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We report a highly efficient generation of singular surface plasmon (SP) field by an achiral plasmonic structure consisting of $\Lambda$-shaped apertures. Our quantitative analysis based on leakage radiation microscopy (LRM) demonstrates that the induced spin-orbit coupling can be tuned by adjusting the apex angle of the $\Lambda$-shaped aperture. Specifically, the array of $\Lambda$-shaped apertures with the apex angle $60^\circ$ is shown to give rise to the directional coupling efficiency. The ring of $\Lambda$-shaped apertures with the apex angle $60^\circ$ realized to generate the maximum extinction ratio (ER=11) for the SP singularities between two different polarization states. This result provides a more efficient way for developing SP focusing and SP vortex in the field of nanophotonics such as optical tweezers.

There are some ∼1,100 known antimicrobial peptides (AMPs), which permeabilize microbial membranes but have diverse sequences. Here, we develop a support vector machine (SVM)-based classifier to investigate ⍺-helical AMPs and the interrelated nature of their functional commonality and sequence homology. SVM is used to search the undiscovered peptide sequence space...

In artificial neural networks, learning from data is a computationally demanding task in which a large number of connection weights are iteratively tuned through stochastic-gradient-based heuristic processes over a cost function. It is not well understood how learning occurs in these systems, in particular how they avoid getting trapped in...

We develop a quantum-mechanical theory for Landau damping of surface plasmons in metal nanostructures larger that the characteristic length for nonlocal effects. We show that the electron surface scattering, which facilitates plasmon decay in small nanostructures, can be incorporated into the metal dielectric function on par with phonon and impurity scattering. The derived surface scattering rate is determined by the plasmon local field polarization relative to the metal-dielectric interface and is highly sensitive to the system geometry. We illustrate our model by providing analytical results for surface scattering rate in some common shape nanostructures.

Nature Photonics 10, 751 (2016). doi:10.1038/nphoton.2016.244

Author: Oliver Graydon

We report on a numerical study of optical chirality. Intertwined gold helices illuminated with plane waves concentrate right and left circularly polarized electromagnetic field energy to sub-wavelength regions. These spots of enhanced chirality can be smoothly shifted in position and magnitude by varying illumination parameters, allowing for the control of light-matter interactions on a nanometer scale.

The emission rate of a point dipole can be strongly increased in presence of a well-designed optical antenna. Yet, optical antenna design is largely based on radio-frequency rules, ignoring e.g.~ohmic losses and non-negligible field penetration in metals at optical frequencies. Here we combine reciprocity and Poynting's theorem to derive a set of optical-frequency antenna design rules for benchmarking and optimizing the performance of optical antennas driven by single quantum emitters. Based on these findings a novel plasmonic cavity antenna design is presented exhibiting a considerably improved performance compared to a reference two-wire antenna. Our work will be useful for the design of high-performance optical antennas and nanoresonators for diverse applications ranging from quantum optics to antenna-enhanced single-emitter spectroscopy and sensing.

I. A. Kolmychek, A. N. Shaimanov, A. V. Baryshev, T. V. Murzina
We present an experimental study of optical second harmonic generation (SHG) from arrays of nanostructures exhibiting collective plasmon resonances in the visible spectral range. Gold nanoparticles with the lateral diameter of 100 nm are packed in a square lattice with the period of 400 nm and are ... [Opt. Lett. 41, 5446-5449 (2016)]

How to defeat dementia

Nature 539, 7628 (2016). http://www.nature.com/doifinder/10.1038/539156a

Author: Elie Dolgin

Three things are needed to turn the tide on the costliest crisis in health care.

We present a boundary integral formulation of electromagnetic scattering by homogeneous bodies that are characterized by linear constitutive equations in the frequency domain. By working with the Cartesian components of the electric, E and magnetic, H fields and with the scalar functions (r*E) and (r*H), the problem is cast as solving a set of scalar Helmholtz equations for the field components that are coupled by the usual electromagnetic boundary conditions at material boundaries. This facilitates a direct solution for E and H rather than working with surface currents as intermediate quantities in existing methods. Consequently, our formulation is free of the well-known numerical instability that occurs in the zero frequency or long wavelength limit in traditional surface integral solutions of Maxwell's equations and our numerical results converge uniformly to the static results in the long wavelength limit. Furthermore, we use a formulation of the scalar Helmholtz equation that is expressed as classically convergent integrals and does not require the evaluation of principal value integrals or any knowledge of the solid angle. Therefore, standard quadrature and higher order surface elements can readily be used to improve numerical precision. In addition, near and far field values can be calculated with equal precision and multiscale problems in which the scatterers possess characteristic length scales that are both large and small relative to the wavelength can be easily accommodated. From this we obtain results for the scattering and transmission of electromagnetic waves at dielectric boundaries that are valid for any ratio of the local surface curvature to the wave number. This is a generalization of the familiar Fresnel formula and Snell's law, valid at planar dielectric boundaries, for the scattering and transmission of electromagnetic waves at surfaces of arbitrary curvature.

We study the properties of dipolar wave propagation in linear chains of isotropic particles with independent electric and magnetic response, embedded in vacuum. It is shown that the chain can support simultaneously right-handed modes (RHM) and left-handed modes (LHM) of transverse-polarization. The LHM are supported by the structure even if the chain's particles possess positive polarizabilities and no Bi-isotropy; the needed structural Bi-isotropy is provided by the propagator instead of by the particle's local properties. In contrast to the transverse modes in chains that consist of purely electric particles that are inherently RHM, the LHM dispersion lacks the light-line branch since their dipolar features are not aligned with the electric and magnetic fields of a right-handed plane-wave solution in free space. Furthermore, it is shown that the spatial width of the LHM is significantly smaller than that of the RHM. Excitation theory is developed, and it is shown that the chain possesses modal and excitation asymmetries that can be used to eliminate reflections from chain's termination.

