Toward theoretically limited SPP propagation length above two hundred microns on ultra-smooth silver surface

We demonstrate the optical medium for surface plasmon - polariton waves (SPP) propagation with ultra low losses corresponding to the theoretically limited values. The unique element of the optical medium is an atomically-flat single-crystalline silver thin film which provides extremely low losses. The SPP excited on the surface of such thin films (at 780 nm) is characterized by a SPP propagation length equals to 200 um, which is twice longer than previously reported experimental results and corresponds to theoretically limited values for silver films.