Absorption and gain saturable nonlinearities in erbium-doped optical microcavities

Nonlinear non-Hermitian systems with saturable nonlinearities give rise to versatile phenomena that have no counterparts in linear systems. Here, we theoretically and experimentally demonstrate saturable nonlinearity (saturable absorption and gain) of erbium ions doped in silica microcavities at different concentrations. Our results show that when the doped concentration is low, the single isolated model agrees well with the experimental results, and also can be used as a basic description of highly doped samples to analyze the saturable nonlinearity. Yet, for highly doped microcavities we find that the clustering effect shall be also taken into account in order to match experimental data and to achieve a reasonable agreement. To confirm our finding, we experimentally characterize the saturable nonlinearity for ion concentrations varying from 1.0 to 5.0 x 10(19)cm(-3). Our experiment shows excellent consistency with the theory, which in turn suggests other potential applications of the theory based on saturable nonlinearity, including optical bistability and nonlinear parity-time symmetry.