Aluminum, with its distinctively favorable dielectric characteristics down to deep ultraviolet (UV) regime, has recently emerged as a broad-band and low-cost alternative to noble metals. However, low Q-factor resonances (Q ∼ 2−4), offered by Al nanostructures, pose a fundamental bottleneck for many practical applications. Here, we show that it is possible to realize Al-nanoantenna with remarkably large extinction cross sections and strong resonance characteristics surpassing those of their noble metal counterparts. By quenching radiation damping through far-field coherent dipolar interactions, we experimentally demonstrate exceptionally narrow line width (∼15 nm) and high Q-factor (∼27) dipolar plasmonic resonances in the blue-violet region of the optical spectrum (∼3 eV) beyond the practical operational limits of traditional plasmonic metals. To realize high Q-factor Al resonators, we introduce a novel space mapping algorithm enabling inverse design of Al nanoantenna arrays at arbitrary sub/superstrate material interfaces with diminished radiative losses. We show that radiatively coupled Al nanoantenna arrays offer remarkably high-Q factor (27 ≤ Q ≤ 53) resonances over the entire visible spectrum and readily outperform similarly optimized silver (Ag) nanoantenna arrays in green-blue-violet wavelengths (≤550 nm) and near UV regime. This report shows that it is possible to realize high Q-factor aluminum resonators by suppressing radiative losses and that Al-based plasmonics holds enormous potential as a viable and low-cost alternative to noble metals. Our inverse-design technique, on the other hand, provides a general and efficient approach in engineering of high Q-factor resonator arrays, independently from the metals and sub/superstrates used.

We study the coupling interactions between a progressively elongated silver nanoparticle and a silver film on a glass substrate. Specifically, we investigate how the coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) is influenced by nanoparticle length. Although the multiple resonances supported by the nanoparticle are effectively standing wave surface plasmons, their interaction with the SPP continuum of the underlying Ag film indicates that their spectral response is still localized in nature. It is found that these LSP-SPP interactions are not limited to small particles, but that they are present as well for extremely long particles, with a transition to the SPP coupling interactions of a bilayer metallic film system beginning at a particle length of approximately 5 μm. Coupling of metallic nanostructures in plasmonic systems has been a topic of major research interest, as it leads to effects such as strong near-field confinement, useful for trapping, sensing, nonlinear interactions, and surface-enhanced Raman scattering [1][2][3][4][5][6]. Coupling between finite-size plasmonic nanostructures and a conductive film is of particular interest as it involves the interaction of discrete localized resonances with a continuum of delocalized surface plasmon polaritons (SPPs) [7][8][9][10][11]. Systems exhibiting coupling of just a few localized resonances with a delocalized continuum have already been researched [7,8]. However, the interaction between an ever-growing number of higher order localized plasmon resonances and a SPP resonance continuum has yet to be discussed. Specifically, it is well known that coupling between a localized surface plasmon (LSP) resonance and a SPP continuum leads to an observable anticrossing at the LSP resonance frequency [7,8,[12][13][14][15][16][17][18][19][20][21]. What is not clear, however, is if this still holds true for a large number of localized resonances, as will be investigated in this Letter. The system studied here consists of a 2D silver nanoparticle of 40 nm height and variable length L spaced 100 nm above a 50 nm thick Ag film on a SiO 2 substrate of refractive index n 1.46 [ Fig. 1(a)]. A 2D geometry was chosen since effects along the third dimension are irrelevant. We use the dielectric data for silver measured by Johnson and Christy [22]. The simulation method carried out in this study is based on Green's tensor formalism in 2D [23]. As shown in Fig. 1(a), a horizontally oriented electric dipole source placed 100 nm to the left of the particle is used for excitation. A study of the system response for a finite length L is made via the same Fourier analysis method as in a previous work [24]. The plots obtained display the magnitude of the parallel k-vector component k x at each given frequency. Note that use of this Fourier analysis method leads to an unavoidable spectral broadening in k-space, which is inversely proportional to the particle length L, but does not pose any limitation on the current study.The sampling...

Abstract:We study a compound plasmonic system composed of a periodic Au grating array placed close to a thin Au film. The study is not limited to normal incidence and dispersion diagrams are computed for a broad variety of parameters. In addition to identifying localized and propagating modes and the coupling/hybridization interactions between them, we go further and identify modes of compound nature, i.e. those exhibiting both localized and propagating characteristics, and discuss which plasmon modes can exhibit such a behavior in the system at hand and how structural parameters play a central part in the spectral response of such modes.

Abstract:The transition from localized to delocalized plasmons (i.e. the transition from a situation where the decay length of a travelling surface plasma wave is greater than its propagation distance to a situation where it is smaller) and hence the onset of plasmon delocalization is studied in a single 2D silver nanoparticle of increasing length. A fourier analysis in the near-field of the nanoparticle is used as the main tool for analysis. This method, along with far-field scattering spectra simulations and the nearfield profile directly above and along the length of the nanoparticle are used to investigate and clearly show the transition from localized to delocalized modes. In particular, it is found that for a finite sized rectangular nanoparticle, both the emerging odd and even delocalized modes are nothing but a superposition of many standing wave plasmon modes. As a consequence, even very short metal films can support delocalized plasmons that bounce back and forth along the film. nanoshells for combined optical imaging and photothermal cancer therapy," Nano Lett. 7(7), 1929-1934 (2007). 10. J. Homola, S. S. Yee, and G. Gauglitz, "Surface plasmon resonance sensors: review," Sens. Actuators B Chem. ©2011 Optical Society of America54(1-2), 3-15 (1999). 11. D. K. Gramotnev, and S. I. Bozhevolnyi, "Plasmonics beyond the diffraction limit," Nat. Photonics 4(2), 83-91 (2010). 12. P. Berini, "Plasmon polariton modes guided by a metal film of finite width," Opt. Lett. 24(15), 1011-1013 (1999). 13. P. Berini, R. Charbonneau, and N. Lahoud, "Long-range surface plasmons on ultrathin membranes," Nano Lett. 4370-4379 (1972). 23. W. Lukosz, and M. Meier, "Lifetimes and radiation patterns of luminescent centers close to a thin metal film,"Opt. Lett. 6(5), 251-253 (1981 874-881 (1957). 26. D. Sarid, "Long-range surface-plasma waves on very thin metal films," Phys.

Compound plasmonic resonances arise due to the interaction between discrete and continuous metallic nanostructures. Such combined nanostructures provide a versatility and tunability beyond that of most other metallic nanostructures. In order to observe such resonances and their tunability, multiple nanostructure arrays composed of periodic metallic gratings of varying width and an underlying metallic film should be studied. Large-area compound plasmonic structures composed of various Au grating arrays with sub-100 nm features spaced nanometers above an Au film were fabricated using extreme ultraviolet interference lithography. Reflection spectra, via both numerical simulations and experimental measurements over a wide range of incidence angles and excitation wavelengths, show the existence of not only the usual propagating and localized plasmon resonances, but also compound plasmonic resonances. These resonances exhibit not only propagative features, but also a spectral evolution with varying grating width. Additionally, a reduction of the width of the grating elements results in coupling with the localized dipolar resonance of the grating elements and thus plasmon hybridization. This newly acquired perspective on the various interactions present in such a plasmonic system will aid in an increased understanding of the mechanisms at play when designing plasmonic structures composed of both discrete and continuous elements.