Amplification Of Evanescent Waves Research Articles

Substrate integrated waveguide filters and mechanical/electrical reconfigurable half-mode substrate integrated waveguide filters

In this paper, novel evanescent mode substrate integrated waveguide (SIW) and half-mode substrate integrated waveguide (HMSIW) filters are proposed, and their applications to mechanical and electrical reconfigurable bandpass filters (BPFs) are studied. SIW and HMSIW loaded with coupled complementary split-ring resonators achieve forward electromagnetic wave transmission below the waveguide cutoff frequency due to evanescent wave amplification. The half-mode configuration of the proposed SIW BPF is an opening structure, and the proposed HMSIW BPF is suitable to load with control elements. The electric/magnetic coupling effects and frequency responses reconfigurable mechanics of the proposed HMSIW structure are studied based on lumped equivalent circuit model. By using floating capacitors with mechanical switches and PIN diodes loaded on the HMSIW resonators, two one-bit HMSIW 500 MHz mechanical and 300 MHz electrical reconfigurable BPFs are demonstrated.

Electrically Tunable Evanescent Mode Half-Mode Substrate-Integrated-Waveguide Resonators

Electrically tunable evanescent mode half mode substrate integrated waveguide (HMSIW) resonators are implemented for S band applications. An HMSIW loaded with a complementary split ring resonator (CSRR) achieves forward electromagnetic wave transmission below the characteristic waveguide cutoff frequency due to evanescent wave amplification. A variable capacitor connected to one of the conductors of the CSRR changes its effective capacitance to ground, resulting in frequency tuning of the resonator. Three different configurations are investigated with a varactor diode connected between the ground and three different contact points of the CSRR. The external Q factor is slightly affected by the frequency tuning. More than 15% tunability is achieved around 3.4 GHz. Full wave structure simulation results are in good agreement with those of measurement.

Limits for superfocusing with finite evanescent wave amplification

Perfect lensing using negative refractive index materials and radiationless electromagnetic interference both provides extreme subwavelength focusing by "amplifying" evanescent wave components that are usually lost. This Letter provides a relation between the achievable focus spot size, the amplification available, and the focal length. This may be considered as a revised version of Abbe's diffraction limit for focusing systems that have evanescent wave amplification. It is useful in comparing the amplification achieved in various subwavelength focusing implementations as well as determining when it is better to use existing near-field techniques, such as simple diffraction from an aperture or slit, than to attempt complicated superfocusing.

Transmission Properties of Electromagnetic Waves Through Left-handed Material: A Revisit

The transmission properties of electromagnetic wave through left-handed material (LHM) have been thoroughly investigated. The characterization of signal propagation and evanescent wave amplification through LHM slab has been done on the basis of analytical model equations. The transmission properties have also been addressed from the study of group and phase indices, and anomalous dispersions regions in LHM and right-handed material. Furthermore, the orthogonality of electric and magnetic fields of classical electromagnetism has been seen to be applicable for electric and magnetic plasma buffered by a left-handed (LH) medium.

Metamaterial Sensor Based on WGM

Sensor utilizing metamaterials have opened up a new field of considerable interest. We extend here the works of our group about metamaterial sensor based on Whispering Gallery Mode (WGM), which is constructed with a microring resonator sensor coated with metamaterial layer. We demonstrate that our sensor possesses higher sensitivity than the traditional sensor since the amplification and penetration of evanescent wave by metamaterials.

Amplification of Acoustic Evanescent Waves Using Metamaterial Slabs

We amplified acoustic evanescent waves using metamaterial slabs with a negative effective density. For the amplifying effect of the slab to overcome the dissipation, it is necessary that the imaginary part of the effective density is much smaller than the real part, a condition not satisfied so far. We report the construction of membrane-based two-dimensional negative-density metamaterials which exhibited remarkably small dissipation. Using a slab of this metamaterial we realized a 17-fold net amplitude gain at a remote distance from the evanescent wave source. Potential applications include acoustic superlensing.

Double Negative Metamaterial Sensor Based on Microring Resonator

Metamaterials are artificial media structured on a size scale smaller than the wavelength of external stimuli, and may provide novel tools to significantly enhance the sensitivity and resolution of the sensors. In this paper, we derive the dispersion relation of cylindrical dielectric waveguide loaded on double negative metamaterial (DNM) layer, and compute the resonant frequencies and electric field distribution of the corresponding Whispering-Gallery-Modes (WGM). Theoretical results of resonant frequency and electric field distribution are in good agreement with the simulation results. We show that the DNM sensor based on microring resonator possesses higher sensitivity than the traditional microring sensor since the amplification of evanescent wave, and with the increase of metamaterial layer thickness, the sensitivity will be increased greatly. It might open an avenue for designing perfect sensors.

