  Optical systems combining balanced loss and gain profiles provide a unique
platform to implement classical analogues of quantum systems described by
non-Hermitian parity-time- (PT-) symmetric Hamiltonians and to originate new
synthetic materials with novel properties. To date, experimental works on
PT-symmetric optical systems have been limited to waveguides in which
resonances do not play a role. Here we report the first demonstration of
PT-symmetry breaking in optical resonator systems by using two directly coupled
on-chip optical whispering-gallery-mode (WGM) microtoroid silica resonators.
Gain in one of the resonators is provided by optically pumping Erbium (Er3+)
ions embedded in the silica matrix; the other resonator exhibits passive loss.
The coupling strength between the resonators is adjusted by using
nanopositioning stages to tune their distance. We have observed reciprocal
behavior of the PT-symmetric system in the linear regime, as well as a
transition to nonreciprocity in the PT symmetry-breaking phase transition due
to the significant enhancement of nonlinearity in the broken-symmetry phase.
Our results represent a significant advance towards a new generation of
synthetic optical systems enabling on-chip manipulation and control of light
propagation.
