Nonlocality-Induced Negative Refraction and Subwavelength Imaging by Parabolic Dispersions in Metal–Dielectric Multilayered Structures with Effective Zero Permittivity

We investigate metal–dielectric multilayered structures with an effectively zero permittivity. Nonlocality induced by the surface plasmons in such structures can produce intriguing dispersions characterized by two crossing branches of parabolas. We obtain the critical conditions to set the two branches of parabolas apart, and reverse the direction of group velocity such that the system becomes capable of negative refraction as well as subwavelength imaging. Such phenomena theoretically exist in the quasistatic limit.     

Analysis and Design of an Object-Independent Superlens

A new design for the multilayer superlens, with the sub-wavelength imaging ability for various 2D objects in the visible range, is introduced and analyzed. The designed superlens will be more versatile for practical applications. A rigorous and efficient approach based on the method of moments is used to study the imaging performance of this structure. The imaging performance of the proposed superlens is evaluated using the correlation coefficient. In this work, the closed-form dyadic Green’s functions in spatial domain, needed for the method of moments solutions, are obtained by applying the complex image method. Besides, the numerical integration is exploited to verify this method. The imaging results obtained via our approach are examined by comparison with the finite element method simulations that reveal good efficiency and accuracy of the proposed method.     

Defining a Superlens Operating Regime for Imaging Fluorescent Molecules

It has been shown that thin metal-based films can at certain frequencies act as planar near-field lenses for certain polarization components. A desirable property of such “lenses” is that they can also enhance and focus some large transverse spatial frequency components which contain sub-diffraction limit details. Over the last decade there has been much work in optimizing designs to reduce effects (such as material losses and surface roughness) that are detrimental to image reconstruction. One design that can reduce some of these undesirable effects, and which has received a fair amount of attention recently, is the stacked metal-dielectric superlens. Here we theoretically explore the imaging ability of such a design for the specific purpose of imaging a fluorescent dye (the common bio-marker GFP) in the vicinity of the superlens surface. Our calculations take into consideration the interaction (damping) of an oscillating electric dipole with the metallic layers in the superlens. We also assume a Gaussian frequency distribution spectrum for the dipole. We treat the metallic-alloy and dielectric-alloy layers separately using an appropriate effective medium theory. The transmission properties are evaluated via Transfer matrix (-matrix) calculations that were performed in the MatLab and MathCad environments. Our study shows that it is in principle possible to image fluorescent molecules using a simple bilayer planar superlens. We find that optimal parameters for such a superlens occur when the peak dipole emission-frequency is slightly offset from the Surface Plasmon resonance frequency of the metal-dielectric interfaces. The best resolution is obtained when the fluorescent molecules are not too close ( nm) or too far ( nm) from the superlens surface. The realization and application of a superlens with the specified design is possible using current nanofabrication techniques. When combined with e.g. a sub-wavelength grating structure (such as in the far-field superlens design previously proposed [1]) or a fast near-field scanning probe, it could provide a means for fast fluorescent imaging with sub-diffraction limit resolution.     

A Super Lens System for Demagnification Imaging Beyond the Diffraction Limit

A super lens system is proposed to achieve subdiffraction limit demagnification imaging. The super lens system consists of a hyperlens with planar input and output surfaces, a metal superlens, and a plasmonic reflector. By employing the hyperlens to transform evanescent waves into propagating waves and employing the metal superlens and the plasmonic reflector to amplify evanescent waves, the super lens system can produce a subdiffraction limit image with relatively high electric field intensity. The reduction factor of the super lens system depends on the geometric parameters of the hyperlens. Simulation results show that an image with a half-pitch resolution of about one tenth the operating wavelength and a reduction factor of about 2.2 can be produced by the super lens system. The proposed super lens system has potential applications in nanolithography.     

