We analyze electromagnetic modes in multi-layered nano-composites and demonstrate that the response of a majority of practical layered structures is strongly affected by the effective nonlocalities, and is not described by conventional effective-medium theories. We develop the analytical description of the relevant phenomena and confirm our results with numerical solutions of Maxwell equations. Finally, we use the developed formalism to demonstrate that multi-layered plasmonic nanostructures support high-index volume modes, confined to deep subwavelength areas. PACS numbers:Nanolayered composites have been recently proposed to serve as negative index systems, super-and hyper-lenses, photonic funnels, and other nanophotonic structures [1,2,3,4,5,6,7,8,9,10,11]. The typical thickness of an individual layer in these "artificial" (meta-) materials is of the order of 10nm. Since this size is much smaller than optical (or IR) wavelength, it is commonly assumed that the properties of the multilayered composites are well-described by the effective medium theory (EMT) [12,13]. In this Letter, we analyze the modes of realistic multi-layered structures and show that the conventional EMT fails to adequately describe these modes due to the metamaterial analog of spatial dispersionstrong variation of the field on the scale of a single layer. We derive a non-local correction to EMT, bridging the gap between metamaterial-and photonic crystal-regimes of multi-layered media, and use numerical solutions of Maxwell equations to verify our results. Finally, we use the developed technique to identify volume metamaterial modes confined to nanoscale areas.While the formalism developed below is applicable to the composites with arbitrary values of permittivities, we illustrate the developed technique on the example of plasmonic nanolayered composite -the major component of a variety of beam-steering and imaging systems [3,4,11]. The schematic geometry of such a structure, containing alternating layers of materials with permittivities, ǫ 1 , ǫ 2 and (average) thicknesses a 1 and a 2 respectively, is shown in Fig.1. In the analytical results presented below, we focus on the propagation of TM waves, which are responsible for plasmon-assisted phenomena, and postpone the straightforward generalization of our approach for TE, and mixed waves as well as for multi-component structures to the future work. In the selected geometry, x coordinate axis is perpendicular to the plane defined by layer interfaces, while y and z axes are parallel to this plane; the direction of z axis is chosen so that that the FIG. 1: Schematic geometry of a planar nanolayer-based meta-material, surrounded by two cladding layers electromagnetic waves propagate in x, z plane.The majority of practical realizations of layered nanoplasmonic structures rely on the metamaterial regime, when the typical layer thickness is much smaller than the free-space wavelength λ so that surface plasmon polaritons propagating on different metal-dielectric interfaces are strongly coupled to ...
We show that a stack of metal-dielectric nanolayers, in addition to the long-and short-range plasmon polaritons, guides also an entire family of modes strongly confined within the multilayer -the bulk plasmon polariton modes. We propose a classification scheme that reflects specific properties of these modes. We report experimental verification of the bulk modes by measuring modal indices in a structure made of three pairs of silica(~29nm)/gold(~25nm) layers.PACS numbers: 42.25. Bs, 42.82.Et, 73.20.Mf, 73.21.Ac Nanoscale confinement of light is of great interest for applications in sensing, imaging, all-optical signal processing and computing. Subwavelength confinement attributed to gap plasmon polaritons (GPP) has been demonstrated in a thin dielectric layer surrounded by metallic claddings.1 Here we present another solution to subdiffraction confinement of light and show that a stack of metal-dielectric nanolayers guides a family of modes strongly confined within the multilayer -the bulk plasmon polariton (BPP) modes. The bulk modes have very short penetration depth into the claddings even if the claddings are made of dielectric materials. Their modal indices (ratio of the light velocity in vacuum to the phase velocity of a guided mode) are typically large in absolute value and may be both positive and negative. 