Superlens Research Articles

Ultrasonic subwavelength imaging with blind structured illumination

The resolution of ultrasonic imaging systems suffers from the diffraction limit. To solve this problem, we develop a blind structured illumination microscopy (blind-SIM) system that achieves far-field subwavelength resolution in underwater ultrasonic imaging systems. By illuminating the object with multiple structured illumination (SI) patterns generated by scattering media with subwavelength features, the spatial frequency mixing between the object and the SIs converts evanescent waves to propagate waves that can reach sensors in the far field. In this work, the acoustic SIs are generated by randomly distributed micro particles in water. The image of the object is then reconstructed from multiple measurements (each produced by a SI pattern) utilizing computational reconstruction algorithms where the precise knowledge of the SIs is not required. The developed ultrasonic blind-SIM system has the potential in improving the ultrasonic imaging resolution in medical diagnoses, underwater acoustic imaging, and communications. Our system largely reduces the imaging hardware complexity and improves the imaging speed compared with existing ultrasonic subwavelength technologies that mostly rely on the precise localization of time-varying contrast agents.

Transformation design of in-plane elastic cylindrical cloaks, concentrators and lenses

We analyse the elastic properties of a class of cylindrical cloaks deduced from linear geometric transforms x→x′ in the framework of the Milton–Briane–Willis cloaking theory [New Journal of Physics 8, 248, 2006]. More precisely, we assume that the mapping between displacement fields u(x)→u′(x′) is such that u′(x′)=A−tu(x), where A is either the transformation gradient Fij=∂xi′/∂xj or the second order identity tensor I. The nature of the cloaks under review can be three-fold: some of them are neutral for a source located a couple of wavelengths away; others lead to either a mirage effect or a field confinement when the source is located inside the concealment region or within their coated region (some act as elastic concentrators squeezing the wavelength of a pressure or shear polarized incident plane wave in their core); the last category of cloaks is classified as an elastic counterpart of electromagnetic perfect cylindrical lenses. The former two categories require either rank-4 elastic tensor and rank-2 density tensor and additional rank-3 and 2 positive definite tensors (A=F) or a rank-4 elasticity tensor and a scalar density (A=I) with spatially varying positive values. However, the latter example further requires that all rank-4, 3 and 2 tensors be negative definite (A=F) or that the elasticity tensor be negative definite (and non fully symmetric) as well as a negative scalar density (A=I). We provide some illustrative numerical examples with the Finite Element package Comsol Multiphysics when A is the identity.

Dispersion‐Enabled Symmetry Switching of Photonic Angular‐Momentum Coupling

AbstractPhotonic spin‐orbit interactions describe the interactions between spin angular momentum and orbital angular momentum of photons, which play essential roles in subwavelength optics. However, the influence of frequency dispersion on photonic angular‐momentum coupling is rarely studied. Here, by elaborately designing the contribution of the geometric phase and waveguide propagation phase, the dispersion‐enabled symmetry switching of photonic angular‐momentum coupling is experimentally demonstrated. This notion may induce many exotic phenomena and be found in enormous applications, such as the spin‐Hall effect, optical calculation, and wavelength division multiplexing systems. As a proof‐of‐concept demonstration, two metadevices, a multi‐channel vectorial vortex beam generator and a phase‐only hologram, are applied to experimentally display optical double convolution, which may offer additional degrees of freedom to accelerate computing and a miniaturization configuration for optical convolution without collimation operation. These results may provide a new opportunity for complex vector optical field manipulation and calculation, optical information coding, light‐matter interaction manipulation, and optical communication.

