Fig. 3: Non-Hermitian resonant systems and their responses.
From: Linear response theory of open systems with exceptional points
a A schematic of a resonant open system, where non-Hermiticity arises due to coupling to external channels and the presence of optical loss or gain. As summarized in (b), the linear response of this system, as described by its resolvent operator \(\hat{G}(\omega )\) can be readily obtained in terms of the right/left eigenvectors of the associated non-Hermitian Hamiltonian when the spectrum of the latter does not contain any EPs. On the contrary, to date, a general expression for \({\hat{G}}_{{{{{{\rm{EP}}}}}}}(\omega )\) in the presence of EPs has not been yet developed.
