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, ...

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-

A new strategy for quantitatively detecting micrococcal nuclease (MNase) is proposed using electrostatic interaction-based fluorescence resonance energy transfer (FRET) between positively charged QDs and negatively charged dye-labeled single-stranded DNA (dye-ssDNA). Herein, we have made our attempt to develop a strategy where the variation of FRET efficiency is due to the change of the electrostatic interaction between QDs and the ssDNA that result from the cleavage of dye-ssDNA by a single-strand-specific nuclease. To demonstrate the feasibility of this design, positively charged QDs (lysozyme modified QDs, Lyz-QDs) are prepared as the energy donor, with the fluorescent dye 6-carboxy-X-rhodamine (ROX) that is labeled to ssDNA serving as the energy acceptor. The ROX-labeled probe ssDNA (ROX-ssDNA) is absorbed to the surface of the QDs through electrostatic interaction, which results in resonance energy transfer between the QDs and the dye. In the presence of MNase which cleaves the ROX-ssDNA into small fragments, the weakened interaction between QDs and the shortened ssDNA causes the decrease of FRET efficiency. At given amounts of donor and acceptor, the ratio of fluorescence intensity of QDs to ROX changes in a MNase concentration-dependent manner. Under optimized conditions, the ratio is linear with MNase concentration over the range of 8 x 10(-3) to 9.0 x 10(-2) unit mL(-1), with a limit of detection of 1.6 x 10(-3) unit mL(-1). This new detection strategy features straightforward design and easy operation, which is capable of expanding the application of the positively charged QDs-based FRET in DNA-related bioassays.

The synthesized Nd fluoride nanocrystals exhibited different upconversion behaviors as dispersed and aggregated samples due to the different energy transfer mechanisms. When they were dispersed in water, the NaNdF(4) nanocrystals exhibited approximately 400 times stronger upconversion fluorescence than the NdF(3) nanocrystals. Remarkable upconversion behaviors were found when the nanocrystals were aggregated in the films. For the NdF(3) nanocrystals, the energy transfer processes (4)I(13/2)<--(4)F(3/2)-->(4)G(7/2) in the films generated avalanche upconversion emissions with a high slope of approximately 12.0, which could be due to the large avalanche cross relaxation rates and spectral broadening effect. In contrast, the spectral broadening effect in the NaNdF(4) NCs films increased the energy transfer (4)I(15/2)<--(4)F(3/2)-->(4)G(5/2) of the Nd(3+) ions, and induced a new upconversion emission at approximately 680 nm with the slope increased from 1.0 to 3.2.

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