A Microring Resonator Based Negative Permeability Metamaterial Sensor

Metamaterials are artificial multifunctional materials that acquire their material properties from their structure, rather than inheriting them directly from the materials they are composed of, and they may provide novel tools to significantly enhance the sensitivity and resolution of sensors. In this paper, we derive the dispersion relation of a cylindrical dielectric waveguide loaded on a negative permeability metamaterial (NPM) layer, and compute the resonant frequencies and electric field distribution of the corresponding Whispering-Gallery-Modes (WGMs). The theoretical resonant frequency and electric field distribution results are in good agreement with the full wave simulation results. We show that the NPM sensor based on a microring resonator possesses higher sensitivity than the traditional microring sensor since with the evanescent wave amplification and the increase of NPM layer thickness, the sensitivity will be greatly increased. This may open a door for designing sensors with specified sensitivity.

Limitations and potentials of metamaterial lenses

The seminal discovery that an ideal negative-index lens may overcome Abbe's diffraction limit has raised enormous interest in the field of metamaterials and of subwavelength focusing. This finding is based on the anomalous wave propagation in ideally isotropic and homogeneous metamaterials with negative index of refraction and low loss, provided they are available. We have designed a metamaterial lens based on one of the simplest metamaterial geometries, a cubic array of spheres, with the aim of verifying its imaging properties in a practical configuration. After a rigorous homogenization, we have shown that, for suitable designs, the effective bulk parameters may indeed provide a quasi-isotropic negative-index response, ideal for imaging applications. We have then tested the imaging properties for finite-size lenses, analyzing challenges and potentials of going beyond the diffraction limit in a practical setup. We have also explored an alternative venue to exploit the negative-index property of the designed metamaterial in a concave lens, in order to resolve subwavelength features in the far-field. Our results indicate that, although subwavelength resolution and evanescent-wave amplification are possible in metamaterial arrays, practical imaging beyond the diffraction limit is challenging and a careful design should consider the granularity, degree of isotropy, and transverse size of the metamaterial lens.

Acoustic field enhancement and subwavelength imaging by coupling to slab waveguide modes

We present a theoretical study on the amplification of evanescent sound waves produced by coupling to trapped modes hosted by a fluidic planar waveguide. Total internal reflection at interfaces of different refractive indexes can be frustrated by the introduction of a slow slab waveguide which is leading to a gigantic field enhancement, useful for sensitive transducers and acoustic shock lithotripsy. The mechanism behind the evanescent field coupling that is also known as tunnelling barrier penetration in quantum mechanics is here adopted for its use in an acoustic superlens. The higher spatial harmonics produced by a subwavelength object can couple to trapped modes of the slow waveguide and be reproduced as an image at a distant plane. We suggest a practical implementation of these ideas by means of a silicone rubber slab containing positive acoustic wave propagation parameters.

Exponential amplification of evanescent acoustic waves and transparency through periodic stacks of planar surfaces.

Two of the most exciting properties of metamaterials are the possibility to sustain “growing” evanescent waves, which may provide sub-wavelength resolution, and transparency or cloaking effects. In this paper, we verify both these effects in periodic stacks of planar surfaces. We investigate this problem using Bloch wave analysis in order to determine the necessary requirements to achieve amplification of evanescent acoustic waves and overall transparency effects. This is achieved analogously to previous work for electromagnetic waves [A. Alù and Engheta, IEEE Antennas Wireless Propag. Lett. 4, 417–420 (2005)] by combining periodic arrays, properly designed to support conjugate Bloch wave properties, even though each stack individually operates in its stopband. Based on this conjugate condition, we have obtained several practical stack designs, formed by lumped passive acoustic elements, that may be able to amplify evanescent Bloch waves and provide complete tunneling and transparency at the frequency of interest. These findings may pave the way for acoustic imaging with sub-wavelength resolution and planar surface transparency.

The Left-Handed Properties of the Composite Medium Composed of Ferrite Cubes’ Array and Copper Net

Left-handed material (LHM), whose permittivity and permeability are both negative, has attracted much attention due to its exotic electromagnetic properties, such as negative refraction, backward wave propagation and evanescent wave amplification. The microwave left-handed property of a composite medium composed of ferrite cubes’ array and copper net was studied using the method of finite difference time domain in this paper. An obvious left-handed pass band emerged when ferrite cubes were combined with longitudinal copper wires, because the ferrite produced negative permeability by the ferromagnetic resonance and the copper wires produced negative permittivity by the plasma. With the addition of horizontal copper wires, the transmission intensity of the pass band was gradually weakened. It can be concluded that the horizontal wires played a negative role in realizing the composite LHM with ferrite cubes and copper net. This work gives an instructive direction to the fabrication of 3D LHMs.