Performers as emotional artisans: Crafting displays in theatre and workload

In this article, we analyse the process of crafting emotional displays developed by performers in theatre work. The systematic approach to emotion regulation and management displayed in theatre provides a perfect lens for researching emotions in the workplace. This, in turn, provides the possibility of developing a toolkit to analyse a wide range of working experiences where emotions and bodily labour are an intrinsic part of the work. This research draws on a sample of 31 in-depth qualitative interviews with professional Spanish performers alongside ethnographic observations and interviews conducted in two theatre companies. We start by exploring the pathways and techniques adopted by performers to address emotional labour and emotional management in theatre work. Afterwards, focusing on the process of displaying emotions live on stage, we analyse the overlap between the performers’ own emotions and the emotions of the characters they portray, revisiting the limitations of the distinction between the notions of the ‘real’ and the ‘fake’ self in the workplace. Finally, we will turn our attention to the workload generated by emotional labour in theatre, trying to ascertain the main positive and negative outcomes for vocational performers with a working experience heavily focused on emotional labour while dealing with a context marked by precarity. We develop a better understanding of the constant desire to work despite the impossibility of making a living through the concept of emotional artisanship.

Wave Propagation Properties of a One‐Dimensional Photonic Crystal with Double Negative Metamaterial

AbstractIn this simulation work, the wave vector and reflectance properties of a 1DPC made of dielectric/metamaterial multilayer structure employing TMM, for TE and TM modes are studied. For TE mode, it is observed that the obtained wide bandgap increases with increase in the angle of incidence, whereas the narrow bandgap also shows a minor increase with increase in the incident angle. It is noted that bandgap increases with increase in the incident angle in TE mode, while bears opposite behavior in TM mode and decreases with increase in the incident angle. If the DPS thickness is increased, the number of bands increases in both TE and TM modes, while the wide bandgaps for TE and TM decreases in width. In both cases of the increased DPS width, it is found that the narrow bandgap obtained in TM mode is showing enhancement with an increase in incident angle, and exhibits ultra‐tunable behavior. Some other researchers have reported earlier that a complementary split‐ring resonator based metamaterial with effective medium ratio has influence on the extraction of effective permittivity, permeability, and refractive index, where DNG regions exist in the ranges 6.34–7.39 and 8.20–9.98 GHz. Comparing with the proposed DNG, the study finds that the aforesaid values correspond to frequency regime taken in our case, 1–10 GHz, particularly the middle of the range 1–10 GHz, where broadband is obtained, which is satisfactory. The insights included in the article can be employed in designing microwave broadband reflectors, tunable mirrors, and can be proved to be useful in photovoltaic cells. Such PC can also be employed in designing super lenses as well as optical cloaking, and the miniaturization of antennas.

Perfect-Lens Theory Enables Metasurface Reflectors for Subwavelength Focusing

Breaking the so-called diffraction limit on the resolution of optical devices and achieving subwavelength focusing requires tailoring the evanescent spectrum of wave fields. There are several possible approaches, all of which have limitations, such as the generation of strong additional scattering, limited focusing power, issues at the implementation step, and the need for a drain at the focal point. This paper presents a feasible strategy based on the concepts of the perfect lens and power flow-conformal metasurfaces. Desired fields for subwavelength focusing are integrated using double-negative media and then the surface profile of a focusing reflector is designed to be tangential to the desired power flow, so that the metasurface can be modeled as a local impedance boundary, and can be easily implemented using passive and lossless elements. Full-wave simulations demonstrate that an example reactive metasurface is able to break the diffraction limit and provide near-field focusing with subwavelength hotspot size. We expect that the outcome will find applications in antennas, beam-shaping devices, nonradiative wireless power transfer systems, microscopy, and lithography.

Wideband 5G Antenna Gain Enhancement Using a Compact Single-Layer Millimeter Wave Metamaterial Lens

This paper presents a very compact, wideband, and enhanced-gain antenna for 5G applications. A simple single-layer millimeter wave (mm-wave) metamaterial lens (meta-lens) is used to improve the gain, aperture efficiency, and gain bandwidth of a slotted-patch antenna over a wide range of frequencies from 25 GHz to 31 GHz. The lens exhibits a metamaterial negative refractive index behavior, which is attributed to a substantial gain enhancement of around 4-5 dBi over the whole band compared to the gain values of the slotted patch antenna alone. The lens’s unit cell comprises a simple single-layer split ring resonator (SRR) whose dimensions are carefully chosen to improve transmitted power and suppress absorbed and reflected power. The meta-lens consists of 8×8 subwavelength SRR unit cells. Each cell has an area of 1.6×1.6 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , it is located in the near-field region closely above a slotted patch antenna to produce a total antenna size of 12.8 ×12.8 ×7.27mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ( 1.2 λ × 1.2 λ × 0.68 λ, where λ is the free space wavelength at 28 GHz). The maximum gain of the proposed antenna is 12.7 dBi, the 1 dB gain bandwidth is 18%, the maximum aperture efficiency is 92%, and the -10 dB impedance bandwidth (10 dB B.W.) is 17%. This excellent combination of essential metrics is hard to realize at mm-wave using narrowband antenna structures (microstrip patch antennas), and the aperture efficiency is the highest thus far for such a class of antennas.

