Recent Progress in Reconfigurable and Intelligent Metasurfaces: A Comprehensive Review of Tuning Mechanisms, Hardware Designs, and Applications.

Intelligent metasurfaces have gained significant importance in recent years due to their ability to dynamically manipulate electromagnetic (EM) waves. Their multifunctional characteristics, realized by incorporating active elements into the metasurface designs, have huge potential in numerous novel devices and exciting applications. In this article, recent progress in the field of intelligent metasurfaces are reviewed, focusing particularly on tuning mechanisms, hardware designs, and applications. Reconfigurable and programmable metasurfaces, classified as space gradient, time modulated, and space-time modulated metasurfaces, are discussed. Then, reconfigurable intelligent surfaces (RISs) that can alter their wireless environments, and are considered as a promising technology for sixth-generation communication networks, are explored. Next, the recent progress made in simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) that can achieve full-space EM wave control are summarized. Finally, the perspective on the challenges and future directions of intelligent metasurfaces are presented.

High efficiency coupling to metal-insulator-metal plasmonic waveguides.

A periodic array of dual-Vivaldi antennas integrated with metal-insulator-metal (MIM) plasmonic waveguides was designed and investigated for its infrared light absorbance efficiency. Full-wave analysis was used to optimize MIM waveguides compatible with parallel and series connected DC leads without sacrificing radiation efficiency. Free-space to MIM waveguide in-coupling efficiency as high as 41% has been obtained in a sub-wavelength unit cell geometry at a wavelength of 1373 nm. Higher efficiency, up to 85%, is predicted with a modified design including a backplane reflector. A nanofabrication process was developed to realize test devices and far-field optical spectroscopy was used as experimental evidence for antenna-waveguide matching.

No Significant Effects of Cellphone Electromagnetic Radiation on Mice Memory or Anxiety: Some Mixed Effects on Traumatic Brain Injured Mice.

Current literature details an array of contradictory results regarding the effect of radiofrequency electromagnetic radiation (RF-EMR) on health, both in humans and in animal models. The present study was designed to ascertain the conflicting data published regarding the possible impact of cellular exposure (radiation) on male and female mice as far as spatial memory, anxiety, and general well-being is concerned. To increase the likelihood of identifying possible "subtle" effects, we chose to test it in already cognitively impaired (following mild traumatic brain injury; mTBI) mice. Exposure to cellular radiation by itself had no significant impact on anxiety levels or spatial/visual memory in mice. When examining the dual impact of mTBI and cellular radiation on anxiety, no differences were found in the anxiety-like behavior as seen at the elevated plus maze (EPM). When exposed to both mTBI and cellular radiation, our results show improvement of visual memory impairment in both female and male mice, but worsening of the spatial memory of female mice. These results do not allow for a decisive conclusion regarding the possible hazards of cellular radiation on brain function in mice, and the mTBI did not facilitate identification of subtle effects by augmenting them.

Scattering by thin shells in fluids: Fast solver and experimental validation.

A numerical model facilitating fast analysis of acoustic scattering by thin shells immersed in fluids is presented. The shell is simulated by an effective boundary condition, in which the inertial properties of the shell are taken into account while the elastic ones are neglected. The problem reduces to a hypersingular surface integral equation, which is solved using the boundary element method accelerated by the multilevel nonuniform grid approach. The validity of the numerical method and the adopted physical approximations were tested by comparison with a water tank experiment on acoustic scattering from cylindrical metallic shells filled with water.

In situ real-time beam monitoring with dielectric meta-holograms.

A novel approach for performing in situ and real-time beam monitoring, based on dielectric meta-hologram, is proposed and demonstrated. The ultrathin dielectric meta-hologram projects a portion of the beam power onto a screen to provide a visual indicator of the spatial intensity distribution of a Gaussian laser beam, as well as its waist position along the optical axis. Specifically, we demonstrate simple monitoring of the spot size, astigmatism, lateral position, and position along the optical axis of the beam. Good agreement is found with both theory and conventional knife-edge beam profiler measurements. This in situ beam monitoring approach could provide a highly useful tool for numerous optical applications.

A fast and stable solver for acoustic scattering problems based on the nonuniform grid approach.

