Observation of two-photon coherence in plasmon-assisted transmission

We experimentally study two-photon coherence in plasmon-assisted transmission with a two-photon Mach–Zehnder (MZ) interferometer. Two collinear photons of identical or orthogonal polarization are simultaneously incident on one optically thick metal film, perforated with a periodic array of subwavelength holes. Interference fringes with two-photon de Broglie wavelength are observed, which indicates that the quantum coherence of biphoton is preserved in surface plasmon assisted transmission.