Amplification Of Evanescent Waves Research Articles

Analysis of near field radiation among lithography-free metal-dielectric-metal perfect absorbers with a combined numerical method

An integrated analysis is developed to determine the far-field and near-field radiation of lithography-free metal-dielectric-metal (MIM) structures. Directional spectral emissivity determined with the integrated analysis shows good agreement with the directional spectral absorptivity from verified full wave simulation. With the integrated analysis, we identified that the condition of Fabry–Perot resonance used to design broadband wide-angle perfect light absorbers/emitters with MIM structures could trigger the waveguide modes of the dielectric layer. The waveguide modes can amplify the thermal electric field for photon tunneling between two MIM structures across a 100 nm level gap. Adding an additional pair of waveguides that can amplify evanescent waves in the gap formed with two MIM structures can further enhance the strength of photon tunneling. The enhanced photon tunneling shows high-intensity quasi-monochromatic near-field radiation in TM mode across a 100 nm gap at specific wavelengths. We expect even stronger photon tunneling for high-intensity quasi-monochromatic near field radiation across a more significant gap can occur when the MIM structure made with lower loss metal is combined with structures providing stronger amplification of evanescent wave.

Subwavelength terahertz imaging via virtual superlensing in the radiating near field

Imaging with resolutions much below the wavelength λ – now common in the visible spectrum – remains challenging at lower frequencies, where exponentially decaying evanescent waves are generally measured using a tip or antenna close to an object. Such approaches are often problematic because probes can perturb the near-field itself. Here we show that information encoded in evanescent waves can be probed further than previously thought, by reconstructing truthful images of the near-field through selective amplification of evanescent waves, akin to a virtual superlens that images the near field without perturbing it. We quantify trade-offs between noise and measurement distance, experimentally demonstrating reconstruction of complex images with subwavelength features down to a resolution of λ/7 and amplitude signal-to-noise ratios < 25dB between 0.18–1.5 THz. Our procedure can be implemented with any near-field probe, greatly relaxes experimental requirements for subwavelength imaging at sub-optical frequencies and opens the door to non-invasive near-field scanning.

Harnessing negative refraction and evanescent waves toward super-resolution Lamb wave imaging

We numerically and experimentally demonstrate super-resolution focusing of the lowest anti-symmetric (A0) mode Lamb waves in a thin aluminum plate. The subwavelength focusing/imaging is achieved by exploiting the anisotropy in phononic crystal (PC) lattices and amplification of evanescent waves. To this end, we embedded a PC flat lens in the aluminum plate, consisting of holes arranged in a square lattice formation. We revealed that the bound slab phonon modes amplify evanescent waves, as previously observed for electromagnetic and acoustic waves. Hence, the slab mode helps propagate subwavelength information through the PC lens to reach the near-field image formed due to negative refraction and result in the high resolution image.

Super-resolution imaging of Maxwell’s fish-eye lens based on surface polaritons

Super-resolution imaging plays a crucial role in the fields of nanolithography, high volume transmission and sensing. Relentless efforts have been made to realize super-resolution imaging in the past decades. In this work, inspired by the mechanism of surface plasmon polaritons (SPPs), we find that Maxwell’s fish-eye lens (MFEL) coated with cylindrical layer of negative permeability can achieve super-resolution imaging. The amplification of evanescent waves in the negative permeability layer facilitates the transmittance of high spatial frequency information from object point to imaging point in the form of magnetic surface polaritons (MSPs). Both analytical calculations and numerical simulations are employed to prove the super-resolution imaging performance of MSPs-assisted MFEL. Our results may pave a new way for super-resolution imaging in metallic systems.

Resonant tunneling compression and evanescent wave amplification by an acoustic metalens

We have demonstrated an acoustic metamaterial (AM) metalens with the anisotropic refractive index cavities based on the space-coiling units. Compared to that based on conventional Fabry-Pérot (FP) resonance, the proposed space-coiling units significantly downsize the thickness of the AM metalens. In the cavities, we further demonstrated a strong resonant tunneling compression effect, which can be used to substantially magnify the evanescent waves to realize subwavelength imaging. Imaging experiments have shown that the objects located at a considerably enlarged distance away from the input surface of the AM metalens can be imaged with a subwavelength spatial resolution. The proposed AM metalens may facilitate potential applications in acoustic super-resolution imaging and focusing devices.

