Ratchet effect in lateral plasmonic crystal: Giant enhancement due to interference of "bright" and "dark" modes

Abstract: We develop a theory of the ratchet effect in a lateral plasmonic crystal (LPC) formed by a two dimensional electron gas under a periodic dual-grating gate. The system is driven by terahertz radiation, and the spatial asymmetry required for the generation of dc photocurrent is introduced by a phase shift between the radiation's near-field modulation and the static electron density profile. In contrast to the commonly used perturbative "minimal model" of the ratchet effect, which assumes weak density modulation, we solve the problem exactly with respect to the static gate-induced potential while treating the radiation field perturbatively. This approach reveals a dramatic enhancement of the plasmonic contribution to the ratchet current due to the interference of "bright" and "dark" plasmon modes, which are excited on an equal footing in the asymmetric LPC. Specifically, we predict a parametric growth of the plasmonic peak as compared with the Drude peak with increasing coupling, and the appearance of a dense super-resonant structure when the spacing between plasmonic sub-bands becomes larger than the damping rate. Hence, the dc response exhibits both resonant and super-resonant regimes observed in recent experiments on the radiation transmission through the LPC. The interplay of bright and dark modes, together with their interference, provides a powerful mechanism for controlling the magnitude and sign of the photocurrent by gate voltages and the radiation frequency.