Based on a recent unified formulation on dichorism and extinction of helicity on scattering by a small particle, dipolar in the wide sense, magnetodielectric or not, chiral or achiral, we show that such extinction is enhanced not only at resonances of the polarizabilities, but also due to interference between left and right circularly polarized components of the incident wave, which contributes with appropriate parameters of the illuminating field, even if the particle is achiral and is placed at points of the incident field at which the local incident helicity density is zero.

This phenomenon goes beyond standard circular dichroism (CD), and we analyze it in detail on account of the values of the several quantities, both of the incident light and the particle, involved in the process. In addition, this interference produces a term in the helicity extinction that remarkably yields information on the real parts of the electric and/or magnetic polarizabilities, which are not provided by CD, of which that helicity extinction phenomenon may be considered a generalization.

Nature Photonics. doi:10.1038/nphoton.2016.217

Authors: Kevin D. Heylman, Niket Thakkar, Erik H. Horak, Steven C. Quillin, Charles Cherqui, Kassandra A. Knapper, David J. Masiello & Randall H. Goldsmith

Although animal models of pain have brought invaluable information on basic processes underlying pain pathophysiology, translation to humans is a problem. This Review will summarize what information has been gained by the direct study of patients with chronic pain. The techniques discussed range from patient phenotyping using quantitative sensory testing to specialized nociceptor neurophysiology, imaging methods of peripheral nociceptors, analyses of body fluids, genetics and epigenetics, and the generation of sensory neurons from patients via inducible pluripotent stem cells. Author: Claudia Sommer

Phenomena such as placebo analgesia or pain relief through distraction highlight the powerful influence cognitive processes and learning mechanisms have on the way we perceive pain. Although contemporary models of pain acknowledge that pain is not a direct readout of nociceptive input, the neuronal processes underlying cognitive modulation are not yet fully understood. Modern concepts of perception—which include computational modeling to quantify the influence of cognitive processes—suggest that perception is critically determined by expectations and their modification through learning. Research on pain has just begun to embrace this view. Insights into these processes promise to open up new avenues to pain prevention and treatment by harnessing the power of the mind. Author: Katja Wiech

Colloidal Quantum dots (CQDs) are nowadays one of the cornerstones of modern photonics as they have led to the emergence of new optoelectronic and biomedical technologies. However, the full characterization of these quantum emitters is currently restricted to the visible wavelengths and it remains a key challenge to optically probe single CQDs operating in the infrared spectral domain which is targeted by a growing number of applications. Here, we report the first experimental detection and imaging at room temperature of single infrared CQDs operating at telecommunication wavelengths. Imaging was done with a doubly resonant bowtie nano-aperture antenna (BNA) written at the end of a fiber nanoprobe, whose resonances spectrally fit the CQD absorption and emission wavelengths. Direct near-field characterization of PbS CQDs reveal individual nanocrystals with a spatial resolution of 75 nm (lambda/20) together with their intrinsic 2D dipolar free-space emission properties and exciton dynamics (blinking phenomenon). Because the doubly resonant BNA is strongly transmissive at both the CQD absorption and emission wavelengths, we are able to perform all-fiber nano-imaging with a standard 20 % efficiency InGaAs avalanche photodiode (APD). Detection efficiency is predicted to be 3000 fold larger than with a conventional circular aperture tip of the same transmission area. Double resonance BNA fiber probes thus offer the possibility of exploring extreme light-matter interaction in low band gap CQDs with current plug-and-play detection techniques, opening up new avenues in the fields of infrared light emitting devices, photodetectors, telecommunications, bio-imaging and quantum information technology.

Artificial intelligence: Deep neural reasoning

Nature 538, 7626 (2016). doi:10.1038/nature19477

Authors: Herbert Jaeger

The human brain can solve highly abstract reasoning problems using a neural network that is entirely physical. The underlying mechanisms are only partially understood, but an artificial network provides valuable insight. See Article p.471

There is a blind spot in AI research

Nature 538, 7625 (2016). doi:10.1038/538311a

Authors: Kate Crawford & Ryan Calo

Fears about the future impacts of artificial intelligence are distracting researchers from the real risks of deployed systems, argue Kate Crawford and Ryan Calo.

The modes of silicon meta-atoms are investigated, motivated by their use as building blocks of Huygens' metasurfaces. A model based on these modes is presented, giving a clear physical explanation of all features in the extinction spectrum. Counter-intuitively, this can show negative contributions to extinction, which are shown to arise from the interference between non-orthogonal modes. The direct and interference contributions to extinction are determined, showing that conservation of energy is preserved. The Huygens' condition of matched electric and magnetic dipole moments leads to strong forward scattering and suppressed back scattering. It is shown that higher order modes with appropriate symmetry generalise this condition, leading to multiple bands of directional scattering. The presented results are obtained using a robust approach to find the modes of nano-photonic scatterers, commonly referred to as quasi-normal modes. By utilising an integral formulation of Maxwell's equations, this work avoids the problem of normalising diverging far-fields, which other approaches require. The model and presented results are implemented in open-source code.

Minseok Kim, George V. Eleftheriades
We propose a highly efficient (nearly lossless and impedance-matched) all-dielectric optical tensor impedance metasurface that mimics chiral effects at optical wavelengths. By cascading an array of rotated crossed silicon nanoblocks, we realize chiral optical tensor impedance metasurfaces that ... [Opt. Lett. 41, 4831-4834 (2016)]

Machine learning is a field of artificial intelligence in which a set of algorithms and training sets can teach a computer to learn and then to independently perform a particular set – [Read More]

Authors: Caroline Ash, Ian S. Osborne