Interference Amplification of Evanescent Waves in a Layered Structure with the Participation of a Hyperbolic Medium

For a planar structure of dielectric and conducting layers, which allows independent propagation of waves of TE- and TM- types, it is shown that for a plane volume wave of the transverse magnetic type incident from the outside, the result of the interaction of partial evanescent waves in each of the layers of the hybrid structure may not be only a sharp increase in the amplitude of the excited interference inhomogeneous wave, but also realization of the effects of both resonant transparency and resonant opacity of the magnetoplasmonic structure.

Effect of polarization in evanescent wave amplification for the enhancement of scattering of nanoparticles on surfaces

We demonstrate the far field detection of low-contrast nanoparticles on surfaces using a technique that is based on evanescent-wave amplification due to a thin dielectric layer that is deposited on the substrate. This research builds upon earlier results where scattering enhancement of 200 nm polystyrene (PSL) particles on top of a glass substrate covered with a ≈ 20 nm InSb layer has been observed by Roy et al. [Phys. Rev. A 96, 013814 (2017)10.1103/PhysRevA.96.013814]. In this paper, the enhancement effect is analyzed using other dielectric materials with lower absorption than the previous one, resulting in a higher signal-to-noise ratio (SNR) for particle detection. We also consider several polarizations of the incoming field, such as linear, circular, azimuthal, and radial. In our experiments, we observe that the optimum enhancement occurs when linear polarization is used. With this new scheme, PSL nanoparticles of 40 nm in diameter have been detected at a wavelength of 405 nm.

Design of Tunable Metasurface Using Deep Neural Networks for Field Localized Wireless Power Transfer

Wireless power transfer (WPT), a convenient method for powering multiple devices, enables a truly wireless connection, eliminating the need for periodic charging and replacing a battery. To further enhance WPT, the unique characteristics of metamaterial, such as its field focusing and evanescent wave amplification, have been successfully utilized. With subwavelength characteristics, computational challenges arise when the number of metamaterial unit cells is increased. In this work, we investigate a deep neural network (DNN)-based design of the tunable metamaterial for WPT. Using structures specifically designed for different tasks, the DNN predicts the frequency spectra and synthesizes the unit cell's design parameters. When trained using a set of ~23000 randomly selected designs, we achieve an accumulated mean square error (MSE) of less than 1.5× 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for 97.3% of the 1929 test set. For synthesizing the unit cell's design parameters, the MSE is less than 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for 95.7% of the test set. The data-driven method is further extended to a generative adversarial network (GAN) to create the WPT paths and predict the frequency spectra of them. To achieve high efficiency, we propose a cost function focusing on the spectra's transmission peak. After training using 80 000 measured data, the GAN can create WPT paths that efficiently connect the transmitter and the receiver on the metasurface. The results show that the DNN provides an alternative and efficient design method for the metamaterial, replacing traditional EM-simulation-based approaches.

Wireless Power Transfer Systems Using Metamaterials: A Review

With the recent advancement and progress in the field of wireless power transfer (WPT), there is an ever increasing demand for high power transfer efficiency (PTE) of the WPT systems and improved transfer distance for the end-users. However, some existing WPT systems have limited PTE and transfer distance as they take the inductive coupling approach, where the PTE dramatically decreases as the distance between (Tx) and receiver (Rx) coils increases. Alternatively, magnetic resonance coupling (MRC) is used as a mid-range WPT approach, for which the insertion of metamaterials (MTMs) between Tx and Rx coils is exploited to improve efficiency. MTMs are artificially engineered materials that show uncommon electromagnetic properties, such as evanescent wave amplification and negative refractive characteristics, which could be utilized for the enhancement of PTE. In this article, a comprehensive review on recent progresses in the MTM-based WPT systems is reported, where previously reported MTM-based WPT systems are compared in terms of various parameters such as configurations, operating frequencies, dimensions and PTE. Also, the PTEs of these systems were plotted as a function of the normalized transfer distance. This review is expected to provide an insight for understanding the trends of the MTM-based WPT systems and serve as a reference for researchers who work on WPT systems and their applications.

