Drift-induced nonreciprocity in nonlocal plasmonic waveguides

Abstract In plasmonic systems under direct current bias, the drift of electrons induces a plasmonic Fizeau drag, leading to nonreciprocal (direction-dependent) spectral shifts of propagating surface plasmon polaritons (SPPs): the wavelength lengthens for propagation along the drift direction and shortens for propagation against it. However, a self-consistent description of this drift-induced nonreciprocity requires a rigorous treatment of nonlocality. Here, using a modified Maxwell-hydrodynamic model that self-consistently treats nonlocal responses arising from both intrinsic mechanisms and electron drift, we present a theoretical study of nonreciprocal SPP propagation in current-carrying plasmonic waveguides, including insulator-plasma (IP), insulator-plasma-insulator (IPI), and plasma-insulator-plasma (PIP) structures. Through complementary complex-frequency and complex-wavenumber dispersion analyses, a finite electron drift current is shown to consistently induce nonreciprocity in all three structures, manifested as a splitting of the forward and backward SPP dispersion branches. Moreover, unidirectional SPP propagation occurs in the current-carrying IP and IPI structures due to the emergence of zero-group-velocity states. Notably, in the current-carrying PIP structure, SPP modes with parallel group and phase velocities undergo conventional Fizeau drag, whereas those characterized by antiparallel group and phase velocities experience anomalous drag, an inverse Fizeau effect that serves as a plasmonic counterpart to the inverse Doppler effect observed in negative-index metamaterials. This work thus elucidates the fundamental mechanisms underlying drift-induced nonreciprocity and lays a theoretical foundation for the design of magnet-free nonreciprocal plasmonic devices.