Uniform optical gain as a non-Hermitian control knob

Abstract

Non-Hermitian optics utilizes judicious engineering of the spatial and spectral distribution of gain and loss in order to tailor the behavior of photonic systems in ways that could not be achieved by modulating only the real part of the refractive index. In this respect, a question that has never been addressed is whether a uniform distribution of gain or loss can also lead to nontrivial non-Hermitian effects in linear systems, beyond just signal amplification or decay. Here, we investigate this problem and demonstrate that the application of uniform gain to a symmetric photonic molecule (PM) can reverse the optical energy distribution inside the structure. For a PM composed of two coupled resonators, this translates into changing the optical energy distribution inside the resonators. For a PM formed through scattering or defect-induced intermodal coupling in a ring resonator, the applied gain, despite being uniform and symmetric, can impose a strong chirality and switch the direction of light propagation from dominantly clockwise to dominantly counterclockwise. These predictions are confirmed by using both coupled mode formalism and full-wave finite-element simulations. Our work establishes a different direction in the field of non-Hermitian optics where interesting behavior can be engineered not only by unbalancing the non-Hermitian parameter but also by changing its average value - a feature that was overlooked in previous works.