Equivalent Circuits and Nanoplasmonics

We show how a circuit analysis, used widely in electrical engineering, finds application to problems of light wave injection and transport in subwavelength structures in the optical frequency range. Lumped circuit and transmission-line analysis may prove helpful in the design of plasmonic devices with standard, functional properties.

Diffraction-managed superlensing using metallodielectric heterostructures

We show that subwavelength diffracted wave fields may be managed inside multilayered plasmonic devices to achieve ultra-resolving lensing. For that purpose we first transform both homogeneous waves and a broad band of evanescent waves into propagating Bloch modes by means of a metal/dielectric (MD) superlattice. Beam spreading is subsequently compensated by means of negative refraction in a plasmon-induced anisotropic effective-medium that is cemented behind. A precise design of the superlens doublet may lead to nearly aberration-free images with subwavelength resolution in spite of using optical paths longer than a wavelength.

Low refractive index gas sensing using a Surface Plasmon Resonance fibre device

A series of surface plasmonic fibre devices were fabricated using multiple coatings deposited on a lapped section of a single mode fibre. Coupling from the guided mode to surface plasmons was promoted following UV laser irradiation of the coated region through a phase mask, which generated a surface relief grating structure. The devices showed high spectral sensitivities and strong coupling for low refractive indices as compared to other grating-type fibre devices. The plasmonic devices were used to detect the variation in the refractive indices of alkane gases with measured wavelength and coupling sensitivity to index of 3400 nm RIU-1 and 8300 dB RIU-1, respectively. As a demonstration of the performance of these gas sensors, a minimum concentration of 2% by volume of butane in ethane was achieved.

Comparison of adhesion layers of gold on silicate glasses for SERS detection

Gold is one of the most widely used metals for building up plasmonic devices. Although slightly less efficient than silver for producing sharp resonance, its chemical properties make it one of the best choices for designing sensors. Sticking gold on a silicate glass substrate requires an adhesion layer, whose effect has to be taken into account. Traditionally, metals (Cr or Ti) or dielectric materials (TiO2 or Cr2O3 ) are deposited between the glass and the nanoparticle. Recently, indium tin oxide and (3-mercaptopropyl)trimethoxysilane (MPTMS) were used as a new adhesion layer. The aim of this work is to compare these six adhesion layers for surface- enhanced Raman scattering sensors by numerical modeling. The near-field and the far-field optical responses of gold nanocylinders on the different adhesion layers are then calculated. It is shown that MPTMS leads to the highest field enhancement, slightly larger than other dielectric materials. We attributed this effect to the lower refractive index of MPTMS compared with the others.

Cu nanoparticles enable plasmonic-improved silicon photovoltaic devices

This work examines the effect of copper nanoparticles (Cu NPs) on the photocurrent efficiency of silicon photovoltaic (Si PV) devices. An optimized synthesis of stable Cu NPs is reported together with a procedure for their immobilization on the Si PV surface. A comprehensive analysis of the photocurrent and power dependence of the Cu NPs surface coverage and size is presented. A decrease in photoconversion was observed for wavelengths shorter than similar to 500 nm, due to the Cu interband absorption. In the low surface coverage limit, where the level of aggregation was found to be low, the surface plasmon resonance absorption dominates leading to a modest effect on the photocurrent response. As the number of aggregates increased with the surface coverage, the photocurrent efficiency also increased, and a maximum enhancement power conversion of 16% was found for a 54 +/- 6 NPs per mu m(2) PV cell. This enhancement was attributed to SPR light scattering and trapping into the Si PV device. Higher surface coverage yielded numerous aggregates which acted as a bulk coating and caused a decrease in both photocurrent and power measurements.