Spatial Mode Selection by the Phase Modulation of Subwavelength Plasmonic Grating

The extraordinary transmission of the subwavelength gold grating has been investigated by the rigorous coupled-wave analysis and verified by the metal–insulator–metal plasmonic waveguide method. The physical mechanisms of the extraordinary transmission are characterized as the excitation of the surface plasmon polariton modes. The subwavelength grating integrated with the distributed Bragg reflector is proposed to modulate the phase to realize spatial mode selection, which is prospected to be applied for transverse mode selection in the vertical cavity surface-emitting laser.     

Ultrahigh Contrast One-Way Optical Transmission Through a Subwavelength Slit

We computationally demonstrate one-way optical transmission characteristics of a subwavelength slit. We comparatively study the effect in single layer and double layer metallic corrugations. We also investigate the effect of a dielectric spacer layer between double corrugations to control the volumetric coupling of plasmon and optical modes. We computationally show unidirectional transmission behavior with an ultrahigh contrast ratio of 53.4 dB at λ?=?1.56 μm. Volumetric coupling efficiency through the nanoslit strongly depends on the efficient excitation of both the surface plasmon resonance and metal–insulator–metal waveguide modes. We show that the behavior is tunable in a wide spectral range.     

Global versus local extinction in a network model of plant–pollinator communities

The loss of a species from an ecological community can trigger a cascade of additional extinctions; the complex interactions that comprise ecological communities make the dynamics and impacts of such a cascade challenging to predict. Previous studies have typically considered global extinctions, where a species cannot re-enter a community once it is lost. However, in some cases a species only becomes locally extinct, and may be able to reinvade from surrounding communities. Here, we use a dynamic, Boolean network model of plant–pollinator community assembly to analyze the differences between global and local extinction events in mutualistic communities. As expected, we find that compared to global extinctions, communities respond to local extinctions with lower biodiversity loss, and less variation in topological network properties. We demonstrate that in the face of global extinctions, larger communities suffer greater biodiversity loss than smaller communities when similar proportions of species are lost. Conversely, smaller communities suffer greater loss in the face of local extinctions. We show that targeting species with the most interacting partners causes more biodiversity loss than random extinctions in the case of global, but not local, extinctions. These results extend our understanding of how mutualistic communities respond to species loss, with implications for community management and conservation efforts.     

Spatial Coherence Conversion with Surface Plasmons Using a Three-slit Interferometer

It has been demonstrated recently that surface plasmons can change the state of coherence of light emanating from a Young’s double-slit interferometer. This suggests the possibility of developing a “coherence-converting” device with a large array of subwavelength holes in a metal plate. We have taken an intermediate approach by considering a three-slit geometry, in which we investigated the effects on the modulation of the spatial coherence when an additional slit is placed between the pair of Young’s slits. Our results show that the amount of modulation (enhancement or suppression) can be increased or decreased in the three-slit geometry, compared to the double-slit configuration. This is promising for achieving coherence converting optical devices with suitable arrays of subwavelength holes.     

Near-infrared Plasmonic Far-field Nanofocusing Effects with Elongated Depth of Focus based on Hybrid Au–Dielectric–Ag Subwavelength Structures

In this paper, we propose a new far-field nanofocusing lens with elongated depth of focus (DOF) under near-infrared (NIR) wavelength. The surface plasmons can be excited by using the hybrid metal–insulator–metal (MIM) subwavelength structure under the NIR wavelength. The constructive interference of surface plasmons launched by the subwavelength MIM structure can form a nanoscale focus that is modulated by the novel metal grating from the near field to the far field. The numerical simulations demonstrated that a nanoscale focal spot (in plane focal area 0.177λ ²) with elongated DOF (3.358λ) and long focal length (5.084λ) can be realized with reasonably designing parameters of the lens. By controlling the positions of the inner radii of each slit ring and the grating width, the focal length, focal spot, and DOF can be tuned easily. This design method, which can obtain the nanoscale focal spot and micron DOF in far field under NIR illumination, paved the road for utilizing the NIR plasmonic lens in superresolution optical microscopic imaging, optical trapping, biosensing, and complex wavefront/beam shaper.     

