The device, however, is not an optical rectifier or diode in the real sense, since it does not break time-reversal symmetry of an electromagnetic wave 14. The reverse halo effect, together with the plasmonic halo, make the structured Ag film work somewhat like an optical rectifier might, with the center of the Ag disk being bright when illuminated from one side and dark when illuminated from the other side. As we show below, the physical origin of this counterintuitive reverse halo phenomenon lies in a three-step coupling process: incident photons are coupled into SPPs, which then propagate and interfere along the metal/dielectric interface, forming standing waves, which couple back into photons via multiple scattering mechanisms. We call this a “reverse halo” effect due to its close relation to the plasmonic halo effect, but with the light direction reversed. In other words, via the step-gap circular drumhead structure, it appears as though photons transmit through an optically thick metal film. This is anomalous because in the central region, where strong far-field light intensity is observed, the Ag film is optically thick (300 nm), forbidding any direct transmission of photons. That is, when light is injected from the top side, one observes that the center of the SPP drumheads always appears bright, in strong contrast to the “plasmonic halo” case, where one sees a dark center of the drumheads. Here, we present another phenomenon related to this step gap drumhead structure, found by simply reversing the direction of illumination. 11, the spectrum of this leaked light is modulated by the formation of resonant SPP standing waves (drumhead modes) on the circular cavity surface, yielding a “plasmonic halo” effect 11. When light is injected from the bottom side of this device, one observes transmitted light leaking into the far field around the step gap circumference. An example of an SPP drumhead cavity surrounded by a circular step gap is shown in Fig. In previous work, we demonstrated SPP drumhead modes within circular structures 11. Our design has axial symmetry and thus has minimum requirements on the polarization state of incident light, such that it works for even unpolarized incidence. We show here a new version of symmetry-broken EM propagation, realized by breaking the geometrical symmetry on different sides of a nanostructured device. Although not in the real sense an optical isolator 14, these effects can have a wide range of optoelectronic applications 12, 13. These works break the symmetry between forward and backward travelling EM waves by generating asymmetric plasmon modes locally in the asymmetric metallic structures. Most recently, researchers are exploring the possibilities of achieving asymmetric propagation of electromagnetic (EM) waves via 2D (or quasi-2D) symmetry-broken structures 12, 13. Due to the advancement of fabrication and calibration techniques, research on novel applications based on SPPs and their cavity modes is expanding, recent examples of which include plasmonic waveguiding 5, 6, focusing 7, 8 and color filtering 9, 10, 11. SPPs have gained significant attention in recent years, perhaps especially since a report of extraordinary optical transmission through sub-wavelength hole arrays on metal films by Ebbesen et al. That plasmons can be excited along a metal-dielectric interface and then propagate as SPPs was noted by Ritchie in 1957 2. in 1929, plasmons are collective excitations of electrons in a metal 1. Surface plasmon polaritons (SPPs) are two dimensional electron density waves that can propagate along and are confined to a metal-dielectric interface.
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