Coupler-free transition from light to surface plasmon polariton

It is widely believed that surface plasmon polaritons (SPPs) at flat metal surfaces cannot be directly excited by free-space lights due to momentum mismatch. Here we propose a way to resonantly excite SPPs by lights without using any coupler and propose a kind of surface-plasmon-resonance (SPR) system which can be simply composed by a metal film and a bottom medium whose real part of permittivity is less than unity. A light, which is incident from vacuum or air to the upper surface of the metal film, can directly excite a SPP at the lower surface of the metal film. Some special media can be selected as the bottom medium, including atomic vapor of electromagnetically induced transparency (EIT) and some special solid media. In the atomic EIT case, the steep dispersion at the transparent window can lead to interesting phenomena; e.g., very slow directional moving of the EIT atoms can lead to unidirectional SPPs (reverse-propagating SPP modes do not exist) and therefore lead to a highly asymmetric SPR angle spectrum which is very sensitive to the velocity of the atoms. We also present feasible and simple designs of all-solid-state coupler-free SPR systems, which are simply composed by a titanium film and a substrate of amorphous aluminum oxide or silicon oxide.