We compare the optical response of periodic nondiffracting metallic nanoparticle and nanohole arrays. Experimental data from both structures show a pronounced minimum in their wavelength-dependent transmittance that, through numerical modeling, we identify as being due to the excitation of localized surface-plasmon resonances associated with the nanoparticles/nanoholes. Our main finding is that, while the optical response of the nanoparticle arrays is largely independent of interparticle separation, the response from nanohole arrays shows a marked dependence on interhole separation. We attribute this effect to coupling between localized surface-plasmon resonances mediated by the symmetric surface plasmon-polaritons associated with the metal film. Further numerical modeling supports this view. [4][5][6] applications that exploit the strongly localized electromagnetic fields associated with these resonant modes. Arrays of nanoparticles have in particular received considerable attention as advances in fabrication techniques allow finer control over structural dimensions. [7][8][9][10][11] It is also known that when metal nanoparticles are brought into close proximity to each other the modes they support may interact, or couple, so as to modify both the resonance shape and frequency of the LSPRs. 12,13 The properties of LSPRs associated with metallic nanoparticles can also be modified by the presence of a nearby metallic surface. 14,15 The extraordinary optical transmission properties of regular arrays of nanoholes in thin metal films were first reported by Ebbesen and co-workers. 16 Since then it has been demonstrated that under appropriate conditions single nanoholes in metallic films may support LSPRs in a manner analogous to that of nanoparticles. [17][18][19][20][21][22] The similarities between the LSPRs of nanoholes and nanodiscs were discussed by Haynes et al. 10 Indeed, Käll and co-workers 17 recently showed that for irregular arrays of such holes the LSPRs of the nanoholes are blueshifted as the hole density is increased, an effect attributed to coupling between LSPRs of neighboring holes. However, as far as we are aware, there has not yet been a comparison of interparticle/interhole coupling in periodic nondiffracting metallic nanoparticle/nanohole arrays.Here we present such a study and show that despite their similarities, these complementary structures show marked differences in their optical response. For the range of periods considered here, we find that the spectral position of the transmittance minimum associated with the nanohole arrays varies with the array period, an effect we attribute to strong LSPR coupling mediated by surface plasmon-polaritons ͑SPPs͒ supported by the intervening flat metallic film. For the complementary nanoparticle arrays there is little shift since SPPs are not supported by this structure. Results from numerical modeling help us identify the role of the symmetric ͑with respect to surface charge distributions 23 ͒ SPP mode supported by the metal film in causing this diffe...
We explore the optical response of two-dimensional (2D) arrays of silver nanoparticles, focussing our attention on structures for which the individual particles in isolation support both dipolar and quadrupolar localised surface plasmon modes. For individual spheres we show that when dipolar and quadrupolar modes are excited simultaneously, interference leads to most of the scattered light being radiated in the forward direction. This is in contrast to what happens when each mode is excited on its own. We further show, using finite-element modelling that when such particles are assembled into square 2D arrays, the dipolar and quadrupolar modes can combine to produce a single peak in the optical density of the array. By simulating the field distributions associated with these modes we are able to illustrate the dual-mode character of this feature in the optical density. We have extended our examination of this effect by considering how the optical density of these arrays changes with incident angle for two polarisations (s and p).
We present composite plasmonic nanostructures designed to achieve cascaded enhancement of electromagnetic fields at optical frequencies. Our structures were made with the help of electron-beam lithography and comprise a set of metallic nanodisks placed one above another. The optical properties of reproducible arrays of these structures were studied by using scanning confocal Raman spectroscopy. We show that our composite nanostructures robustly demonstrate dramatic enhancement of the Raman signals when compared to those measured from constituent elements. [7], and data storage [8]. Normally, optical fields are concentrated by focusing light with appropriate lenses, the minimum volume of the enhanced field ultimately being determined by the wavelength of the light used. It is well known that metal nanostructures allow one to concentrate light in smaller volumes through the excitation of localized surface plasmons, thereby enhancing the strength of the electric field over that available otherwise [9]. Individual metallic nanoparticles allow a modest field enhancement, of the order of the quality factor Q of the plasmonic resonance [10]. The field strength can be increased further in particle conglomerates [11,12] and particle pairs [13]. The gap between particles is particularly important in many systems [11][12][13][14]. Recently, it was suggested that a new class of composite metallic nanostructures might be used to provide very high and well-controlled optical-field enhancements [10].The confinement of fields using metallic nanostructures, in volumes well below the diffraction limit, down to just several tens of nanometers, involves a process mediated by the electron plasma of the metal [10]. The deeply subwavelength volumes of field confinement and strongly enhanced optical fields that are predicted for composite metallic nanostructures offer many intriguing prospects, for example, 3D optical near-field trapping [15,16]. Surface-enhanced Raman scattering (SERS) from single molecules immobilized on ''hot'' colloidal nanoparticle structures has been reported [17][18][19] with field enhancements of $100 being invoked to account for this extraordinary sensitivity [20]. These results provide an incentive to find ways to produce such high field enhancements in a controlled way, rather than relying on nondeterministic colloidal synthesis techniques.The self-similar plasmonic structure suggested by Li, Stockman, and Bergman [10] presents an appealing design for a composite nanostructure in which the near fields produced by an illuminated large metallic nanoparticle play the role of the exciting field for a smaller metallic nanoparticle, the result of which is to enhance the field further. The electromagnetic field may thus undergo a cascaded enhancement by a factor of up to g tot $ g n , where g $ Q $ Re"ð!Þ=Im"ð!Þ [10] ["ð!Þ is the metal permittivity; e.g., for gold Q $ 7 at a wavelength of 630 nm] and n is the number of cascades. Although a self-similar nanoassembly has been demonstrated [21], no experimental verif...
We present results from composite plasmonic nanostructures designed to achieve the cascaded enhancement of electromagnetic fields at optical frequencies. Our structures comprise a small metallic nanodisc suspended above a larger disk. We probe the optical properties of these structures by coating them with a layer of a visible-light fluorophore and observing fluorescence signals with the help of scanning confocal microscopy. A 43 +/- 5-fold increase in the far-field fluorescence signal has been observed for two-tier composite nanostructures, when compared to the signal obtained from individual nanodiscs. Our results offer the prospect of using such nanostructures for field concentration, optical manipulation of nanoobjects, chemical and biological sensing.
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