Abnormal in-plane epitaxy and formation mechanism of vertically aligned Au nanopillars in self-assembled CeO2-Au metamaterial systems.

Metamaterials present great potential in the applications of solar cells and nanophotonics, such as super lenses and other meta devices, owing to their superior optical properties. In particular, hyperbolic metamaterials (HMMs) with exceptional optical anisotropy offer improved manipulation of light-matter interactions as well as a divergence in the density of states and thus show enhanced performances in related fields. Recently, the emerging field of oxide-metal vertically aligned nanocomposites (VANs) suggests a new approach to realize HMMs with flexible microstructural modulations. In this work, a new oxide-metal metamaterial system, CeO2-Au, has been demonstrated with variable Au phase morphologies from nanoparticle-in-matrix (PIM), nanoantenna-in-matrix, to VAN. The effective morphology tuning through deposition background pressure, and the corresponding highly tunable optical performance of three distinctive morphologies, were systematically explored and analyzed. A hyperbolic dispersion at high wavelength has been confirmed in the nano-antenna CeO2-Au thin film, proving this system as a promising candidate for HMM applications. More interestingly, a new and abnormal in-plane epitaxy of Au nanopillars following the large mismatched CeO2 matrix instead of the well-matched SrTiO3 substrate, was discovered. Additionally, the tilting angle of Au nanopillars, α, has been found to be a quantitative measure of the balance between kinetics and thermodynamics during the depositions of VANs. All these findings provide valuable information in the understanding of the VAN formation mechanisms and related morphology tuning.

Method of Designing a Planar Scan Angle Enhancing Metalens Featuring Lossless Wide Scanning at mmWave

For the first time, a method to design a planar scan angle enhancing lens with minimum beam distortion for automotive radar at millimeter-wave is presented. The proposed lens is not only advantageous regarding its planar topology but also offers a solution to the chronic problem surrounding planar phased array antenna and planar lenses: scan loss. In this article, ray optics and ray transfer matrix are applied to Galilean lens configuration from which the concept of scan enhancement and compensating parameters are conceived. The basic idea of compensating lies in re-focusing the portion of the beams and suppressing transmission at extreme angles. Visual analysis of the wave refraction and designing of the lens system is conducted using 2-D ray-tracing code. Full-wave simulation validates the effectiveness of the proposed idea by extending the limited scan range of the phased array antenna from 45° to 58° with gain fluctuation below 1.1 dB at E-band. The radiation efficiency is calculated to be 0.86 at 76.5 GHz. Furthermore, the proposed lens system is fabricated using low-temperature co-fired ceramic (LTCC) technology and experimentally verified. Series-fed array antennas integrated with scan angle enhancing meta lens (SAEMLs) confirmed the shift of the gain peak from 46° to 58°, proving that SAEML can effectively extend the scanning range of phased arrays with small gain variation.