A fast and stable boundary element method (BEM) algorithm for solving external problems of acoustic scattering by impenetrable bodies is developed. The method employs the Burton-Miller integral equation, which provides stable convergence of iterative solvers, and a generalized multilevel nonuniform grid (MLNG) algorithm for fast evaluation of field integrals. The MLNG approach is used here for the removal of computational bottlenecks involved with repeated matrix-vector multiplications as well as for the low-order basis function regularization of the hyper-singular integral kernel. The method is used for calculating the fields scattered by large acoustic scatterers, including nonconvex bodies with piece-wise smooth surfaces. As a result, the algorithm is capable of accurately incorporating high-frequency effects such as creeping waves and multiple-edges diffractions. In all cases, stable convergence of the method is observed. High accuracy of the method is demonstrated by comparison with the traditional BEM solution. The computational complexity of the method in terms of both the computation time and storage is estimated in practical computations and shown to be close to the asymptotic O(N log N) dependence.

An auxilliary grid method for the calculation of electrostatic terms in density functional theory on a real-space grid.

In this work we show the implementation of a linear scaling algorithm for the calculation of the Poisson integral. We use domain decomposition and non-uniform auxiliary grids (NGs) to calculate the electrostatic interaction. We demonstrate the approach within the PARSEC density functional theory code and perform calculations of long 1D carbon chains and other long molecules. Finally, we discuss possible applications to additional problems and geometries.

Highly efficient and broadband wide-angle holography using patch-dipole nanoantenna reflectarrays.

We demonstrate wide-angle, broadband, and efficient reflection holography by utilizing coupled dipole-patch nanoantenna cells to impose an arbitrary phase profile on the reflected light. High-fidelity images were projected at angles of 45 and 20° with respect to the impinging light with efficiencies ranging between 40-50% over an optical bandwidth exceeding 180 nm. Excellent agreement with the theoretical predictions was found at a wide spectral range. The demonstration of such reflectarrays opens new avenues toward expanding the limits of large-angle holography.

Quantum nonreciprocity of nanoscale antenna arrays in timed Dicke states.

We predict a linear nonreciprocal effect that is based on the timed Dicke states in an ensemble of dipole-dipole coupled oscillators. This effect is examined on a nanoscale antenna array comprising two-level identical emitters. The studied nonreciprocity, which has no analogs in classical antennas, manifests itself in strong characteristic asymmetry of the radiation pattern, even for a single-photon laser pumping. Promising applications of our results for remotely tunable nanoantennas and nanocircuit elements are discussed.

High load sensitivity in wideband infrared dual-Vivaldi nanoantennas.

Dual-Vivaldi nanoantenna (DVA) arrays were designed, fabricated, and optically characterized in the infrared (IR) and visible regimes. The antenna arrays were characterized by measuring the scattered light at IR (1450-1640 nm) and visible (780 nm) spectral ranges. The radiation efficiency and the spectral response of the antennas were found to be in good agreement with numerical simulations. The results presented here demonstrate the extremely wideband nature of the DVAs and the strong impact of load at the antenna terminals on its scattering response. These properties, as well as their many degrees of freedom for design, render the DVAs excellent candidates for optical sensing applications.

Fast direct solution of 3-D scattering problems via nonuniform grid-based matrix compression.

A fast non-iterative algorithm for the solution of large 3-D acoustic scattering problems is presented. The proposed approach can be used in conjunction with the conventional boundary element discretization of the integral equations of acoustic scattering. The algorithm involves domain decomposition and uses the nonuniform grid (NG) approach for the initial compression of the interactions between each subdomain and the rest of the scatterer. These interactions, represented by the off-diagonal blocks of the boundary element method matrix, are then further compressed while constructing sets of interacting and local basis and testing functions. The compressed matrix is obtained by eliminating the local degrees of freedom through the Schur's complement-based technique procedure applied to the diagonal blocks. In the solution process, the interacting unknowns are first determined by solving the compressed system equations. Subsequently, the local degrees of freedom are determined for each subdomain. The proposed technique effectively reduces the oversampling typically needed when using low-order discretization techniques and provides significant computational savings.

The role of the cantilever in Kelvin probe force microscopy measurements.

The role of the cantilever in quantitative Kelvin probe force microscopy (KPFM) is rigorously analyzed. We use the boundary element method to calculate the point spread function of the measuring probe: Tip and cantilever. The calculations show that the cantilever has a very strong effect on the absolute value of the measured contact potential difference even under ultra-high vacuum conditions, and we demonstrate a good agreement between our model and KPFM measurements in ultra-high vacuum of NaCl monolayers grown on Cu(111). The effect of the oscillating cantilever shape on the KPFM resolution and sensitivity has been calculated and found to be relatively small.