2 We propose a classification scheme that reflects specific properties of BPPs. We verify BPPs experimentally by measuring their modal indices in a structure made of three pairs of silica/gold nano-layers.When considering the light confinement in a waveguide, the modal index n * (rather than group index) is of interest because it defines the modal profile. The field penetration length into the cladding with permittivity, where λ is the vacuum wavelength. For given ε cl , the larger modal index leads to shorter penetration length and stronger confinement. This justifies interest to the highindex modes. In all-dielectric waveguides, the modal index is smaller than the core index, which limits the confinement scale to ~λ/7 in a silicon-on-insulator waveguide with silicon core (n≈3.5) and silica or vacuum claddings. is slightly above the index of the dielectric. In the visible and near infrared spectral region, the permittivity of metal is typically negative and |ε m |>>ε d leading to subwavelength field penetration into metal, while the penetration into the dielectric can be as large as several wavelengths. A remarkable exception is the case of resonant SPPs 5 when permittivities of materials at different sides of the interface are exactly opposite: ε m +ε d →0 and thus n * →∞. In homogeneous media, the field distribution of the resonant SPP is expected to be confined infinitely close to the interface. Actual scale of the field distribution is defined by the applicability of the concept of dielectric permittivity, that is, by the discrete atomic structure of materials. At optical frequencies, the resonant SPPs are possible if the dielectric has huge optical gain. 6 A metallic film of thickness t m betw...
Long-range surface-plasmon-polariton (LR–SPP) waveguiding along thin gold stripes embedded in polymer is investigated in the wavelength range of 1510–1620 nm. LR–SPP intensity distributions at the output are measured for different stripe widths and thicknesses. Coupling loss of ∼0.5 dB is achieved when exciting the fundamental LR–SPP mode along 10-nm-thick stripes of 6–10 μm width with a polarization maintaining fiber. LR–SPP propagation loss of 6–8 dB/cm is estimated (at 1550 nm) and attributed to scattering from inhomogeneities of the metal stripe and polymer cladding.
An array of design strategies have been targeted toward minimizing failure of implanted microelectrodes by minimizing the chronic glial scar around the microelectrode under chronic conditions. Current approaches toward inhibiting the initiation of glial scarring range from altering the geometry, roughness, size, shape, and materials of the device. Studies have shown materials which mimic the nanotopography of the natural environment in vivo will consequently result in an improved biocompatible response. Nanofabrication of electrode arrays is being pursued in the field of neuronal electrophysiology to increase sampling capabilities. Literature shows a gap in research of nanotopography influence in the reduction of astrogliosis. The aim of this study was to determine optimal feature sizes for neural electrode fabrication, which was defined as eliciting a nonreactive astrocytic response. Nanopatterned surfaces were fabricated with nanoimprint lithography on poly(methyl methacrylate) surfaces. The rate of protein adsorption, quantity of protein adsorption, cell alignment, morphology, adhesion, proliferation, viability, and gene expression was compared between nanopatterned surfaces of different dimensions and non-nanopatterned control surfaces. Results of this study revealed that 3600 nanopatterned surfaces elicited less of a response when compared with the other patterned and non-nanopatterned surfaces. The surface instigated cell alignment along the nanopattern, less protein adsorption, less cell adhesion, proliferation and viability, inhibition of glial fibrillary acidic protein, and mitogen-activated protein kinase kinase 1 compared with all other substrates tested.
We describe the concept of a super compact diffractive imaging spectrometer, with optical components a few millimeters across in all dimensions, capable of detecting optical fluorescence spectra within the entire visible spectral range from 400 nm to 700 nm with resolution of the order of 2 nm. In addition, the proposed spectrometer is capable of working simultaneously with multiple, up to 35, independent input optical channels. A specially designed diffractive optical element integrated with a planar optical waveguide is the key component of the proposed device. In the preliminary experimental tests, a uniform waveguide grating with a microlens was used to mimic operation of the diffractive optical element. A microspectrometer with optical components measured below 1 cm in all dimensions covers the spectral range from 450 nm to 650 nm and shows a spectral resolution of 0.5 nm at wavelengths close to 514 nm and 633 nm.
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