Research On Polarization States In Surface Plasmonic Nanophotonics

Surface plasmons are electromagnetic mode existing between metal and dielectric, including propagating surface plasmon polaritons (SPPs) and localized surface plasmons resonance (LSPR). Surface plasmons are highly confined in the metal surface with a sub-wavelength attenuation length in dielectric, making them an important research topic in nanophotonics. They have been used in all-optical control, super-resolution imaging, optical sensing, data storage, metamaterials, photonic integrated circuits, enhancing fluorescence/Raman scattering and other applications. Recently, the 2D nanoscale fabrication technology has made significant progress, driven by the micro-electronics manufacture. This technology is very suitable for fabricating the surface plasmons structures, making the surface Plasmonics one of the most important fields.This paper is aimed at the study of nanoscale surface Plasmonics. Using the sub-wave length SPPs and polarization-sensitive coupling to light in free space, we designed a new miniature polarization analyzer for measuring arbitrary states of polarization (SOP) of light. For metal particles with local surface plasmons resonance, they behave as an antenna in optical frequency region. We made an optical rotator for linear polarized light, which made it possible to control the SOP of light in sub-wavelength scale. Surface plasmons should be fully studied by the near-field method because that they are highly confined to the surface of metal. We summarized the theory of near-field microscope and put forward a new simplified scattering-type scanning near-field optical microscope (s-SNOM) model. Furthermore, using s-SNOM we investigated several nano-materials, especially noble-metal nano-materials. We give a summary of the near-field imaging contrast and measured SPPs mode in monocrystalline silver nanowires. The work about s-SNOM is valuable for theory and applications.1. The SOP of arbitrary light can be described by four Stokes parameters, which can be measured by linear polarizer and a set of quarter wave plate and linear polarizer (SQAL). The function of SQAL is equivalent to a circular polarizer. The measurement of the Stokes parameters consists of four experimental operations at least. For each of the operations a particular polarization component of incident light can be obtained. However, it’s very hard to miniaturize the setup for the bulky quarter wave plates and linear polarizers. We made a new miniature polarization analyzer (MPA), based on our designed linear polarizer and SQAL using the "bulls’ eye" structures in Plasmonics. It demonstrated that MPA has well behavior in SOP measurement and low crosstalk between adjacent MPAs in the experiments. These advantages made it possible to fabricate MPAs array for real-time polarization imaging.2. Optical activity has very high practical value, originating from circular birefringence which is an optical phenomenon for all chiral materials. Mostly the circular birefringence in natural materials is very weak. Usually the polarization rotation can only be observed in bulk natural material. We realized such a polarization rotator by using surface plasmons on noble metal. The optical rotator is based on optical circular polarized antenna, which is feed by dual-SPPs sources. To adjust the phase of SPPs propagating from two directions, we introduced a phase difference between left- and right-polarization light radiated by the antenna. This results in a polarization rotation of linear polarized light.3. Different from the old-type based on tapered fiber as probe, s-SNOM is a new-type SNOM using normal atomic force microscope (AFM) probe. It collects the scattering light from the tip of AFM probe as the near-field optical signal. For the radius of curvature of probe tip can be very small, the s-SNOM has a higher optical resolution (about 10nm). However, it needs more technology to get the near-field information from various signals scattering from probe, making it more complicated to analyze s-SNOM image. A good near-field model is of great importance to interpret the near-field image. At present, the most important model for s-SNOM is the "dipole moder" in quasi-static approximation, which is an excellent one for infrared light, not suited very well to visible light. Here, we put forward a new simplified s-SNOM model using dyadic Green’s function method, which gives a reference to interpret the experimental result.4. Monocrystalline silver nanowires is one of important surface plasmons materials, which is usually used as SPPs waveguide and SPPs cavity. Using the s-SNOM we investigated the SPPs mode on monocrystalline silver nanowires. We found an interesting interference pattern with a period equal to one SPPs wavelength, differing from the normal SPPs standing wave pattern with a period half the SPPs wavelength. According to FDTD simulation and our s-SNOM model, we found that this effect is mainly due to the interference between incident exciting light and weakly resonant SPPs cavity mode. Furthermore, we measured several nano-materials. We found that the near-field imaging contrast can be classified by optical property contrast and near-field contrast. This is agreed with our model.