New sensing platforms based on nanoscopic devices for probing protein-membrane interactions

A novel nanosized and addressable sensing platform based on membrane coated plasmonic particles for detection of protein adsorption using dark field scattering spectroscopy of single particles has been established. To this end, a detailed analysis of the deposition of gold nanorods on differently functionalized substrates is performed in relation to various factors (such as the pH, ionic strength, concentration of colloidal suspension, incubation time) in order to find the optimal conditions for obtaining a homogenous distribution of particles at the desired surface number density. The possibility of successfully draping lipid bilayers over the gold particles immobilized on glass substrates depends on the careful adjustment of parameters such as membrane curvature and adhesion properties and is demonstrated with complementary techniques such as phase imaging AFM, fluorescence microscopy (including FRAP) and single particle spectroscopy. The functionality and sensitivity of the proposed sensing platform is unequivocally certified by the resonance shifts of the plasmonic particles that were individually interrogated with single particle spectroscopy upon the adsorption of streptavidin to biotinylated lipid membranes. This new detection approach that employs particles as nanoscopic reporters for biomolecular interactions insures a highly localized sensitivity that offers the possibility to screen lateral inhomogeneities of native membranes. As an alternative to the 2D array of gold nanorods, short range ordered arrays of nanoholes in optically transparent gold films or regular arrays of truncated tetrahedron shaped particles are built by means of colloidal nanolithography on transparent substrates. Technical issues mainly related to the optimization of the mask deposition conditions are successfully addressed such that extended areas of homogenously nanostructured gold surfaces are achieved. Adsorption of the proteins annexin A1 and prothrombin on multicomponent lipid membranes as well as the hydrolytic activity of the phospholipase PLA2 were investigated with classical techniques such as AFM, ellipsometry and fluorescence microscopy. At first, the issues of lateral phase separation in membranes of various lipid compositions and the dependency of the domains configuration (sizes and shapes) on the membrane content are addressed. It is shown that the tendency for phase segregation of gel and fluid phase lipid mixtures is accentuated in the presence of divalent calcium ions for membranes containing anionic lipids as compared to neutral bilayers. Annexin A1 adsorbs preferentially and irreversibly on preformed phosphatidylserine (PS) enriched lipid domains but, dependent on the PS content of the bilayer, the protein itself may induce clustering of the anionic lipids into areas with high binding affinity. Corroborated evidence from AFM and fluorescence experiments confirm the hypothesis of a specifically increased hydrolytic activity of PLA2 on the highly curved regions of membranes due to a facilitated access of lipase to the cleavage sites of the lipids. The influence of the nanoscale gold surface topography on the adhesion of lipid vesicles is unambiguously demonstrated and this reveals, at least in part, an answer for the controversial question existent in the literature about the behavior of lipid vesicles interacting with bare gold substrates. The possibility of formation monolayers of lipid vesicles on chemically untreated gold substrates decorated with gold nanorods opens new perspectives for biosensing applications that involve the radiative decay engineering of the plasmonic particles.

Site-controlled Ag nanocrystals grown by molecular beam epitaxy-Towards plasmonic integration technology

We demonstrate site-controlled growth of epitaxial Ag nanocrystals on patterned GaAs substrates by molecular beam epitaxy with high degree of long-range uniformity. The alignment is based on lithographically defined holes in which position controlled InAs quantum dots are grown. The Ag nanocrystals self-align preferentially on top of the InAs quantum dots. No such ordering is observed in the absence of InAs quantum dots, proving that the ordering is strain-driven. The presented technique facilitates the placement of active plasmonic nanostructures at arbitrarily defined positions enabling their integration into complex devices and plasmonic circuits.

Subwavelength beams with polarization singularities in plasmonic metamaterials

We investigated the diffraction behavior of plasmonic Bessel beams propagating in metal-dielectric stratified materials and wire media. Our results reveal various regimes in which polarization singularities are selectively maintained. This polarization-pass effect can be controlled by appropriately setting the filling factor of the metallic inclusions and its internal periodic distribution. These results may have implications in the development of devices at the nanoscale level for manipulation of polarization and angular momentum of cylindrical vector beams.

Microfluidic Paper-based Devices For Bioanalytical Applications.

Paper has become increasingly recognized as a very interesting substrate for the construction of microfluidic devices, with potential application in a variety of areas, including health diagnosis, environmental monitoring, immunoassays and food safety. The aim of this review is to present a short history of analytical systems constructed from paper, summarize the main advantages and disadvantages of fabrication techniques, exploit alternative methods of detection such as colorimetric, electrochemical, photoelectrochemical, chemiluminescence and electrochemiluminescence, as well as to take a closer look at the novel achievements in the field of bioanalysis published during the last 2 years. Finally, the future trends for production of such devices are discussed.

Triboelectric Effect As A New Strategy For Sealing And Controlling The Flow In Paper-based Devices.