A Comparison of Equivalent Source Method and Monopole Time Reversal Method for Noise Source Localization

The equivalent source method (ESM) and monopole time reversal method (MTRM) are two popular techniques for noise source localization. These two methods have some similar characteristics, such as using the pressure field measured by a microphone array as the input and using similar propagation matrices obtained from the Green's function. However, the spatial resolutions of results obtained by these two methods are different. The aim of this paper is to reveal the reason resulting in this difference from a theoretical analysis and compare the performance of these two methods using results from numerical simulations and experiments. Using the singular value decomposition (SVD) technique, the difference between the two methods is found to be only the diagonal matrices of singular values, and the two methods are equivalent after simply replacing the diagonal matrix in the MTRM with its inverse. Comparison of the results demonstrates that the ESM can calculate the real source strength and obtain a high spatial resolution due to the significant amplification of evanescent waves in the inverse process. However, it does not work when the signal-to-noise ratio (SNR) is low or the measurement distance is large. The performance of ESM under these situations can be significantly improved by introducing a regularization procedure. While the MTRM fails to calculate the real source strength and locate the source at low frequencies due to the loss of information of evanescent waves, it works well at high frequencies even with a low SNR and a large measurement distance.

Exploiting evanescent-wave amplification for subwavelength low-contrast particle detection

The classical problem of subwavelength particle detection on a flat surface is especially challenging when the refractive index of the particle is close to that of the substrate. We demonstrate a method to improve the detection ability several times for such a situation, by enhancing the ``forbidden'' evanescent waves in the substrate using the principle of super-resolution with evanescent waves amplification. The working mechanism of the system and experimental validation from a design with a thin single dielectric layer is presented. The resulting system is a simple but complete example of evanescent-wave generation, amplification, and the consequent modulation of the far field. This principle can have far reaching impact in the field of particle detection in several applications ranging from contamination control to interferometric scattering microscopy for biological samples.

Mushroom-type structures with the wires connected through diodes: Theory and applications

In this paper, we establish a general formalism to quantify the interaction of electromagnetic waves with mushroom-type structures (high impedance surface and bi-layer) with diodes inserted along the direction of the wires. The analysis is carried out using the nonlocal homogenization model for the mushroom structure with the generalized additional boundary conditions at the connection of the wires to diodes. We calculate numerically the magnitude and phase of the reflected/transmitted fields in the presence of an ideal and realistic PIN diodes. It is observed that the reflection/transmission characteristics of the mushroom-type structures can be controlled by tuning the working states of the integrated PIN diodes. We realize a structure with a multi-diode switch to minimize the undesired transmission for a particular incident angle. In addition, a dual-band subwavelength imaging lens is designed based on the resonant amplification of evanescent waves, wherein the operating frequency can be tuned by changing the states of the PIN diodes. The analytical results are verified with the full-wave electromagnetic solver CST Microwave Studio, showing a good agreement.

Super-resolution imaging of a single metal layer: high loss but superior resolution

In this work, we re-analyze the influence of the loss on the super-resolution imaging of a single metal layer superlens system and reveal its positive role of the imaging. The analysis is based on the surface plasmon polariton (SPP) theory. We show that SPP mode with high loss could suppress the amplification of evanescent waves and concentrate the energy, thus contribute to the imaging. We propose to surround the metal layer with high index medium to increase the loss of the SPP modes. The proposed structure shows better performance in super-resolution imaging than the low loss cases. Numerical simulations are performed to demonstrate the results by using two-dimensional finite element method.

Scaling of plane-wave functions in statistically optimized near-field acoustic holography.