Design of 1 × 2 MIMO Palm Tree Coplanar Vivaldi Antenna in the E-Plane with Different Patch Structure

In this paper, 1 × 2 MIMO of Palm Tree Coplanar Vivaldi Antenna is presented that simulated at 0.5–4.5 GHz. Some GPR applications require wideband antennas starting from a frequency below 1 GHz to overcome high material loss and achieve deeper penetration. However, to boost the gain, antennas are set up in MIMO and this is costly due to the large size of the antenna. When configuring MIMO antenna in the E-plane, there is occasionally uncertainty over which antenna model may provide the optimum performance in terms of return loss, mutual coupling, directivity, beam squint, beam width, and surface current using a given substrate size. However, the configuration of E-plane antenna in MIMO has an issue of mutual coupling if the distance between elements is less than 0.5λ. Furthermore, it produces grating lobes at high frequencies.We implement several types of patch structures by incorporating the truncated, tilt shape, Hlbert and Koch Fractal, Exponential slot, Wave slot, the lens with elips, and metamaterial slot to the radiator by keeping the width of the substrate and the shape of the feeder. The return loss, mutual coupling, directivity, beam squint, beamwidth, and surface current of the antenna are compared for 1 × 2 MIMO CVA. A continuous patch MIMO has a spacing of 0.458λ at 0.5 GHz, which is equivalent to its element width. From the simulation, we found that Back Cut Palm Tree (BCPT) and Horizontale Wave Structure Palm Tree (HWSPT) got the best performance of return loss and mutual scattering at low-end frequency respectively. The improvement of directivity got for Metamaterial Lens Palm Tree (MLPT) of 4.453 dBi if compared with Regular Palm Tree-Coplanar Vivaldi Antena (RPT) at 4 GHz. Elips Lens Palm Tree (ELPT) has the best beam squint performance across all frequencies of 0°. It also gots the best beamwidth at 4.5 GHz of 3.320. In addition, we incorporate the MLPT into the radar application.

Sub-wavelength passive single-shot computational super-oscillatory imaging

Super-oscillatory (SO) imaging presents a passive solution to the sub-diffraction imaging challenge. The point spread functions (PSFs) of SO systems are band-limited and locally oscillate faster than their highest Fourier frequency. Because of this property, SO imaging systems are not bound by the Nyquist limit, allowing them to acquire details finer than the diffraction limit. In this work, we present a comprehensive theoretical analysis of passive incoherent SO imaging, leading to two key results. First, we show that the SO property of an imaging system is preserved when the system is combined with a general system of lenses. This opens the door for integrating SO imaging into existing microscopes and telescopes. Second, we show that incoherent SO imaging cannot resolve feature sizes below λ / 2 because of the inability to achieve incoherence at finer scales. This establishes the operational limits of the approach. We demonstrate our theory experimentally with a compound SO system that achieves sub-wavelength resolutions using high-NA lenses and Fourier spectrum modulation.

Subwavelength Terahertz Resonance Imaging (STRING) for Molecular Fingerprinting.

Subwavelength terahertz (THz) imaging methods are highly desirable for biochemical sensing as well as materials sciences, yet sensitive spectral fingerprinting is still challenging in the frequency domain due to weak light-matter interactions. Here, we demonstrate subwavelength THz resonance imaging (STRING) that overcomes this limitation to achieve ultrasensitive molecular fingerprinting. STRING combines individual ring-shaped coaxial single resonators with near-field spectroscopy, yielding considerable sensitivity gains from both local field enhancement and the near-field effect. As an initial demonstration, we obtained spectral fingerprints from isomers of α-lactose and maltose monohydrates, achieving sensitivity that was enhanced by up to 10 orders of magnitude compared to far-field THz measurements with pelletized samples. Our results show that the STRING platform could enable the development of THz spectroscopy as a practical and sensitive tool for the fingerprinting and spectral imaging of molecules and nanoparticles.

Bifunctional metasurface for cross-polarization conversion and ultra-broadband absorption in terahertz range