We reported here for the first time that triboelectric charges on PET sheets can be used to seal and control the flow rate in paper-based devices. The proposed method exhibits simplicity and low cost, provides reversible sealing and minimizes the effect of sample evaporation.

Nanostructured thin films obtained by electrodeposition over a colloidal crystal template: applications in electrochemical devices

Colloidal particles have been used to template the electrosynthesis of several materials, such as semiconductors, metals and alloys. The method allows good control over the thickness of the resulting material by choosing the appropriate charge applied to the system, and it is able to produce high density deposited materials without shrinkage. These materials are a true model of the template structure and, due to the high surface areas obtained, are very promising for use in electrochemical applications. In the present work, the assembly of monodisperse polystyrene templates was conduced over gold, platinum and glassy carbon substrates in order to show the electrodeposition of an oxide, a conducting polymer and a hybrid inorganic-organic material with applications in the supercapacitor and sensor fields. The performances of the resulting nanostructured films have been compared with the analogue bulk material and the results achieved are depicted in this paper.

Nanochromics: old materials, new structures and architectures for high performance devices

Due to the development of nanoscience, the interest in electrochromism has increased and new assemblies of electrochromic materials at nanoscale leading to higher efficiencies and chromatic contrasts, low switching times and the possibility of color tuning have been developed. These advantages are reached due to the extensive surface area found in nanomaterials and the large amount of organic electrochromic molecules that can be easily attached onto inorganic nanoparticles, as TiO2 or SiO2. Moreover, the direct contact between electrolyte and nanomaterials produces high ionic transfer rates, leading to fast charge compensation, which is essential for high performance electrochromic electrodes. Recently, the layer-by-layer technique was presented as an interesting way to produce different architectures by the combination of both electrochromic nanoparticles and polymers. The present paper shows some of the newest insights into nanochromic science.

Conducting polymer- hydrogel blends for electrochemically controlled drug release devices

Blends formed by electrochemical polymerization of polypyrrole (PPy) into polyacrylamide (PAAm) hydrogels were used as devices for controlled drug release. The influence of several parameters in the synthesis, such as type of hydrogel matrix and polymerization conditions was studied by using a fractional factorial design. The final goal was to obtain an adequate device for use in controlled release tests, based on electrochemical potential control. For controlled release tests, Safranin was used as model drug and release curves (amount of drug vs. time) have shown that these blends are promising materials for this use. The optimized blends obtained were characterized by cyclic voltammetry and Raman spectroscopy.

Programmable diagnostic devices made from paper and tape

This paper describes three-dimensional microfluidic paper-based analytical devices (3-D mu PADs) that can be programmed (postfabrication) by the user to generate multiple patterns of flow through them. These devices are programmed by pressing single-use 'on' buttons, using a stylus or a ballpoint pen. Pressing a button closes a small space (gap) between two vertically aligned microfluidic channels, and allows fluids to wick from one channel to the other. These devices are simple to fabricate, and are made entirely out of paper and double-sided adhesive tape. Programmable devices expand the capabilities of mu PADs and provide a simple method for controlling the movement of fluids in paper-based channels. They are the conceptual equivalent of field-programmable gate arrays (FPGAs) widely used in electronics.

Automatic tuning method for the design of supplementary damping controllers for flexible alternating current transmission system devices

The design of supplementary damping controllers to mitigate the effects of electromechanical oscillations in power systems is a highly complex and time-consuming process, which requires a significant amount of knowledge from the part of the designer. In this study, the authors propose an automatic technique that takes the burden of tuning the controller parameters away from the power engineer and places it on the computer. Unlike other approaches that do the same based on robust control theories or evolutionary computing techniques, our proposed procedure uses an optimisation algorithm that works over a formulation of the classical tuning problem in terms of bilinear matrix inequalities. Using this formulation, it is possible to apply linear matrix inequality solvers to find a solution to the tuning problem via an iterative process, with the advantage that these solvers are widely available and have well-known convergence properties. The proposed algorithm is applied to tune the parameters of supplementary controllers for thyristor controlled series capacitors placed in the New England/New York benchmark test system, aiming at the improvement of the damping factor of inter-area modes, under several different operating conditions. The results of the linear analysis are validated by non-linear simulation and demonstrate the effectiveness of the proposed procedure.