Dual-Vivaldi wideband nanoantenna with high radiation efficiency over the infrared frequency band.

A dual-Vivaldi nanoantenna is proposed to demonstrate the possibility of wideband operation at IR frequencies. The antenna geometry design is guided by the material properties of metals at IR frequencies. According to our numerical results, this nanoantenna has both high radiation efficiency and good impedance-matching properties over a wide frequency band (more than 122%) in the IR frequency band. The design is based on the well-known Vivaldi antenna placed on quartz substrate but operating as a pair instead of a single element. Such a pair of Vivaldi antennas oriented in opposite directions produces the main lobe in the broadside direction (normal to the axes of the antennas) rather than the usual peak gain along the axis (end fire) of a single Vivaldi antenna. The dual-Vivaldi nanoantenna is easy to fabricate in a conventional electron-beam lithography process, and it provides a large number of degrees of freedom, facilitating design for ultra-wideband operation.

Multilevel nonuniform grid algorithm for acceleration of integral equation-based solvers for acoustic scattering.

A fast algorithm for the evaluation of acoustic fields produced by given source distributions is developed with the aim of accelerating iterative boundary element method (BEM) solvers. The algorithm is based on field smoothing by phase and amplitude compensation, which allows for sampling of the fields radiated by finite-size source distributions over coarse nonuniform (spherical) grids (NGs). Subsequently, the fields at the desired target points can be obtained by an interpolation and phase and amplitude restoration. Combining this approach with the divide-and-conquer strategy, the total field is computed via a hierarchical decomposition of the source domain. In this computational scheme, the phase and amplitude compensated fields produced by neighboring subdomains are gradually aggregated through a multilevel process involving interpolation between increasingly dense NGs and the scatterer surface. This multilevel NG algorithm is used to reduce the computational cost of applying the field evaluation operator and its adjoint, as required in each iteration of the conjugate gradient solver based on the BEM-discretized integral representation of scattering problems. Accuracy and computational efficiency of the NG algorithm are demonstrated on representative examples of elongated, quasi-planar, and full 3-D scatterers.

Dielectric inserts for sensitivity and RF magnetic field enhancement in NMR volume coils.

A method for enhancing the signal to noise ratio (SNR) in NMR volume coils is described. By introducing inserts made of low-loss, high dielectric constant material into specific locations in the coil, the SNR can often be enhanced by up to 20%, while B(1) homogeneity is hardly affected. A model for predicting the limit of the SNR improvement is also presented. The model accurately predicts the SNR gain obtained in both numerical simulations and experiment. An experiment was conducted on a mini-MRI system. Experimental results are in very good agreement with the simulations in regard to both SNR improvement and B(1) enhancement in transmission. Inserts made of ultra high dielectric constant materials can be as thin as few millimeters, thus, conveniently fitting into existing coil-sample gaps in volume coils.

Two-dimensional Green's function theory for the electrodynamics of a rotating medium.

We derive an exact spectral representation for the Green's function of Maxwell equations in a two-dimensional homogeneous and rotating environment. The formulation is developed in the medium (noninertial) rest frame, and it represents the response to a point source, where both the source and observation points rotate together with the medium. The closed form expression for the Green's function is derived for (nonrelativistic) slowly rotating media at finite distances. An approximate expression for the efficient evaluation of the Green's function, that avoids laborious summation of rotating-medium spherical harmonics, is provided and tested against the exact expression. Furthermore, it is shown that our spectral theory can provide a broad view of the optical response of rotating systems, from which the classical Sagnac effect is obtained as a special case.

Sensitivity analysis of narrowband photonic crystal filters and waveguides to structure variations and inaccuracy.

Photonic crystal microcavities, formed by local defects within an otherwise perfectly periodic structure, can be used as narrowband optical resonators and filters. The coupled-cavity waveguide (CCW) is a linear array of equally spaced identical microcavities. Tunneling of light between microcavities forms a guiding effect, with a central frequency and bandwidth controlled by the local defects' parameters and spacing, respectively. We employ cavity perturbation theory to investigate the sensitivity of microcavities and CCWs to random structure inaccuracies. For the microcavity, we predict a frequency shift that is due to random changes in the lattice structure and show an approximate linear dependence between the standard deviation of the structure inaccuracy and that of the resonant frequency. The effect of structural inaccuracy on the CCW devices, however, is different; it has practically no effect on the CCW performance if it is below a certain threshold but may destroy the CCW if this threshold is exceeded.