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    ABSTRACT: We demonstrate that the surface second-harmonic generation can lead to the formation of nonlinear plasmonic whispering-gallery modes (WGMs) in microcavities made of metallic nanowires. Since these WGMs are excited by induced surface nonlinear dipoles, they can be generated even when they are not coupled to the radiation continuum. Consequently, the quality factor of these nonlinear modes can be as large as the theoretical limit imposed by the optical losses in the metal. Remarkably, our theoretical analysis shows that nonlinear plasmonic WGMs are characterized by fractional azimuthal modal numbers. This suggests that the plasmonic cavities investigated here can be used to generate multi-color optical fields with fractional angular momentum. Applications to plasmonic sensors are also discussed.
    Physical Review Letters 10/2013; 111(20). DOI:10.1103/PhysRevLett.111.203903
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    ABSTRACT: Resistive switching offers a promising route to universal electronic memory, potentially replacing current technologies that are approaching their fundamental limits. In many cases switching originates from the reversible formation and dissolution of nanometre-scale conductive filaments, which constrain the motion of electrons, leading to the quantisation of device conductance into multiples of the fundamental unit of conductance, G0. Such quantum effects appear when the constriction diameter approaches the Fermi wavelength of the electron in the medium - typically several nanometres. Here we find that the conductance of silicon-rich silica (SiOx) resistive switches is quantised in half-integer multiples of G0. In contrast to other resistive switching systems this quantisation is intrinsic to SiOx, and is not due to drift of metallic ions. Half-integer quantisation is explained in terms of the filament structure and formation mechanism, which allows us to distinguish between systems that exhibit integer and half-integer quantisation.
    Scientific Reports 09/2013; 3:2708. DOI:10.1038/srep02708
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    ABSTRACT: Objective. Detonation nanodiamond monolayer coatings are exceptionally biocompatible substrates for in vitro cell culture. However, the ability of nanodiamond coatings of different origin, size, surface chemistry and morphology to promote neuronal adhesion, and the ability to pattern neurons with nanodiamonds have yet to be investigated. Approach. Various nanodiamond coatings of different type are investigated for their ability to promote neuronal adhesion with respect to surface coating parameters and neurite extension. Nanodiamond tracks are patterned using photolithography and reactive ion etching. Main results. Universal promotion of neuronal adhesion is observed on all coatings tested and analysis shows surface roughness to not be a sufficient metric to describe biocompatibility, but instead nanoparticle size and curvature shows a significant correlation with neurite extension. Furthermore, neuronal patterning is achieved with high contrast using patterned nanodiamond coatings down to at least 10 µm. Significance. The results of nanoparticle size and curvature being influential upon neuronal adhesion has great implications towards biomaterial design, and the ability to pattern neurons using nanodiamond tracks shows great promise for applications both in vitro and in vivo.
    Journal of Neural Engineering 09/2013; 10(5):056022. DOI:10.1088/1741-2560/10/5/056022
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    ABSTRACT: In this paper, we propose a heterogeneous-prioritized spectrum sharing policy for coordinated dynamic spectrum access networks, where a centralized spectrum manager coordinates the access of primary users (PUs) and secondary users (SUs) to the spectrum. Through modeling the access of PUs and multiple classes of SUs as continuous-time Markov chains, we analyze the overall system performance with consideration of a grade-of-service guarantee for both the PUs and the SUs. In addition, two new call admission control (CAC) strategies are devised in our models to enhance the maximum admitted traffic of SUs for the system. Numerical results show that the proposed heterogeneous-prioritized policy achieves higher maximum admitted traffic for SUs. The trade-off between the system's serving capability and the fairness among multiple classes of SUs is also studied. Moreover, the proposed CAC strategies can achieve better performance under max-sum, proportional, and max-min fairness criteria than the conventional CAC strategies. Copyright © 2011 John Wiley & Sons, Ltd.
    Wireless Communications and Mobile Computing 04/2013; 13(5). DOI:10.1002/wcm.1118
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    ABSTRACT: To address the question of surface oxidation in organic electronics the chemical composition at the surface of single crystalline rubrene is spatially profiled and analyzed using Time of Flight - Secondary Ion Mass Spectroscopy (ToF-SIMS). It is seen that a uniform oxide (C42H28O) covers the surface while there is an increased concentration of peroxide (C42H28O2) located at crystallographic defects. By analyzing the effects of different primary ions, temperature and sputtering agents the technique of ToF-SIMS is developed as a valuable tool for the study of chemical composition variance both at and below the surface of organic single crystals. The primary ion beams C603+ and Bi3+ are found to be most appropriate for mass spectroscopy and spatial profiling respectively. Depth profiling of the material is successfully undertaken maintaining the molecular integrity to a depth of ∼5 μm using an Ar cluster ion source as the sputtering agent.