Statistically Optimized Near-field Acoustic Holography (SONAH) is a Patch Holography method, meaning that it can be applied in cases where the measurement area covers only part of the source surface. The method performs projections directly in the spatial domain, avoiding the use of spatial discrete Fourier transforms and the associated errors. First, an inverse problem is solved using regularization. For each calculation point a multiplication must then be performed with two transfer vectors--one to get the sound pressure and the other to get the particle velocity. Considering SONAH based on sound pressure measurements, existing derivations consider only pressure reconstruction when setting up the inverse problem, so the evanescent wave amplification associated with the calculation of particle velocity is not taken into account in the regularized solution of the inverse problem. The present paper introduces a scaling of the applied plane wave functions that takes the amplification into account, and it is shown that the previously published virtual source-plane retraction has almost the same effect. The effectiveness of the different solutions is verified through a set of simulated measurements.

Perfect tunneling of a structure containing a left-handed material layer

Tunneling of monochromatic radiation obliquely incident on a plane-layered structure is investigated. In the structure, a left-handed material layer (with a negative refractive index) is surrounded by layers with positive refractive indexes. Conditions for resonance amplification of evanescent waves that occurs simultaneously with perfect tunneling, i.e., reflection-free transmission of radiation through the plane-layered structure, are revealed.

Evanescent wave amplification and subwavelength imaging by ultrathin uniaxial μ-near-zero material

We demonstrate strong evanescent wave amplification by a thin slab of uniaxial μ-near-zero (UMNZ) material. It is found that while retaining the same amplification effect, the slab can be made arbitrarily thin when the negative permeability along the axis of anisotropy approaches zero. Numerical results show that using a single layer of split-ring resonators (SRRs) with its thickness equal three thousandth of the incident wavelength (λ/3000), a subwavelength source distribution with λ/4 resolution can be transferred to a distance of λ/3.

A dynamical crossover regime during evanescent-wave amplification

A dynamical crossover regime is revealed when exposing a classical two-dimensional–ordered Josephson junction (JJ) array to evanescent waves and tuning the incident microwave power. At the lowest possible temperature for these experiments, 1.1 K, and at the lowest power setting, , evanescent waves are transmitted without loss and the resonance exhibits a quality factor of . A second, smaller resonance, which evolves with increasing power from the main resonance, is also investigated. In contrast to the behavior of the main resonance, this second peak grows as the incident power is increased and does not maintain a fixed resonant frequency for temperatures less than the superconducting critical temperature of niobium. The tunability of both resonances is studied as a function of temperature and microwave power. Finally, we speculate that this dynamical crossover regime is evidence of a transition between two states of phase coherence where at low microwave power the JJ arrays are phase locked and at high microwave power the JJ arrays are unlocked.

A spoof surface WGM sensor based on a textured PEC cylinder

Based on the negative permittivity properties of a two-dimensional (2D) textured perfect electric conductor (PEC) cylinder, we design a spoof surface whispering gallery mode (WGM) sensor at microwave band. The sensitivity of the proposed sensor is much higher than that of the traditional dielectric sensor in the same geometry size. Importantly, we show that the physical origin for the improvement of sensitivity is the evanescent-wave amplification and local field enhancement effects. This may open up an avenue for designing metamaterial sensors with superior sensitivity.

Enhancing evanescent wave intensity in a ferroelectrics plate (Kretschmann geometry)

Conditions for the fourfold amplification of evanescent wave (s- or p-type) intensity are discussed for a nonmagnetic dielectric-ferroelectric plate-vacuum structure (Kretschmann geometry).

Metamaterial Sensors

Metamaterials have attracted a great deal of attention due to their intriguing properties, as well as the large potential applications for designing functional devices. In this paper, we review the current status of metamaterial sensors, with an emphasis on the evanescent wave amplification and the accompanying local field enhancement characteristics. Examples of the sensors are given to illustrate the principle and the performance of the metamaterial sensor. The paper concludes with an optimistic outlook regarding the future of metamaterial sensor.