Focusing surface plasmon polaritons in archimedes' spiral nanostructure

Surface plasmon polaritons (SPPs) are a hybrid mode of a light field and metallic collective electrons oscillated resonantly and excited at the metal/dielectric interface. Recently extensive research has been carried out due to its technological potential in nano-optics. The SPPs coupling, focusing, waveguiding and resonance enhancement are hot spots in this field. In particular, to find a simple method that can focus SPPs into a highly confined spot with the size beyond the diffraction limit is still a big challenge. In this work, we have fabricated the Archimedes' spiral structures with different structural parameters on an Au film by using focused ion beam etching technique. Through changing the chiralities of the incident circularly polarized light and the spiral structure, we have studied theoretically and experimentally the focusing properties of the Archimedes spiral structures with different parameters. We find that besides the chiralities of the incident light and the spiral structure, the pitch of screw of the spiral structure and the wavelength of the excited light also affect the surface plasmon field. The resulting surface plasmon fields inside the structure are the zero-order, first-order, and high-order evanescent Bessel beams. By using a phase analysis and a finite-difference time-domain simulation method, we calculate the electric field and phase distribution in different spiral structures. A near-field vortex mode with different spin-dependent topological charges can be obtained in the structures. Furthermore, the results of the scanning near-field optical microscopy measurements verify the theory and simulation results. The method of using an Archimedes' spiral structure to focus SPPs provides a new route to manipulate the SPPs optical field in nanoscale. Based on theoretical calculation and FDTD simulation in this work, we have studied the physical process of the optical field manipulation in spiral structures. The significant and innovated points of this work are: a) We have developed the phase theory, and analyzed the field manipulation process of spiral structures with different parameters and chiralities at different circular polarization and wavelengths. b) A more effective and convenient way is used for SPPs focusing in linearly polarized light and circularly polarized light. c) A near-field vortex surface mode with different spin-dependent topological charges is obtained for the structure. This work can be considered to have applications in SPPs tweezers, highly integrated photonic devices.