Single-plasmon interferences

Abstract

Surface plasmon polaritons are electromagnetic waves coupled to collective electron oscillations propagating along metal-dielectric interfaces, exhibiting a bosonic character. Recent experiments involving surface plasmons guided by wires or stripes allowed the reproduction of quantum optics effects, such as antibunching with a single surface plasmon state, coalescence with a two-plasmon state, conservation of squeezing, or entanglement through plasmonic channels. We report the first direct demonstration of the wave-particle duality for a single surface plasmon freely propagating along a planar metal-air interface. We develop a platform that enables two complementary experiments, one revealing the particle behavior of the single-plasmon state through antibunching, and the other one where the interferences prove its wave nature. This result opens up new ways to exploit quantum conversion effects between different bosonic species as shown here with photons and polaritons.

Keywords: physics; plasmonic device; quantum optics; surface plasmon; wave-particle duality.

Fig. 1. The plasmonic platform.

(A) Scanning electron microscope top view of the photon-to-SPP launcher. It is made of 11 grooves of asymmetric dimensions (29). (B) Scanning electron microscope top view of the plasmonic chip. Striped rectangles 1 and 2 are the SPP launchers as shown in (A). The groove doublet forms a plasmonic BS. The characterized splitter gives T₁ = 29 ± 1% and R₁ = 18 ± 1% when shining from coupler 1 and T₂ = 32 ± 1% and R₂ = 15 ± 1% when shining from coupler 2. For both input ports, the losses of the BS are measured to be approximately 53%. The SPPs propagate from launcher 1 or 2 to the BS and finally reach the large slits (black rectangles) where they are converted into photons in the silica substrate. (C) Line shape of the sample. It exhibits how an SPP can be generated with a Gaussian beam focused orthogonally to the photon-to-SPP converter. The SPP reaches the grooves of the plasmonic BS and finally propagates to the slit. The slit allows the SPP to couple out as photon in the substrate at 42° with an efficiency of about 50%.

Fig. 2. Experiments on SSPs showing the unicity of the SPP state and its wave behavior.

(A) Sketch of the SPP experiments. The orientation of the first half-wave plate (HWP0) determines the polarization state impinging on the PBS cube and allows choosing between the HBT and MZ configurations of the SPP setup. HWP1 and HWP2 are half-wave plates that control the polarization of the incident beams on the photon-to-SPP couplers. For both experiments, we recorded the heralding rate R_C and the heralded rates R_A|C, R_B|C, and R_AB|C. (B) Intensity correlation function at zero delay g⁽²⁾(0) as a function of the mean photon number produced in the gating window ΔT = 10 ns. The lowest measured value of g⁽²⁾(0) obtained is 0.03 ± 0.06, which is well below the classical limit and is a signature of a single SPP state. The data points were obtained with 20 min of integration. (C) The single SPP source was used at g⁽²⁾(0) = 0.25 to perform interferences in an MZ interferometer for SPPs. We plotted the heralded photon output rates R_A|C (red circles) and R_B|C (blue squares) of the MZ interferometer for a varying delay in one arm of the interferometer. The solid lines are the sine fit functions of our experimental data.