Plasmonic Y-splitters of High Wavelength Resolution Based on Strongly Coupled-Resonator Effects

Based on the strongly coupled-resonator effects, a high wavelength-resolution plasmonic Y-splitter, consisting of a Y-branch metal–insulator–metal waveguide with a baffle in each channel, is numerically investigated using the finite element method. Due to the coupling of different resonators (with nearly equal bandwidths) in the Y-splitter, sharp and asymmetric transmission spectra occur. This greatly increases the wavelength resolution of the Y-splitter to be Δλ?≈?15 nm, which is significantly narrower than the bandwidth of the single resonator (Δλ _FWHM?≈?110 nm). An analytic model based on the scattering matrix theory is provided to describe and explain this phenomenon.     

Electromagnetically Induced Transparency and Refractive Index Sensing for a Plasmonic Waveguide with a Stub Coupled Ring Resonator

A plasmonic refractive index sensor based on electromagnetically induced transparency (EIT) composed of a metal-insulator-metal (MIM) waveguide with stub resonators and a ring resonator is presented. The transmission properties and the refractive index sensitivity are numerically studied with the finite element method (FEM). The results revealed an EIT-like transmission spectrum with an asymmetric line profile and a refractive index sensitivity of 1057 nm/RIU are obtained. The coupled mode theory (CMT) based on transmission line theory is adopted to illustrate the EIT-like phenomenon. Multiple EIT-like peaks are observed in the transmission spectrum of the derived structures based on the MIM waveguide with stub resonator coupled ring resonator. To analyze the multiple EIT-like modes of the derived structures, the H _z field distribution is calculated. In addition, the effect of the structural parameters on the EIT-like effect is also studied. These results provide a new method for the dynamic control of light in the nanoscale.     

Independently Formed Multiple Fano Resonances for Ultra-High Sensitivity Plasmonic Nanosensor

We proposed a plasmonic nanosensor with an ultra-high sensitivity based on groove and ring resonator. Simulation results show that these sharp Fano profiles originate from the interference between the groove and ring resonator. The profile can be easily tuned by changing the parameters of the structure. Moreover, we introduce a new way to achieve multiple Fano resonances through independent processes by adding a side-coupled stub cavity, and the Fano resonances can be tuned independently. These characteristics offer flexibility in the design of the devices. This nanosensor yields an ultra-high sensitivity of ～2000 nm/RIU, which is rarely seen in the previous report. Our structures may have potential applications for nanosensors, slow light, and nonlinear devices in highly integrated circuits.     

Plasmonically Induced Absorption and Transparency Based on Stub Waveguide with Nanodisk and Fabry-Perot Resonator

A novel metal-insulator-metal (MIM) plasmonic waveguides structure, which is composed by stub waveguide with nanodisk and Fabry-Perot (F-P) resonator, has been proposed and numerically simulated with the finite-difference time-domain (FDTD). Based on the three-level system, the extreme destructive interference between bright and dark resonators gives rise to the distinct plasmonically induced absorption (PIA) response with the abnormal dispersion and novel fast-light feature. Simultaneously, the dramatic double plasmonically induced transparency (PIT) effect with slow-light characteristic can also be achieved in the system. The relationship between the transmission characteristics and the geometric parameters is studied in detail. By optimum design, the modulation depth of the PIA transmission spectrum of 90 % with 0.145 and 0.14 ps fast-light effect can be gained simultaneously, and the peak transmissivity of the double PIT system of 75.2 and 72.8 % with ?0.38 ps slow light-effect can be achieved. The simulated transmission features are in agreement with the temporal-coupled mode theory (CMT). The characteristics of the system indicate an important potential application in integrated optical circuits such as slow-light and fast-light devices, high-performance filter, and optical storage.     

Coupled-Resonator-Induced Fano Resonances for Plasmonic Sensing with Ultra-High Figure of Merits

Fano resonances are numerically predicted in an ultracompact plasmonic structure, comprising a metal-isolator-metal (MIM) waveguide side-coupled with two identical stub resonators. This phenomenon can be well explained by the analytic model and the relative phase analysis based on the scattering matrix theory. In sensing applications, the sensitivity of the proposed structure is about 1.1?×?10³ nm/RIU and its figure of merit is as high as 2?×?10⁵ at λ?=?980 nm, which is due to the sharp asymmetric Fano line-shape with an ultra-low transmittance at this wavelength. This plasmonic structure with such high figure of merits and footprints of only about 0.2 μm² may find important applications in the on-chip nano-sensors.     

