We investigate the optical response of a gold nanorod array coupled with a semicontinuous nanoparticle film. We find that, as the gold nanoparticle film is adjusted to the percolating regime, the nanorod-film hybrids are tuned into plasmonic Fano resonance, characterized by the coherent coupling of discrete plasmonic modes of the nanorod array with the continuum band of the percolating film. Consequently, optical transmission of the percolating film is substantially enhanced. Even more strikingly, electromagnetic fields around the nanorod array become much stronger, as reflected by 2 orders of magnitude enhancement in the avalanche multiphoton luminescence. These findings may prove instrumental in the design of various plasmonic nanodevices.KEYWORDS Plasmonic Fano resonance, plasmon hybrid, gold percolating film, gold nanorod array, enhanced transmission, enhanced photoluminescence E xtensive research efforts have been devoted recently to utilizing metal nanostructures to manipulate the propagation, intensity, and polarization of light, [1][2][3][4][5][6] leading to the emergence of nanophotonics as a major new direction in photonics. In this emerging field, one central physical entity is plasmon, characterizing the collective excitation of conduction electrons in metal nanostructures. Many intriguing phenomena discovered recently, such as the squeezing of light into subwavelength nanoholes, 7-9 and the detection of molecules trapped between nanogaps via surface-enhanced Raman scattering (SERS) with single molecule sensitivity, [10][11][12] are tied to the coupling of incident light with plasmon modes. Such studies not only broaden our fundamental understanding of photon interaction with nanoscale systems, but also may have far-reaching technological impacts.In exploration of various intriguing plasmonic phenomena at the nanoscale, a widely studied and distinctive research emphasis is the exploitation of the coupling and hybridization of different plasmon modes supported by various elegantly fabricated metal nanostructures. [13][14][15][16][17][18][19][20][21][22] Compelling examples include the plasmon coupling of a discrete mode to a continuum band, known as the "plasmonic Fano resonance". The Fano type absorption spectra were first reported in hole arrays in thin metal films and coaxial metallic arrays due to interferences of localized and delocalized plasmon modes.23-25 A more vivid picture of the plasmonic Fano model with three interaction regimes was convincingly demonstrated in metallic nanoparticle-film systems by tuning the film thickness. 26 Recently, multiple Fano resonances in a metallic ring/disk dimer and twinned Fano resonances in the Au-Ag heteronanorod dimer were also reported. 27 Most research focused on the asymmetric Fano line-shape in the absorption spectra, but enhanced emissions and Raman scattering induced by constructive interferences via the plasmonic Fano effect are seldom explored, which is of great importance for both passive and active plasmonic nanosystems.In this Letter, ...

The chemical growth of silver nanorings that possess singly twinned crystals and a circular cross section via a reductive reaction solution is reported. The wire and ring diameters of the synthesized nanorings are in the ranges 80–200 nm and 4.5–18.0 μm, respectively. By lighting up the multipolar dark plasmons with slanted illumination, the silver nanoring exhibits unique focused scattering and large local‐field enhancement. We also demonstrate strong exciton–plasmon interactions between a monolayer of CdSe/ZnS semiconductor quantum dots and a single silver antenna‐like nanoring (nanoantenna) at the “hot spots” located at the cross points of the incident plane and nanoring; the position of these spots are tunable by adjusting the incidence angle of illumination. The tunable plasmonic behavior of the silver nanorings could find applications as optical nanoantennae or plasmonic nanocavities.

D irectional control over the excitation energy transfer between different nanosystems is of critical importance for the emerging field of nanophotonics and has various prospective applications ranging from biological detections to quantum information processing. 1Ϫ14 For instance, the excitation energy transfer between semiconductor quantum dots (QDs) is employed to demonstrate quantum operations, 4 and the stimulated interactions between active optical dipoles and surface plasmons are used to generate plasmonic lasing. 5Ϫ8 Comparing with nonradiative energy transfer (such as Dexter and Fö rster processes), 15,16 radiative energy transfer has sufficient distance range but poor efficiency and directionality. The surface plasmons of the exquisitely designing and optimizing metal nanostructures are powerful tools to enhance the efficiency of both radiative and nonradiative energy transfers. 1,17Ϫ19 The Ag films have been used by Andrew et al. to first demonstrate plasmon-mediated radiative energy transfer from donor to acceptor dye molecules over distances longer than 100 nm, 1 which proceeds in three processes, converting optical dipole of the donors to the surface plasmon on one interface of a Ag film, then cross coupling of two surface plasmons on the opposite interfaces of the film, and finally transferring excitation energy to the acceptors on the opposite side. On the basis of this principle, the corrugated nanostructures have been used by Feng et al. to enhance the cross coupling of the two surface plasmons on the opposite interfaces. 17 A two-dimensional (2D) standing metal nanowire array could be a good candidate to assist radiative energy transfer due to its near-field coupling and imaging behaviors. 20,21 The excitation energy of nanoemitters can be efficiently converted to the surface plasmons through strong coupling near the tips of the Ag nanowires, 10 and the corresponding Purcell factor, P (the ratio of the spontaneous emission rate into the plasmon modes over emission into other channels), is predicted to be as high as 10 3 . 22,23 Metal nanorods support both longitudinal and transverse surface plasmon resonances (abbreviated by LSPRs and TSPRs, respectively) by the free electrons near the metal surface oscillating perpendicularly to and along the long axis of the nanorods. Resonant transmission through a Au nanorod array with a far-field excitation is reported by Lyvers et al.,24 which is found to be caused by the half-

We fabricated dark-bright-dark plasmon resonators, investigated enhanced-transmission induced by plasmon transfer from the bright mode to the dark modes. Furthermore, we demonstrated interferences between dark plasmons excited by the near-field bright plasmons and directly excited by slantwise incident light, which leads to selective storage of excited energy in one of the two dark plasmon cells in the resonators by adjusting the phase between signal and control sources. This could find the applications in all-optical modulations.

Ag nanowire (NW) arrays with NW diameter d NW =12-120 nm were electrodeposited in anodic aluminum oxide templates. Strong avalanche photoluminescence (PL) from Ag NW arrays with small d NW were observed near 914 nm by using picosecond laser at the excitation wavelength 808 nm, which is originated from the plasmonenhanced radiative intraband transitions. The peak PL intensity of the avalanche PL from the sample with small diameter d NW =12 nm is about 10 2 times stronger than that of the linear PL from the sample with large diameter d NW =120 nm. The opposite excitation polarization dependence and emission polarization distribution of the PL from Ag NW array with d NW =12 nm and d NW =120 nm were also observed.