A bifunctional terahertz (THz) metasurface based on vanadium dioxide (VO2) was proposed, which can achieve near-perfect cross-polarization conversion (CPC) and ultra-broadband absorption. When VO2 is in insulator state, the metasurface acts as a cross-polarization converter, converting linearly polarized (LP) and circularly polarized (CP) waves into the corresponding cross-polarized waves. The polarization conversion ratio (PCR) exceeds 99.5% in the frequency range of 4.1∼6.7 THz at normal incidence, and can be maintained more than 98.3% even at an incident angle of 20°. When VO2 is in metal state, the metasurface becomes a THz broadband absorber with absorption efficiency above 90% in a wide frequency range of 2.9 THz∼8.4 THz. The absorption efficiency is almost unchanged when the incident angle increases from 0° to 15° for TE wave. In addition, the absorption efficiency can maintain above 90% at 4.0∼7.1 THz when the incident angle is up to 60° for TM wave. Moreover, the interference theory model was utilized for verifying theoretically the validity of the numerical simulation results of our proposed metasurface, and the theoretical calculation results are in good agreement with the simulation results. We believe that this proposed metasurface has potential applications in many fields, such as sensing, imaging, non-destructive detectors, super lenses, and telecommunications.

Single-layer near-zero refractive index metamaterial lens based on non-complete periodic arrays.

In this study, an electric inductive-capacitive (ELC) resonator with a refractive index of around 0.08+i0.08 at 5.8 GHz serves as the unit cell. Based on the ELC unit cell, together with the electromagnetic wave (EMW) single knife-edge diffraction model, a near-zero refractive index metamaterial (NZIM) lens with a non-complete periodic array and small dimension of 0.8λ0 2 is proposed to weaken the blocking of feed source radiation, and bring more significant gain enhancement compared to that with a complete periodic array. To demonstrate the proposed lens performance, the lens-loaded antenna formed by the proposed NZIM lens and a rectangular microstrip patch antenna (MPA) operating at the center frequency of 5.8GHz is provided. The measured results show that the relative bandwidth (RBW) of the lens-loaded antenna is 9.83% (5.51-6.08 GHz), which is expanded by 3.6 times contrasted with that of MPA (2.75%). Moreover, the average and maximum gains enhancement of 4.77 dB and 6.88 dB are obtained. Due to improving the radiation characteristic, this work can be used to drive the application of MPAs in wireless communication systems.

Impact of anomalous surface boundary conditions on the planar negative‐refractive index lens

It is shown that anomalous boundary conditions at the surface of a negative-refractive-index metamaterial planar lens severely diminishes the resolution of the lens when its relative permittivity and permeability are both −1. Anomalous boundary conditions arise in practical microwave metamaterials that are typically a periodic array of identical unit cells comprising dielectric and conducting elements. For a unit-cell size much smaller than the wavelength, homogenised permittivity and permeability are the constituent parameters of the average fields. Average fields vary on a scale larger than the unit cell size compared to localised fields associated with the constituent elements. Unlike the tangential components of the localised field, tangential components of the average fields are discontinuous across the surface of such materials. This anomalous boundary condition at the lens surface must be described by generalised sheet transition conditions. This study develops expressions for the negative-refractive-index lens optical transfer functions, as a function of spatial frequency, for transverse-electric waves, that account for anomalous surface boundary conditions. Both the simulations and experimental data are used to verify the expressions at a frequency of 3 GHz.

Metallic Structures Based on Zinc Oxide Film for Enzyme Biorecognition

Two structures (Ag/ZnO/ITI/glass: #1 sample and Ag/ZnO/SiO2/Si: #2 sample) are investigated, on the one hand, from the point of view of the formation of evanescent waves in the gratings of metal strips on the structures when the incident TEz wave in the radio frequency range is used. The simulation of the formation of evanescent waves at the edge of the Ag strips, with thicknesses in the range of micrometers, was carried out before the test in the subwavelength regime, with the help of a new improved transducer with metamaterial (MM) lenses. By simulation, a field snapshot was obtained in each sequence of geometry. The evanescent waves are emphasized in the plane XY, due to the scattering of the field on the edge of the strips. On the other hand, ZnO nanoparticles are investigated as a convenient high-efficiency biodetection material, where these structures were used as a biosensitive element to various enzymes (glucose, cholesterol, uric acid, and ascorbic acid). The obtained results demonstrate that the investigated structures based on ZnO nanostructures deposited on different supports are fast and sensitive for enzyme detection and can be successfully incorporated into a device as a biosensing element.