Asymmetric Binary Plasmon Resonator Arrays for Perfect Trapping of Light

We propose to achieve perfect trapping of light with asymmetric binary plasmon resonator arrays on metal substrates, in which antisymmetrically coupled resonance modes are excited in each subwavelength period to eliminate any leaky radiation. The specific structure in the study is an ultrathin binary metal stripe array on a flat metal substrate interspaced with a dielectric layer. The antiphase resonance modes are excited underneath the binary metal stripes in each period, resulting in perfect trapping of light under appropriate difference of the metal stripe widths. The trapped light is fully absorbed by metals, accompanied with an improved enhancement of the local field compared to those in symmetric structures with equal metal stripe widths. The work suggests a new way in designing optical metamaterials to manipulate light for enhanced light-matter interactions.     

Ultra-High Sensitivity Nanosensor Based on Multiple Fano Resonance in the MIM Coupled Plasmonic Resonator

This paper proposes a compact plasmonic structure that is composed of a metal-insulator-metal (MIM) waveguide coupled with a groove and stub resonators, and then investigates it by utilizing the finite element method (FEM). Simulation results show that the interaction between the local discrete state caused by the stub resonator and the continuous spectrum caused by the groove resonator gives rise to one of the two Fano resonances, while the generation of the other resonance relies only on the groove. Meanwhile, the asymmetrical linear shape and the resonant wavelength can be easily tuned by changing the parameters of the structure. By adding stubs on the groove, we excited multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of 2000 nm/RIU and a figure of merit of about 3.04?×?10³, which can find extensive applications for nanosensors.     

Ultra-Broadband Linear Polarization Conversion via Diode-Like Asymmetric Transmission with Composite Metamaterial for Terahertz Waves

In this paper, a tri-layer metamaterial composed of a split-disk structure array sandwiched with two layers of twisted sub-wavelength metal grating is proposed and investigated numerically in terahertz region. The numerical results exhibit that linear polarization conversion via diode-like asymmetric transmission for terahertz waves within ultra-broadband frequency range is achieved due to Fabry-Perot-like resonance. In our design, the conversion polarization transmission coefficient for normal incidence is greater than 90 % in the range of 0.23–1.17 THz, equivalent to 134.3 % relative bandwidth. The physical mechanism of the broadband linear polarization conversion effect is further illustrated by simulated electrical field distributions.

     

The flow of bile in the human cystic duct

Clinical studies suggest that the flow of bile in the biliary system may be a contributing factor in the pathogenesis of cholelithiasis, but little is known about its transport mechanism. This paper reports a numerical study of steady flow in human cystic duct models. In order to obtain parametric data on the effects of various anatomical features in the cystic duct, idealised models were constructed, first with staggered baffles in a channel to represent the valves of Heister and lumen. The qualitative consistency of these findings are validated by modelling two of the real cystic ducts obtained from operative cholangiograms. Three-dimensional (3D) models were also constructed to further verify the two-dimensional (2D) results. It was found that the most significant geometric parameter affecting resistance is the baffle clearance (lumen size), followed by the number of baffles (number of folds in the valves of Heister), whilst the least significant ones are the curvature of the cystic duct and the angle between the neck and the gallbladder. The study presented here forms part of a larger project to understand the functions of the human cystic duct, especially the influence of its various anatomical structures on the resistance to bile flow, so that it may aid the assessment of the risk of stone formation in the gallbladder.     

Resonant Mode Analysis of the Nanoscale Surface Plasmon Polariton Waveguide Filter with Rectangle Cavity

The resonant mode characteristics of the nanoscale surface plasmon polaritons (SPP) waveguide filter with rectangle cavity are studied theoretically. By using the finite difference time domain method, both the band-stop- and band-pass-type rectangle SPP filters are analyzed. The results show that the whispering gallery mode (WGM) and the Fabry–Perot (FP) mode can be supported by the rectangle SPP resonator. Furthermore, both traveling-wave mode and standing-wave mode can be realized by the WGM, while only standing-wave mode can be introduced by the FP mode. The traveling-wave mode can only be realized by the square-shaped SPP resonator, and the traveling-wave mode is splitted into two standing-wave modes by transforming the cavity shape from square to rectangle. Also, the effects of the cavity shape, cavity size, and coupling gap size on the transmission spectra of the SPP resonators are analyzed in detail. This simple SPP waveguide filter is very promising for the high-density SPP waveguide integrations.     

