Study On The Properties Of Off-axially Excited SPPs Self-interference

Surface Plasmon Polaritons(SPPs) are surface electromagnetic wave excited by evanescent field on the interface of the metal/dielectric, with the properties of near field enhancement, sub-wavelength confinement and anomalous dispersion relation, which makes the photons can be manipulated in the wavelength scale. SPPs have been widely used in many fields.The resolution of the existing optical instruments is difficult to be further improved because of the diffraction limit in classical optics. As a result, higher integration of photoelectric device cannot be achieved, failing to satisfy the needs of social development. Surface plasmon polaritons provide an important breakthrough for problem. In addition, paraxial incident light beam is a significant condition to guarantee the imaging quality of optical instruments designed on the basis of the paraxial principle. Off-axis of incident beams will bring the problems like optics aberrations, resulting in the reduce of the imaging resolution, which directly affects the application effect of SPPs. It has great theoretical and practical significant to studying the properties of SPPs excited by off-axis beams.Exciting SPPs by normal incident beams has the advantages of symmetry and time saving. When the beam deviated from optic axis, the distribution of wave vectors in focus field will change in the overall, causing the distortion of the transmission field, decrease of the intensity and fringe visibility, and the like. In this paper, the mechanism and distribution characteristics of SPPs excited by tightly focused off-axis beams are analyzed in theoretical and simulation in detailed. Some basic rules of SPPs excited by off-axis beams are revealed. In the theoretical calculation, we use the angular spectrum analysis method. In simulation, we adopt the commercial calculation software----FDTD Solutions, which based on finite difference time domain method. In this paper, the concept of "critical off-axis Angle" and "equivalent numerical aperture" are first proposed. The intensity distribution of transmission field is modulated by the longitudinal resonance component. The modulation effect will be weakened with the increase of the off-axis angle, and a theoretical critical angle exists, whose value is found to be smaller than the actual one. Under the condition of linear polarized, when the tilt direction of incidence beams is parallel or perpendicular to the polarization direction, the interference field of SPPs is still axially symmetry, but with only one symmetry axis.S wave and P wave components in the focus field will redistribute when the incident beams tilt perpendicular to the polarization, reducing the excitation efficiency of SPPs. Under the same angle, efficiency of oblique excitation is higher than off-axis excitation. The oblique and off-axis of an incident beams is actually weakening the modulation effect of the resonance components, decreasing the instrument imaging resolution. Field distribution derived from angular spectrum and FDTD simulation are basically coincide, but the fringe contrast of the latter is higher. MSE of the two fields is less than 0.02, proving that the angular spectrum formula deduced from this paper is reasonable.