Super-resolution imaging using a three-dimensional metamaterials nanolens

Super-resolution imaging beyond Abbe’s diffraction limit can be achieved by utilizing an optical medium or “metamaterial” that can either amplify or transport the decaying near-field evanescent waves that carry subwavelength features of objects. Earlier approaches at optical frequencies mostly utilized the amplification of evanescent waves in thin metallic films or metal-dielectric multilayers, but were restricted to very small thicknesses (⪡λ, wavelength) and accordingly short object-image distances, due to losses in the material. Here, we present an experimental demonstration of super-resolution imaging by a low-loss three-dimensional metamaterial nanolens consisting of aligned gold nanowires embedded in a porous alumina matrix. This composite medium possesses strongly anisotropic optical properties with negative permittivity in the nanowire axis direction, which enables the transport of both far-field and near-field components with low-loss over significant distances (>6λ), and over a broad spectral range. We demonstrate the imaging of large objects, having subwavelength features, with a resolution of at least λ/4 at near-infrared wavelengths. The results are in good agreement with a theoretical model of wave propagation in anisotropic media.

Resonant amplification of frustrated evanescent waves by single dielectric coating

Abstract Resonant amplification and enhancement of evanescent wave’s frustration across an optical barrier by using single dielectric coating is numerically demonstrated in this article. With further tuning of the thickness values of the second and the third medium (optical barrier) of the proposed stratified four-media configuration, it is shown that it may be possible to achieve unity transmittance at discrete, but closely spaced incident angles within the full range defined by the theoretical cut-off limit. The designed configuration may have other potential applications than lithography and waveguide design is one such example.

Waveguide Probes Using Single Negative Media

In this work, we show that by using single negative (SNG) media, the sensitivity of open-ended near-field waveguide probes can be enhanced. The enhancement is due to the evanescent wave amplification property of SNG media. As a demonstration of the efficacy of the proposed SNG-enhanced probe, we present a numerical study of the case of detecting precursor pitting in aluminum plates. We show that the SNG-enhanced probe achieves 35 times increase in the phase shift due to the presence of the target in comparison to the case of a classical probe. We show that there is an optimum SNG media thickness and an optimum standoff distance that maximize the sensitivity.

Study of active superlens and evanescent wave amplification using an active metamaterial model

We propose a Π -shaped circuit model for active metamaterials using periodic circuit structures with series inductors and shunt capacitors (LC) and series capacitors and shunt inductors (CL). Using the circuit model, we develop a general theory for active metamaterials, revealing the role of active components and demonstrating the principle for metamaterials to be active. By inserting negative and positive resistances in the LC and CL structures, we first attain the active superlens using three slabs of effective media made by such metamaterial circuit structures and show that the amplitude of voltage will be augmented on two interfaces after adding the active/lossy components. Then we realize the active amplification of evanescent waves and show that the transmission coefficient will be larger than 0 dB near the resonant frequency, which evidently illustrates the active feature of the medium. Both of these two applications have to satisfy the anti-matching conditions to fulfill the resonance and tunneling effect.

Modelling and Analysis of Ω-shaped Double Negative Material-assisted Microwave Sensor

A novel Ω-shaped double negative (DNG) material-assisted microwave sensor is presented, and modelled using a full wave electromagnetic commercial simulator CST. Simulating results show that the amplified evanescent wave is sensitive to the permittivity of dielectric material in the detecting zone. Meanwhile, mechanism of the evanescent wave amplification is explained. Comparing with conventional microwave resonant sensor, the Ω-shaped DNG materials-assisted microwave sensor allows for much higher sensitivity.

Simple demonstration of visible evanescent-wave enhancement with far-field detection

We demonstrate the fabrication of a simple metamaterial-diffraction grating device that both amplifies and converts evanescent waves into propagating ones. This is the most accessible example of such phenomena reported until now (to our knowledge), as it requires few thin film layers, operates in the visible, and can be probed with traditional optics. The metamaterial exhibits amplification of evanescent waves generated from total internal reflection. The device was fabricated over 1 cm2 using pulsed-laser deposition and a pattern-replication technique.

Experimental and Theoretical Evidence for Subwavelength Imaging in Phononic Crystals

We show experimentally and theoretically that super resolution can be achieved while imaging with a flat lens consisting of a phononic crystal exhibiting negative refraction. This phenomenon is related to the coupling between the incident evanescent waves and a bound slab mode of the phononic crystal lens, leading to amplification of evanescent waves by the slab mode. Super resolution is only observed when the source is located very near to the lens, and is very sensitive to the location of the source parallel to the lens surface as well as to site disorder in the phononic crystal lattice.

Subwavelength internal imaging by means of a wire medium

Evanescent wave amplification is observed, for the first time to our knowledge, inside a half-wavelength-thick wire medium slab used for subwavelength imaging. The wire medium is analyzed using both a spatially dispersive finite-difference time-domain (FDTD) method and a full-wave commercial electromagnetic simulator CST Microwave Studio. In this work we demonstrate that subwavelength details of a source placed at a distance of one-tenth of a wavelength from a wire medium slab can be detected inside the slab with a resolution of approximately one-tenth of a wavelength in spite of the fact that they cannot be resolved at the front interface of the device, due to the rapid decay of evanescent spatial harmonics in free space.