Observation of the Spin-Based Plasmonic Effect in Nanoscale Structures

Observation of surface-plasmon phenomena that are dependent upon the handedness of the circularly polarized incident light (spin) is presented. The polarization-dependent near-field intensity distribution obtained in our experiment is attributed to the presence of a geometric phase arising from the interaction of light with an anisotropic and inhomogeneous nanoscale structure. A near-field vortex surface mode with a spin-dependent topological charge was obtained in a plasmonic microcavity. The remarkable phenomenon of polarization-sensitive focusing in a plasmonic structure was also demonstrated.

Figure 1

Spin-dependent geometric phase in the spiral plasmonic structure. (a) The geometry of the structure (side view) and the optical setup. The quarter wave plate (QWP) was used to generate a circularly polarized illumination, which was partially coupled to the SPP mode in the cavity, $E_z$. (b) Scanning electron microscope (SEM) picture of the spiral plasmonic cavity. (c),(d) Intensity distribution in the cavity measured by a NSOM for $|{\sigma }_{+}⟩$ and $|{\sigma }_{-}⟩$ illumination, respectively. (e),(f) Calculated intensity in the cavity for $|{\sigma }_{+}⟩$ and $|{\sigma }_{-}⟩$ illumination. (g),(h) Calculated phase in the cavity for $|{\sigma }_{+}⟩$ and $|{\sigma }_{-}⟩$ illumination.

Figure 2

Plasmonic field distribution in the circular cavity. (a) SEM picture of the circular structure. (b) Measured intensity distribution in the near field for $|{\sigma }_{+}⟩$ illumination. An identical intensity distribution was obtained for $|{\sigma }_{-}⟩$. (c) Calculated intensity distribution for $|{\sigma }_{+}⟩$. (d),(e) Calculated $E_z$ field phase inside the cavity for $|{\sigma }_{+}⟩$ and $|{\sigma }_{-}⟩$ illumination, respectively.

Figure 3

Graphical representation of the geometric phase on the Majorana sphere. Geometric phase induced by the interaction of light with an anisotropic inhomogeneous structure is equal to half of the area enclosed by the two geodesics that correspond to different propagation directions $k_p$ of the excited SPPs (gray area). The variation of the propagation wave vector ${\mathbf{k}}_p(\phi )$ along the element is depicted in the small picture by blue arrows.

Figure 4

Spin-dependent plasmonic lens based on a geometric phase. (a),(b) The intensity distributions measured by a NSOM for $|{\sigma }_{+}⟩$ and $|{\sigma }_{-}⟩$ illumination, respectively. (c) The transverse cross sections of the measured intensity distributions in the focal plane of the lens for $|{\sigma }_{+}⟩$ (blue squares) and $|{\sigma }_{-}⟩$ (red circles) illumination, respectively. FDTD calculation is plotted for each polarization (solid blue line, $|{\sigma }_{+}⟩$; dashed red line, $|{\sigma }_{-}⟩$). The cross sections in (c) were measured along the horizontal dashed lines depicted in (a) and (b). (d) The intensity distribution measured by NSOM for $|{\sigma }_{+}⟩+|{\sigma }_{-}⟩$ (linearly polarized) illumination. (e) The SEM picture of the element.