Plasmonic Waveguide Filters Based on Tunneling and Cavity Effects

This work presents a bandstop plasmonic filter that comprises a metal–insulator–metal (MIM) waveguide and a few pairs of strip cavities that are embedded in the metal. The strip cavity acts as both a near-field antenna and an MIM resonator. The central frequency and the bandwidth of the forbidden band are inversely related to the cavity length and the cavity-to-waveguide distance, respectively. These results correlate with the predictions of the ring resonator model but only under the resonant condition that double the effective length of cavity is an integer multiple of the guiding wavelength in the cavity.     

Measurement and theory of wheezing breath sounds

We measured the time and frequency domain characteristics of breath sounds in seven asthmatic and three nonasthmatic wheezing patients. The power spectra of the wheezes were evaluated for frequency, amplitude, and timing of peaks of power and for the presence of an exponential decay of power with increasing frequency. Such decay is typical of normal vesicular breath sounds. Two patients who had the most severe asthma had no exponential decay pattern in their spectra. Other asthmatic patients had exponential patterns in some of their analyzed sound segments, with a range of slopes of the log power vs. log frequency curves from 5.7 to 17.3 dB/oct (normal range, 9.8-15.7 dB/oct). The nonasthmatic wheezing patients had normal exponential patterns in most of their analyzed sound segments. All patients had sharp peaks of power in many of the spectra of their expiratory and inspiratory lung sounds. The frequency range of the spectral peaks was 80-1,600 Hz, with some presenting constant frequency peaks throughout numerous inspiratory or expiratory sound segments recorded from one or more pickup locations. We compared the spectral shape, mode of appearance, and frequency range of wheezes with specific predictions of five theories of wheeze production: 1) turbulence-induced wall resonator, 2) turbulence-induced Helmholtz resonator, 3) acoustically stimulated vortex sound (whistle), 4) vortex-induced wall resonator, and 5) fluid dynamic flutter. We conclude that the predictions by 4 and 5 match the experimental observations better than the previously suggested mechanisms. Alterations in the exponential pattern are discussed in view of the mechanisms proposed as underlying the generation and transmission of normal lung sounds. The observed changes may reflect modified sound production in the airways or alterations in their attenuation when transmitted to the chest wall through the hyperinflated lung.     

Tunable Multiple Fano Resonances and Stable Plasmonic Band-Stop Filter Based on a Metal-Insulator-Metal Waveguide

Plasmonics - A metal-insulator-metal (MIM) waveguide consisting of two stub resonators and a ring resonator is proposed, which can be used as refractive index sensor and stop-band filter at the...     

Ability of phytoplankton trait sensitivity to highlight anthropogenic pressures in Mediterranean lagoons: A size spectra sensitivity index (ISS-phyto)

Size spectra exhibit common patterns of variation and predictable responses to pressures across ecosystem types, functional guilds and taxonomic groups. Here, we extend the size spectra approach to phytoplankton ecological status assessment in transitional waters by developing, testing and validating a multi-metric index of size spectra sensitivity (ISS-phyto), which integrates size structure metrics with others such as phytoplankton diversity, biomass and sensitivity of size classes to anthropogenic disturbance. The ability of various theoretical models of size spectra sensitivity to discriminate between disturbed and undisturbed ecosystems and levels of anthropogenic stress was evaluated. We used data on phytoplankton samples collected in 14 Mediterranean and Black sea transitional water ecosystems (coastal lagoons) from Italy, Albania, Greece, Bulgaria and Romania, and compared the models’ efficiency by looking at their pressure–impact response along salinity and enrichment gradients, the latter quantified as variations in dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). Data from a fifteenth Mediterranean lagoon was used for external validation purposes. Right asymmetric models of size class sensitivity, implying higher sensitivity of smaller cell size classes, were found to contribute to the ISS-phyto multimetric tool more effectively than symmetric and left asymmetric models, distinguishing disturbed from undisturbed lagoons and disturbed from undisturbed stations within the same lagoon. When based on right asymmetric sensitivity models, i.e., those that were most efficient in identifying anthropogenic impacts, ISS-phyto also showed the best fit of pressure–response relationships along the salinity and enrichment gradients; at low to high levels of impact ISS response was driven by size class sensitivity and at very high impacts by phytoplankton biomass. A scheme for the classification of Ecological Quality Status based on ISS-phyto is proposed and validated. The validation procedure found that ISS-phyto is an effective and sensitive monitoring tool, robust, easy to apply and to inter-calibrate among laboratories.     

