Non-Hermitian gauged laser arrays with localized excitation: Anomalous threshold and generalized principle of selective pumping

We investigate non-Hermitian skin modes in laser arrays with spatially localized excitation. Intriguingly, we observe an unusual threshold behavior when selectively pumping either the head or the tail of these modes: Both cases exhibit the same lasing threshold and hence defy the conventional principle of selective pumping, which aims to maximize the overlap between the pump profile and the target lasing mode. To shed light on this enigma, we reveal a previously overlooked phenomenon, i.e., energy exchange at non-Hermitian coupling junctions with the photonic environment, which does not occur with uniform gain or loss. Utilizing a transfer matrix approach, we elucidate the mechanism of this anomalous threshold behavior, which is determined by the specific physical realization of the non-Hermitian gauge field (i.e., using gain, loss, or their mixture). Finally, we derive a generalized principle of selective pumping in non-Hermitian arrays, which shows that the decisive spatial overlap is given by the tripartite product of the pump, the lasing mode, and its biorthogonal partner. Our study provides a glimpse into how the two forms of non-Hermiticity, i.e., asymmetric couplings and a complex onsite potential, interact synergetically in laser arrays, which may stimulate further explorations of their collective effects in photonics and related fields.