Deterministic control of surface plasmon polariton excitation at the metal–atom interface via off-resonant light–matter interaction in a non-ideal EIT medium

Abstract

Quantum coherence controlled by the resonant condition of electromagnetically induced transparency (EIT) has attracted considerable attention for the excitation of surface plasmon polaritons (SPPs) in various metal–dielectric hybrid structures. However, for all coherent fields operating resonantly in the system, the exact extent to which perfect EIT is effective in establishing coupler-free SPP excitation has not yet been well examined. Here, we investigate the excitation of SPPs in an air–metal–atom composite structure incorporating a three-level Λ-type atomic model that operates in the strong ground-state coherence-dephasing regime. Such decoherence limits the establishment of true EIT under the condition of two-photon Raman resonance. It is found that, although the characteristic narrow resonant field enhancement associated with EIT mediated by the resonant fields disappears, significant SPP-assisted field enhancement survives over a broad off-resonant spectral region. This behavior originates from the preservation of the momentum-matching condition together with the fulfillment of the dielectric requirements for surface plasmon resonance (SPR) in the normal-dispersion regime. If the coupling field is tuned to the SPR condition, the detuned two-photon Raman resonance plays a vital role in structuring a doublet in the spectral response of an initially single-peaked broadband SPP excitation. This observation demonstrates that, although true EIT does not occur in the system, an EIT-like plasmonic spectral signature appears through the off-resonant SPP excitation mechanism. All-optical switching can be realized by turning the detuned condition of the coupling field on and off, thereby exhibiting this coherence feature. Furthermore, we show that the present model remains robust in the Doppler-broadened regime, providing a realistic route toward coherence-controlled coupler-free SPP excitation beyond the conventional ideal-EIT framework.