Impedance generalization for plasmonic waveguides beyond the lumped circuit model

We analytically derive a rigorous expression for the relative impedance ratio between two photonic structures based on their electromagnetic interaction. Our approach generalizes the physical meaning of the impedance to a measure for the reciprocity-based overlap of eigenmodes. The consistence with known cases in the radiofrequency and optical domain is shown. The analysis reveals where the applicability of simple circuit parameters ends and how the impedance can be interpreted beyond this point. We illustrate our approach by successfully describing a Bragg reflector that terminates an insulator-metal-insulator plasmonic waveguide in the near-infrared by our mpedance concept

Prompt-specificity in scenario-based assessments: Associations with personality versus knowledge and effects on predictive validity

Many scenario-based assessments (e.g., interviews, assessment center exercises, work samples, simulations, and situational judgment tests) use prompts (i.e., cues provided to respondents to increase the likelihood that the information received from them is clear, sufficient, and job-related). However, a dilemma for practitioners and researchers is how general or specific one should prompt people's answers. We posit that such differences in prompt-specificity (i.e., extent to which prompts cue performance criteria) have important implications for the predictive validity of scenario-based assessment scores. Drawing on the interplay of situation construal and situational strength theory, we propose that prompt-specificity leads to differential relationships between scenario-based scores and external constructs (personality traits vs. knowledge), which in turn affects the predictive validity of scenario-based assessments. We tested this general hypothesis using intercultural scenarios for predicting effectiveness in multicultural teams. Using a randomized predictive validation design, we contrast scores on these scenarios with general (N = 157) versus specific (N = 158) prompts. As a general conclusion, prompt-specificity mattered: Lesser prompt-specificity augmented the role of perspective taking and openness-to-experience in the intercultural scenario scores and their validity for predicting intercultural performance, whereas greater prompt-specificity increased the role of knowledge in these scores and their validity for predicting in-role performance. This study's theoretical and practical implications go beyond a specific assessment procedure and apply to a broad array of assessment and training approaches that rely on scenarios

Validity of effective material parameters for optical fishnet metamaterials

Although optical metamaterials that show artificial magnetism are mesoscopic systems, they are frequently described in terms of effective material parameters. But due to intrinsic nonlocal (or spatially dispersive) effects it may be anticipated that this approach is usually only a crude approximation and is physically meaningless. In order to study the limitations regarding the assignment of effective material parameters, we present a technique to retrieve the frequency-dependent elements of the effective permittivity and permeability tensors for arbitrary angles of incidence and apply the method exemplarily to the fishnet metamaterial. It turns out that for the fishnet metamaterial, genuine effective material parameters can only be introduced if quite stringent constraints are imposed on the wavelength/unit cell size ratio. Unfortunately they are only met far away from the resonances that induce a magnetic response required for many envisioned applications of such a fishnet metamaterial. Our work clearly indicates that the mesoscopic nature and the related spatial dispersion of contemporary optical metamaterials that show artificial magnetism prohibits the meaningful introduction of conventional effective material parameters

Self-Assembled Arrays of Gold Nanorod-Decorated Dielectric Microspheres with a Magnetic Dipole Response in the Visible Range for Perfect Lensing and Cloaking Applications

Photonic nanostructures made of a dielectric sphere covered with many metallic nanospheres fabricated by self-assembly constitute a basic building block for optical metamaterials with a magnetic response in the visible. However, they suffer from limited degrees of freedom to tune their response. Once the involved materials are chosen, the response is mostly determined. To overcome such a limitation, we design, fabricate, and characterize here a bottom-up metamaterial in which metallic nanorods are used instead of nanospheres. Nanorods offer the ability to tune the spectral position of the resonances by changing their aspect ratio. Building blocks consisting of dielectric spheres covered with metallic nanorods are fabricated and characterized. They are also deposited in densely packed arrays on a substrate using a blade coating deposition of the dielectric spheres first and a subsequent deposition of the metallic nanorods. Full-wave optical simulations support the spectroscopic characterization. These simulations also indicate a dominant magnetic dipolar response of the building blocks. These arranged core–shell structures are promising materials for applications such as perfect lensing and cloaking

A simple and versatile analytical approach for planar metamaterials

We present an analytical model which permits the calculation of effective material parameters for planar metamaterials consisting of arbitrary unit cells (metaatoms) formed by a set of straight wire sections of potentially different shape. The model takes advantage of resonant electric dipole oscillations in the wires and their mutual coupling. The pertinent form of the metaatom determines the actual coupling features. This procedure represents a kind of building block model for quite different metaatoms. Based on the parameters describing the individual dipole oscillations and their mutual coupling the entire effective metamaterial tensor can be determined. By knowing these parameters for a certain metaatom it can be systematically modified to create the desired features. Performing such modifications effective material properties as well as the far field intensities remain predictable. As an example the model is applied to reveal the occurrence of optical activity if the split ring resonator metaatom is modified to L- or S-shaped metaatoms.Comment: 5 figures, 1 tabl

Inverse Design of All-dielectric Metasurfaces with Bound States in the Continuum

Metasurfaces with bound states in the continuum (BICs) have proven to be a powerful platform for drastically enhancing light-matter interactions, improving biosensing, and precisely manipulating near- and far-fields. However, engineering metasurfaces to provide an on-demand spectral and angular position for a BIC remains a prime challenge. A conventional solution involves a fine adjustment of geometrical parameters, requiring multiple time-consuming calculations. In this work, to circumvent such tedious processes, we develop a physics-inspired, inverse design method on all-dielectric metasurfaces for an on-demand spectral and angular position of a BIC. Our suggested method predicts the core-shell particles that constitute the unit cell of the metasurface, while considering practical limitations on geometry and available materials. Our method is based on a smart combination of a semi-analytical solution, for predicting the required dipolar Mie coefficients of the meta-atom, and a machine learning algorithm, for finding a practical design of the meta-atom that provides these Mie coefficients. Although our approach is exemplified in designing a metasurface sustaining a BIC, it can, also, be applied to many more objective functions. With that, we pave the way toward a general framework for the inverse design of metasurfaces in specific and nanophotonic structures in general.Comment: 20 pages, 5 figures, Supplementary Materia