Tailoring Absorption in Metal Gratings with Resonant Ultrathin Bridges

We present a theoretical analysis of the effects of short range surface plasmon polariton excitation on subwavelength bridges in metal gratings. We show that localized resonances in thin metal bridges placed within the slit of a free-standing silver grating dramatically modify transmission spectra and boost absorption regardless of the periodicity of the grating. Additionally, the interference of multiple localized resonances makes it possible to tailor the absorption properties of ultrathin gratings, regardless of the apertures’ geometrical size. This tunable, narrow band, enhanced–absorption mechanism triggered by resonant, short-range surface plasmon polaritons may also enhance nonlinear optical processes like harmonic generation, in view of the large third-order susceptibility of metals.     

Local Field Spectroscopy of Metal Dimers by TPL Microscopy

We report on two-photon photoluminescence (TPL) spectroscopy on metal dimers made of two gold nanoparticles separated by subwavelength distances. A direct comparison with far-field scattering measurements shows that TPL provides additional data on the structure modes of major importance for their use in surface-enhanced Raman scattering, enhanced fluorescence, and sensing.     

A New GaAs Metal-Semiconductor-Metal Photodetector Based on Hybrid Plasmonic Structure to Improve the Optical and Electrical Responses

In this article, a new hybrid plasmonic based metal-semiconductor-metal photodetector (MSM-PD) is proposed. A subwavelength slit, the metal nanoscale gratings, and the metal pads which are extended into the absorption layer are used in a basic hybrid plasmonic structure to enhance the absorption coefficient. The finite-difference time-domain (FDTD) method is used to simulate the new structure. The absorption coefficient of the hybrid plasmonic MSM-PD becomes 42 times greater than that of the conventional plasmonic MSM-PD made of only subwavelength slit, which is known as the reference structure. This result is equivalently about 1.5 times greater than that of a recently reported structure. It is also demonstrated that the quantum efficiency of the proposed structure is 10 times more, if compared with the reference one. Moreover, considering the incident light modulation frequency, the frequency response of the hybrid plasmonic MSM-PD is improved, where the cutoff frequency is increased 22 times greater than that of the reference MSM-PD.     

Automatic classification of the greatly amplified ECG in relation to time and frequency

The non-invasive recording of cardiac micro-potentials from the surface of the body, in particular of the so-called ventricular late potentials from the highly amplified ECG, makes it possible to identify those patients at high risk from severe arrhythmics or sudden death. By using more extensive automatic parameter extraction methods, we were able to increase the detection rate of pathological ECGs. We were also able to show that the calculation of the power density spectrum, in particular on the basis of maximum entropy spectral estimation, in combination with the variance extraction method of eliminating the influence of residual noise, is capable of providing separate additional valuable information.     

On the relationship between common amplitude surface roughness parameters and surface area: Implications for the study of cell–material interactions

In order to show that surface area is not always a quantity proportional to the surface roughness, we have constructed simple surfaces consisting of boxes of the same height equally spaced, and rms roughness and surface area have been computed. We have shown how we can get examples of surface configurations for which an increment in the surface roughness corresponds to a decrease in the surface area, although this is observed only for surfaces having similar rms roughness. We have also shown that even in the more intuitive situations where an increase in the surface roughness leads to an increase in the surface area, this increase is not necessarily equivalent. Analogous conclusions have been found when roughness was evaluated through the average roughness. These results could be interesting when analyzing interfacial phenomena such as cell adhesion, especially from a microscopic point of view, where the exact contact area between interacting phases governs these phenomena, and an exact-as-possible approximation to its real value is desirable. Also, the results of this paper could be of interest in various biomedical applications where the modulation of material surface-by-surface roughness may play a significant role. It can be concluded that care should be taken when using roughness parameters as estimators or indicators of the contact area between phases, since the relationship is not always simple.     

