Symmetry‐Broken Metamaterial Absorbers as Reflectionless Directional Couplers for Surface Plasmon Polaritons in the Visible Range

Abstract

directional coupling between propagating photons and surface plasmon polaritons (SPPs) in the visible range is a growing pursuit, due to its perceived essential role in nanoplasmonic and nanophotonic applications. [11][12][13] Here, we demonstrate that asymmetric, two-element, periodic, metal-insulator-metal (MIM) structures, perhaps representing the simplest gradient-index scenario, can serve as refl ectionless directional converters between free photons and SPPs operable in the visible frequency range. The simplicity of this two-element MIM structure allows for the fabrication of testable samples for operation with visible light. Simulated dispersion relations of this structure reveal the nature of the high-effi ciency coupling process: interactions between propagating modes caused by the broken refl ection symmetry (RS) and localized modes generated by individual MIM elements that are well understood in the language of MM "perfect absorbers". [ 14,15 ] There are regions in k -space where these interactions are very strong, such that the two modes hybridize to two new quasi-particles, [ 16,17 ] with concomitant Rabi splitting. [ 18,19 ] Experiments on samples with tuned parameters confi rm the simulated dispersion maps, and demonstrate a large Rabi splitting. Absolute refl ectance measurements under normal incidence with small aperture angle (3.75°) on samples with areas less than 60 × 60 µm 2 were carried out using a modifi ed microscope design. This scheme avoids the common problem of averaging over large incident angles when measuring optical properties of small-area samples by conventional methods. [ 20 ] Direct experimental evidence is obtained for refl ectionless (less than 8% measured refl ectance) directional SPP coupling in the visible range, with more than 88.8% of the channeled SPP energy being directed to the designed direction. Our results are meaningful for integrated nanoplasmonics, plasmonic logic, and plasmonic light harvesting, among others. Results and Discussions Localized Mode AnalysisUsing the measurement scheme described in Experimental Section, we performed refl ectance measurements to study Gradient-index meta-surfaces have been shown to have the ability to manipulate wavefronts at will in a refl ectionless manner in the GHz range, including the extreme example of converting freely propagating waves into surface waves with high effi ciency. Upon approaching the optical regime, the gradient-index concept encounters diffi culties due to fabrication limitations. Here, it is theoretically and experimentally demonstrated that asymmetric, periodic, two-element metal-insulator-metal structures can serve as refl ectionless directional converters between freely propagating visible photons and surface plasmon polaritons (SPPs). Coupling between propagating modes caused by the broken symmetry and localized modes generated by individual elements is shown to be the mechanism of this high-effi ciency process, yielding an observed Rabi splitting of ∼135 meV. Direct experimental evidence is obtained fo...

Plasmonic plano-semi-cylindrical nanocavities with high-efficiency local-field confinement

“…For d  < 6 nm, the hybrid plasmons “A” and “B” are nearly overlapped and cannot be resolved completely by their spectral features, as marked by empty triangles, and both exhibit a distinct blue shift as the space layer increases. This is based on the quickly reduced near-field interaction between SCNSs and the Au film591011121314. However, for d  > 6 nm, the hybrid plasmons split into “B” and “A”, which shift to opposite directions with “B” to the red and “A” to the blue, as the thickness of the spacer layer was increased further, as shown in Fig.…”

“…Localized surface plasmon resonance (LSPR) as a result of collective oscillation of free electrons in noble metals have been investigated extensively either in single nanostructures1234 or in hybrid systems like dimmers, oligomers, core-shells, particle-film systems, and etc 5678910111213141516171819202122232425262728…”

“…A typical hybrid design consisting of metallic nanoparticles and a metallic film with a dielectric spacer, which is known as the metal-insulator-metal (MIM) system, has been extensively investigated56789101112131415161718192021. The near-field interaction between particle plasmons and delocalized surface plasmons confines huge optical energy in the dielectric spacer and subsequently induces the out-of-plane plasmonic nanocavity resonance.…”

“…The near-field interaction between particle plasmons and delocalized surface plasmons confines huge optical energy in the dielectric spacer and subsequently induces the out-of-plane plasmonic nanocavity resonance. Such a resonance mode depends strongly on the thickness of the spacer, where strong confinement was observed for a spacer thickness smaller than 50 nm567891011121314151617. With increasing the spacer thickness, the confinement becomes weaker and part of the hybrid resonance mode radiates out of the system, decreasing spectral selectivity with broadened line width18192021.…”

Surface‐Plasmon‐Polariton Diode by Asymmetric Plano‐Concave Nanocavities

“…Much effort has been used to produce a variety of plasmonic designs, in order to break through these challenges. Slit‐grating structures, subwavelength asymmetric slits, aperiodic grooves, gradient nanoantennas or metasurfaces, slant gratings, and asymmetric gratings have been demonstrated as directional SPP structures. The subwavelength asymmetric slits or cavities usually had ultrasmall sizes beyond the diffraction limit and satisfied the wide operation bandwidth to excite unidirectional SPPs .…”