Observation of energy localization in nonlinear non-Hermitian systems

Energy localization in Hermitian systems has been utilized to generate ultra-sensitivity. Here, we report the interplay between non-Hermitian parity-time (PT) symmetry breaking and the mode localization transition. In our scheme, a PT-symmetric system consists of two coupled LC (inductor–capacitor) resonators: one has a linear loss and the other has a saturated gain described by a nonlinear model. The nonlinear gain is initially set to be slightly higher than the loss, and the system is operated at the exact PT-symmetric phase close to an exceptional point. The capacitance variation applied on the loss resonator, i.e., perturbation, causes PT-symmetry to break, generating complex frequencies. As a result of nonlinear gain, the resonator will grow to reach its steady state and saturate out the gain. This stable oscillation eliminates the complex frequencies, and the mode is ultimately localized at the gain side. We have observed that the voltage amplitude of the gain resonator due to the perturbation has experienced drastic changes. The amplitude ratio before and after the perturbation is sensitive to the perturbation. Our results provide an approach to study perturbation-driven localization phenomena in a PT-symmetric system and pave the way for sensors with ultrahigh sensitivity.