlinear optics often
involves a resonant cavity to amplify the light intensity. Microresonators have proven
to be an ideal platform for this kind of experiment since the cavity mode area can be
as small as a few µm2 and their high Q-factor traps light for many round trips while
more light is coupled in. Also, light interacts with the nonlinear material, of which
the resonator is made, for the whole round trip. However, the interaction between
counter-propagating light in microresonators is still a relatively unexplored field.
This thesis reports on the first observation of Kerr-induced spontaneous symmetry breaking in a microresonator, whereby light can circulate in only one direction
inside the resonator. I develop a theoretical model describing the steady-state solutions and the dynamics of how the symmetry-broken regime responds to the input
changes. I show experimentally how the symmetry breaking can be used to realise all-optical isolator, circulators, memories and logic gates. These devices, based
on the Kerr-nonreciprocity, represent a promising alternative for the realisation of
integrated all-optical passive photonics circuits
