ABSTRACT
We investigated the process of focusing a radially polarized (RP) light beam through a sub-wavelength annular aperture (SAA). We found that the result was a non-diffraction doughnut-shaped light beam which propagates in free space. After analyzing the electric field component of the focus generated by the SAA structure, we identified the relationship between the focal field generated by the SAA. We then compared it to a case with a traditional objective lens. From our findings, we propose that a SAA structure can be viewed as a continuous numerical aperture optical element.
Subject(s)
Computer-Aided Design , Lenses , Lighting/instrumentation , Optical Devices , Refractometry/instrumentation , Surface Plasmon Resonance/instrumentation , Computer Simulation , Equipment Design , Equipment Failure Analysis , Light , Models, Theoretical , Scattering, RadiationABSTRACT
We examined the optical properties such as propagation modes, focal length, side lobes, etc. of metallic subwavelength annular apertures (SAA) and used finite-difference time-domain (FDTD) simulation to compare our experimental findings. Using two different metals, silver and tungsten, we examined the different optical transmission properties of the two metallic SAA structures. The far-field propagation of the silver SAA structure was found to be a type of quasi-Bessel beam when compared with a quasi-Bessel beam generated by a perfect axicon. The propagation characteristics of these two beams were found to match qualitatively. The far-field transmitted light generated by the silver SAA structure was found to possess a 390 nm sub-micron focal spot with a 24 microm depth of focus, which was much smaller than the focal spot generated by a perfect axicon. We also found that a silver SAA structure can generate a sub-micron quasi- Bessel beam that has a much lower far-field side-lobe when compared to that of non-diffraction beams generated by a tungsten SAA structure.
Subject(s)
Nanostructures/chemistry , Nanotechnology/instrumentation , Refractometry/instrumentation , Silver/chemistry , Surface Plasmon Resonance/instrumentation , Tungsten/chemistry , Computer-Aided Design , Equipment Design , Equipment Failure Analysis , Light , Nanostructures/ultrastructure , Refractometry/methods , Reproducibility of Results , Scattering, Radiation , Sensitivity and Specificity , Surface Plasmon Resonance/methodsABSTRACT
We propose a direct experimental set-up to observe the directional beaming effect of surface plasmon. A single diffracted beam from an asymmetric-sided surface corrugation is demonstrated. A single subwavelength slit with an asymmetric structure was fabricated using a focused ion beam (FIB) onto a metal surface with a glass substrate. By means of surface plasmon (SP) diffraction, the directionality of the light can be changed by the period of the metallic gratings. We show corresponding numerical simulations achieved by a Rigorous Coupled-Wave Analysis (RCWA) method and a Finite-Difference Time-Domain (FDTD) method. The simulation results were in agreement with the experimental data.