Electrical noise from lipid bilayer membranes in the presence of hydrophobic ions

In the presence of the hydrophobic ion dipicrylamine, lipid bilayer membranes exhibit a characteristic type of noise spectrum which is different from other forms of noise described so far. The spectral density of current noise measured in zero voltage increases in proportion to the square of frequency at low frequencies and becomes constant at high frequencies. The observed form of the noise spectrum can be interpreted on the basis of a transport model for hydrophobic ions in which it is assumed that the ions are adsorbed in potential-energy minima at either membrane surface and are able to cross the central energy barrier by thermal activation. Accordingly, current-noise results from random fluctuations in the number of ions jumping over the barrier from right to left and from left to right. On the basis of this model the rate constant ki for the translocation of the hydrophobic ion across the barrier, as well as the mean surface concentration Nt of adsorbed ions may be calculated from the observed spectral intensity of current noise. The values of ki obtained in this way closely agree with the results of previous relaxation experiments. A similar, although less quantitative, agreement is also found for the surface concentration Nt.     

Attenuation of noise in ultrasensitive signaling cascades

Ultrasensitive cascades often implement thresholding operations in cell signaling and gene regulatory networks, converting graded input signals into discrete all-or-none outputs. However, the biochemical and genetic reactions involved in such cascades are subject to random fluctuations, leading to noise in output signal levels. Here we prove that cascades operating near saturation have output signal fluctuations that are bounded in magnitude, even as the number of noisy cascade stages becomes large. We show that these fluctuation-bounded cascades can be used to attenuate the noise in an input signal, and we find the optimal cascade length required to achieve the best possible noise reduction. Cascades with ultrasensitive transfer functions naturally operate near saturation, and can be made to simultaneously implement thresholding and noise reduction. They are therefore ideally suited to mediate signal transfer in both natural and artificial biological networks.     

Neuroimaging biomarkers for epilepsy: Advances and relevance to glial cells

Glial cells play an important role in normal brain function and emerging evidence would suggest that their dysfunction may be responsible for some epileptic disease states. Neuroimaging of glial cells is desirable, but there are no clear methods to assess neither their function nor localization. Magnetic resonance imaging (MRI) is now part of a standardized epilepsy imaging protocol to assess patients. Structural volumetric and T2-weighted imaging changes can assist in making a positive diagnosis in a majority of patients. The alterations reported in structural and T2 imaging is predominately thought to reflect early neuronal loss followed by glial hypertrophy. MR spectroscopy for myo-inositol is a being pursued to identify glial alterations along with neuronal markers. Diffusion weighted imaging (DWI) is ideal for acute epileptiform events, but is not sensitive to either glial cells or neuronal long-term changes found in epilepsy. However, DWI variants such as diffusion tensor imaging or q-space imaging may shed additional light on aberrant glial function in the future. The sensitivity and specificity of PET radioligands, including those targeting glial cells (translocator protein) hold promise in being able to image glial cells. As the role of glial function/dysfunction in epilepsy becomes more apparent neuroimaging methods will evolve to assist the clinician and researcher in visualizing their location and function.     

Extraordinary Optical Transmission Performances of Nanosandwiched Grating for Wideband Multi-Function Integration

In this paper, we present a peculiar metal-dielectric-metal (MDM) nanosandwich grating structure that can achieve extraordinary optical transmission performances at normal incidence in the ultraviolet-visible-near infrared (UV-VIS-NIR) regions. The proposed structure shows three obvious spectrum characteristics: it can obtain high transmittance up to 80 % in NUV region and efficiently blocking visible wavelengths for transverse-magnetic (TM) polarized incidence; a broadband NIR polarizer can be inspired in the wavelength range from 950 to 1400 nm; more surprisingly, these performances do not deteriorated until 30° tilting angle. Compared to other grating structures with single metal overlayer, it shows wider band-stop characteristics and higher broadband transmission transmittance and extinction ratio (ER) in the investigated wavebands. We analyze the underlying physical mechanism by using numerical simulation, which is primarily attributed to metal ultraviolet transparency, surface plasmon polariton (SPP) at metal/dielectric interface, Fabry–Perot (FP)-like cavity mode within this dielectric grating, and optical magnetic resonance especially in the dielectric interlayer of the MDM sandwiched structure. This structure is very important for developing high-performance subwavelength multifunctional integrated optical devices.