Generation of Multiple Fano Resonances in Plasmonic Split Nanoring Dimer

We present a computational study of the plasmonic response of a split nanoring dimer resonator which supports multiple plasmonic Fano-like resonances that arises by the coupling and interference of the dimer plasmon modes. For the generation of Fano resonances with large modulation depths, numerous configurations of the dimer resonator are analyzed which are observed to be highly dependent on the polarization of incident light. Moreover, the influence of dimension of the split nanoring structure on the spectral positions and intensities of the higher order Fano resonances are also investigated, and it is found that the asymmetric Fano line shapes can be flexibly tuned in the spectrum by varying various geometrical parameters. Such Fano resonators are also discovered to offer high values of figure of merit and contrast ratio due to which they are suitable for high-performance biological sensors.     

Chromatin models. The ionic strength dependence of model histone-DNA interactions: circular dichroism studies of lysine-leucine polypeptide-DNA complexes.

The ionic strength dependence of the complexes between DNA and both random, (Lysx, Leuy)n, and block copolymers, (Lysx)n(Leuy)m, of lysine and leucine, with different amino acid compositions, was studied using circular dichroism (CD) as the probe to detect conformational differences in these complexes relative to native DNA. It was found that the CD spectra of complexes of both the random (Lys84, Leu16)n and block (Lys85)n(Leu15)m copolymers with DNA show a very sharp ionic strength dependence. The maximum altered CD spectrum for the complexes with the block copolymer was found to occur at the same ionic strength as that for poly(L-lysine)-DNA complexes, while the maximum CD change for the random copolymer complex occurred at a slightly lower ionic strength. This sharp dependence of the CD change on the ionic strength was found to be independent of the polymer/DNA ratio, r, for each individual copolymer. The CD spectra for these complexes at optimum NaCl concentration resemble those of the psi spectra of DNA [Jordan, C. F., Lerman, L.S., and Venable, J.H. (1972), Nature (London), New Biol. 236, 67]. The complexes of the random copolymer, (Lys68, Leu32)n, with DNA (r=0.25) at 0.15 M NaCl and below have CD spectra that resemble the A-form DNA spectra. The ionic strength dependence of the CD spectra of this complex is not as sharp as observed with the above polymers and has a broad positive plateau. It is suggested that both the CD spectra of these complexes reflect the phenomena of DNA condensation into a higher order asymmetric structure (folded and compact). The block copolymer, (Lys77)n(Leu23)m, complexes with DNA show very slight alterations in the CD spectra, with respect to native DNA. It appears that the long Leu sequence at one end of such copolymers may be unpropitious for causing the polypeptide-DNA complex to condense into a higher order asymmetric structure. Thus the importance of the distribution of hydrophobic residues, in the copolypeptides of Lys, is shown for causing condensation of complexes with DNA. The relevance of these findings to histone-DNA complexes in chromatin is discussed.     

A rhodamine‐based fluorescent probe for Cu(II) determination in aqueous solution

An ‘off–on’ rhodamine‐based fluorescence probe for the selective detection of Cu(II) has been designed, exploiting the guest‐induced structure transform mechanism. This system shows a sharp Cu(II)‐selective fluorescence enhancement response in an aqueous system under physiological pH, and possesses high selectivity against a background of environmentally and biologically relevant metal ions. Under optimum conditions, the fluorescence intensity enhancement of this system is linearly proportional to the Cu(II) concentration from 50 nM to 6.0 μM with a detection limit of 29 nM. Copyright © 2014 John Wiley & Sons, Ltd.     

Circular dichroism of helices formed by purine monomers with pyrimidine polynucleotides

The CD spectra of a number of helical complexes formed by purine monomers and complementary pyrimidine polyribonucleotides have been observed over the range 200–400 nm. Each of these spectra is quite similar to that of the corresponding polymer–polymer helix. The spectra are evidently determined by the geometry of the asymmetric array of bases, largely unperturbed by the ribose–phosphate backbone. The helix structure (A-form), on the other hand, is determined by the backbone of the pyrimidine homopolymer. Data on the monomer–polymer complexes support the conclusion that the CD spectra of ribohomopolymer helices depend primarily on interastrand interactions of the same transition within a given base and are relatively unaffected by transitions of the complementary base.     

The synthesis and characterization of nucleotide complexes involving an asymmetric metal

The synthesis of cobalt and chromium complexes of H₄ATP and H₄GTP in which the metal is asymmetric are reported. These compounds were characterized by visible spectroscopy, fast atom bombardment mass spectroscopy (FAB MS), and ³¹P NMR. The mass spectral data allow identification of the complexes to be made from ions in the molecular weight region. The effect of an asymmetric metal greatly alters the appearance of the ³¹P NMR spectra in comparison to complexes which do not have this feature. Complexes of uridine diphosphoglucose, UDPG, are also reported. The effect of an asymmetric metal ion on the chromatographic and spectral properties of the complexes are discussed.