    Physical Chemistry Chemical Physics 03/2013; DOI:10.1039/c3cp50310k
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    ABSTRACT: Coupling between tuneable broadband modes of an array of plasmonic metamolecules and a vibrational mode of carbonyl bond of poly(methyl methacrylate) is shown experimentally to produce a Fano resonance, which can be tuned in situ by varying the polarization of incident light. The interaction between the plasmon modes and the molecular resonance is investigated using both rigorous electromagnetic calculations and a quantum mechanical model describing the quantum interference between a discrete state and two continua. The predictions of the quantum mechanical model are in good agreement with the experimental data and provide an intuitive interpretation, at the quantum level, of the plasmon-molecule coupling.
    Physical Review Letters 02/2013; 110(8):087402. DOI:10.1103/PhysRevLett.110.087402
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    ABSTRACT: Ubiquitous in-network caching is one of the key aspects of information-centric networking (ICN) which has received widespread research interest in recent years. In one of the key relevant proposals known as Content-Centric Network-ing (CCN), the premise is that leveraging in-network caching to store content in every node along the delivery path can enhance content delivery. We question such an indiscriminate universal caching strategy and investigate whether caching less can actually achieve more. More specifically, we study the problem of en route caching and investigate if caching in only a subset of nodes along the delivery path can achieve better performance in terms of cache and server hit rates. We first study the behavior of CCN's ubiquitous caching and observe that even naïve random caching at a single intermediate node along the delivery path can achieve similar and, under certain conditions, even better caching gain. Motivated by this, we propose a centrality-based caching algorithm by exploiting the concept of (ego network) betweenness centrality to improve the caching gain and eliminate the uncertainty in the performance of the simplistic random caching strategy. Our results suggest that our solution can consistently achieve better gain across both syn-thetic and real network topologies that have different structural properties. We further find that the effectiveness of our solution is correlated to the precise structure of the network topology whereby the scheme is effective in topologies that exhibit power law betweenness distribution (as in Internet AS and WWW networks).
    Computer Communications 02/2013; DOI:10.1016/j.comcom.2013.01.007
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    ABSTRACT: The design of a new non-invasive hybrid microwave-optical tissue oxygenation probe is presented, which consists of a microwave biocompatible antenna and an optical probe. The microwave antenna is capable of inducing localised heat in the deep tissue, causing tissue blood flow and therefore tissue oxygenation to change. These changes or thermal responses are measured by the optical probe using near-infrared spectroscopy. Thermal responses provide important information on thermoregulation in human tissue. The first prototype of the biocompatible antenna was developed and placed on the human calf for in vivo experiments. The measured results include oxy-, deoxy- and total haemoglobin concentration changes (ΔHbO2/ΔHHb/ΔHbT), tissue oxygenation index and the normalised tissue haemoglobin index for two human subjects. Both ΔHbO2 and ΔHbT show an increase during 5 min of microwave exposure. The thermal response, defined as the ratio of the increase in ΔHbT to the time duration, is 7.7 μM/s for subject 1 (fat thickness = 6.8 mm) and 18.9 μM/s for subject 2 (fat thickness = 5.0 mm), which may be influenced by the fat thicknesses. In both subjects, ΔHbO2 and ΔHbT continued to increase for approximately another 70 s after the microwave antenna was switched off.
    Advances in Experimental Medicine and Biology 01/2013; 789:371-7. DOI:10.1007/978-1-4614-7411-1_49
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    ABSTRACT: Exact closed-form analytical expressions are derived for the average bit error probability of multibranch switched diversity systems over independent and identically Nakagami-m distributed fading channels. Practical schemes that use noncoherent or differentially coherent symbol detection are considered. The general bit error probability expression derived in this paper includes as particular cases the following signaling formats: orthogonal binary signaling, correlated binary signaling, differential phase-shift keying, and differential quadrature phase-shift keying. Finally, we apply our analytical results to study the impact of the switching threshold selection on the system performance. Copyright © 2011 John Wiley & Sons, Ltd.
    International Journal of Communication Systems 01/2013; 26(1):127-137. DOI:10.1002/dac.1337
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    ABSTRACT: A simple chemical route to fabricating highly linear, well-aligned and unbranched one-dimensional arrays of silver nanoparticles with ultra-high aspect ratios is demonstrated. Starting with a polydisperse nanoparticle material, the application of ultrasound was found to promote digestive ripening and induce a thiolate-driven self-assembly process. The assembly method discovered has been extended to other systems, with initial findings showing the successful templating of gold nanoparticles into chains using Cu(I)-thiolates. This opens the possibility of a generic route which can be applied to many nanoparticle systems.
    Advanced Materials 12/2012; 24(38):5227-35. DOI:10.1002/adma.201202005
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