Strong resonant coupling between light and plasmons in noble metal particles of nanometer dimensions leads to a number of striking and technologically important optical effects, among them surface enhanced Raman scattering (SERS) [1] and the enhancement of fluorescence from nearby molecules. [2] While each of these show great promise for the development of highly sensitive biochip detectors, [3,4] fluorescence is the technique of choice for many biological assays. Significant enhancement here would greatly enhance the sensitivity of these assays to a host of target biomolecules. To date, the maximum enhancement available in fluorescence has not been established. This is largely due both to difficulties in controlling the size and shape of the particles, and to the multiplicity of contributing factors: increased radiative decay rate and enhanced electric fields at resonance, ''hot spots'', i.e. regions of high field between closely spaced particles. The substrate is known to play a role as well; in particular there have been suggestions that certain substrates might play an active role in light-plasmon coupling rather than merely shifting the resonance frequency, as would a conventional glass substrate. Here we report observations of just such an effect, in which the size dependence of the enhancement of fluorescence from monodisperse silver nanoparticles is profoundly altered by the use of a Si substrate. Comparing fluorescence measurements with calculations of the response of the silver nanoparticles to incident light, we show that unlike what is commonly assumed, the variation of the fluorescence enhancement with nanoparticle diameter does not simply follow that of plasmon excitation as measured by the optical extinction. Instead we find that it is the generation of regions of high electrical field intensity near the particle which dominates the fluorescence enhancement we observe, and that the silicon substrate plays an active role in this regard: sweeping these regions out from beneath the particles as their size approaches the optimum for fluorescence. ResultsIn Figure 1 we compare scanning electron microscopy images and fluorescence microscopy images from size-selected, fluorescently-tagged silver nanoparticles on a silicon substrate over a range of particle diameters. In Figure 1(a)-(e) the SEM images are shown in red, and fluorescence images for an excitation wavelength of 514 nm (exciting the Cy3 fluorophore), scanned from precisely the same regions are superposed in green. The SEM images show that particles are spherical, and randomly distributed. There are rare occasions where two or more particles are in contact; this occurs more frequently on samples with larger average particle diameter, as shown in Figure 1(e). In general, however, individual particles are well separated from their neighbors. The fluorescence intensity shows a strong and systematic variation from image to image, i.e. as the average diameter of the Ag nanoparticles is varied. Highest intensities are obtained from samples with partic...

By embedding NaYF 4 :Yb 3+ ,Tm 3+ nanocrystals into the top cladding layer of a resonant waveguide grating structure, we demonstrate that the upconversion fluorescence of Tm 3+ ions can be greatly enhanced, by a factor of up to 10 4 . The resonant waveguide grating structure consists of an SU8 bottom layer with sinusoidal grating morphology coated with a thin TiO 2 waveguide layer and then covered with a poly(methyl methacrylate) cladding layer doped with NaYF 4 :Yb 3+ ,Tm 3+ nanocrystals. The giant enhancement of the upconversion fluorescence is achieved first by coupling the excitation light with a guided mode of the resonant waveguide grating structure and then the fluorescent light with a second guided mode. Our numerical simulation results obtained by rigorous coupled-wave analysis indicate that the electric field of the incident light is strongly enhanced near the interface of the TiO 2 layer and the poly(methyl methacrylate) layer at guided mode resonance, and this is the major effect of the observed enhancement of the upconversion fluorescence of the nanocrystals. The resonance between the fluorescent emission and the waveguide structure further enhances the intensities of the fluorescent signal. We also find that the lifetime of upconversion fluorescence at 480 nm wavelength from the rare-earth nanocrystals is reduced about 1.34-fold when both excitation and extraction resonance occurs in the waveguide structure.

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In this work, we present efficient, robust, and transparent electromagnetic interference (EMI) shielding by a hybrid material comprised of a nickel (Ni) mesh and a conformal graphene coating. We demonstrate that a 20 nm-thick graphene/Ni hybrid mesh can provide EMI shielding effectiveness (SE) exceeding 12.1 dB (∼93.6% power attenuation) in the decimeter band while retaining a high visible transmittance of ∼83%. Its maximum achieved SE value was 26.6 dB (∼99.5% power attenuation) at 0.75 GHz. Furthermore, the thicker Ni mesh exhibited a higher EMI SE. Compared to a conventional Ni mesh, the hybrid mesh exhibits a higher SE and a greatly improved corrosion resistance. The graphene coating is directly grown on a Ni mesh via rapid annealing of solid carbon precursors under low vacuum. Scalable fabrication of the mesh was achieved by a self-formed TiO2 crack network template. Our results not only provide a promising material for high-performance EMI shielding in optoelectronics devices but also enable applications of EMI shielding in harsh environments.

We explore the role of coupling between silver nanowires and an underlying silver film in fluorescence enhancement from proximal molecules. Variation of the thickness of an oxide layer separating nanowire arrays from the Ag film causes an alternation in the incident light polarization that produces the highest enhancement. Finite difference time domain calculations show that it results from an alternation of regions of high field above and between nanowires as the spacer thickness is increased.