A quantum graph approach to metamaterial design

Since the turn of the century, metamaterials have gained a large amount of attention due to their potential for possessing highly nontrivial and exotic properties—such as cloaking or perfect lensing. There has been a great push to create reliable mathematical models that accurately describe the required material composition. Here, we consider a quantum graph approach to metamaterial design. An infinite square periodic quantum graph, constructed from vertices and edges, acts as a paradigm for a 2D metamaterial. Wave transport occurs along the edges with vertices acting as scatterers modelling sub-wavelength resonant elements. These resonant elements are constructed with the help of finite quantum graphs attached to each vertex of the lattice with customisable properties controlled by a unitary scattering matrix. The metamaterial properties are understood and engineered by manipulating the band diagram of the periodic structure. The engineered properties are then demonstrated in terms of the reflection and transmission behaviour of Gaussian beam solutions at an interface between two different metamaterials. We extend this treatment to N layered metamaterials using the Transfer Matrix Method. We demonstrate both positive and negative refraction and beam steering. Our proposed quantum graph modelling technique is very flexible and can be easily adjusted making it an ideal design tool for creating metamaterials with exotic band diagram properties or testing promising multi-layer set ups and wave steering effects.

Plasmonic Bound States in the Continuum in Compact Nanostructures

AbstractOptical bound states in the continuum (BICs) have become a unique way to produce the extreme localization and enhancement of light waves, however, it remains challenging to realize them in single‐particle systems. This work shows the existence of optical BICs in a compact corrugated metallic cylinder, and quasi‐BICs are observed in multiple channels when adjacent grooves have different depths. It is revealed that such BICs are purely plasmonic, resulting from the destructive interference of two localized surface plasmon modes in the nanostructures. Benefiting from the surface plasmon effects, the findings extend optical BICs and the associated physics to subwavelength optics, providing a route to break the diffraction limit and boost light–matter interactions.

Low-Loss Flexible Polarization-Maintaining Hollow Waveguide for Linearly Polarized 100 GHz Radiation Transmission and Subwavelength Imaging

Terahertz imaging has been developed rapidly in recent years. However, conventional terahertz imaging systems are often bulky due to the free space propagation. Various subwavelength sized probes have been developed to realize high resolution. However the tiny probes often suffer from high loss. A flexible metallic polarization-maintaining (PM) hollow waveguides with elliptical cross section is investigated and fabricated. The waveguide is used for transmitting linearly polarized terahertz (THz) radiation and flexible imaging. Bore size of the waveguide is on the order of wavelength. Transmission loss is as low as 0.92 dB/m. High birefringence of 0.08 and measured polarization ratio of 97.2% are achieved by the waveguide at 100 GHz. Moreover, a scanning 100 GHz imaging system is established by using the waveguide as a flexible probe. The PM capability of the fabricated waveguide brings benefits compared to conventional non-PM circular waveguide. The linearly polarized fundamental modes are easily excited and can be propagated through the fabricated waveguides to one meter away with polarization maintenance, which results in compressing of output beam and realizes a high resolution. A resolution better than λ/3 is achieved. It is further experimentally verified that the waveguide is robust to bending and twisting deformation, and maintained the imaging quality during flexible imaging. A subwavelength bore-sized single-polarization single-mode waveguide is also fabricated to simultaneously achieve high horizontal and vertical resolutions. The discussed waveguide is easy to fabricate and may find important applications in THz microscopy and flexible polarization imaging.

Mesotronics: Some New, Unusual Optical Effects

The recently emerged field of Mesotronics provides novel opportunities for subwavelength magnetic and electric field localization and giant enhancement by mesoscale dielectric particles and structures from low-index to high-index materials, supported by novel optical phenomena. In this case, two regions: non-resonant and resonant, can be distinguished. In this short review, which is a direct continuation of our recently published study, we continue to present the authors’ point of view on some new optical effects in dielectric mesotronics. Among them are anomalous apodization effect in phase mesoscale gratings, new effects on high order Fano-resonances and extreme effects in field localization, mesoscale particle-based super-resolution and high-speed communications, photonic hook-based high-contrast subwavelength imaging, and reverse optical energy flow in a perforated resonant spherical particle.