Superlens Research Articles

Study on the Propagation and Conversion of Evanescent Lamb Waves in Discontinuous Plate Structures

In conventional Lamb wave based non-destructive testing (NDT), subwavelength imaging cannot be performed for very small damages due to diffraction limits, which may pose safety hazards to engineering structures and large equipment, and even cause major safety accidents, leading to property damage and casualties. However, in evanescent Lamb waves, there is the necessary subwavelength information that can be converted into propagable Lamb waves, which contain subwavelength damage information that can be received to form super-resolution imaging. Therefore, this article investigates the propagation and transformation characteristics of evanescent Lamb waves in a discontinuous plate structure, providing a research approach for further super-resolution imaging, which has certain scientific research and practical application value.

Application of a capacitively-loaded split-ring metamaterial lens in a 0.3 T magnetic resonance imaging system

In this work, it is demonstrated that metamaterial lenses can help to increase the signal-to-noise ratio of magnetic resonance imaging coils in low-field magnetic resonance imaging systems. In low-field magnetic resonance imaging systems, the dominant source of noise in the signal-to-noise ratio comes from the metallization of the coil and the noise sample or tissue is negligible, unlike high-field magnetic resonance imaging systems. A metamaterial lens consisting of a three-dimensional array of capacitively loaded split rings was fabricated and tested on a 0.3 T MRI system. The experiment was performed for a configuration in which the metamaterial lens is far enough from the coil so that the additional losses that can be introduced into the coil by the metamaterial lens are also negligible, as are the tissue losses.

Far-field mid-infrared microscopy via spatial frequency shifting of evanescent waves in photorefractive nematic liquid crystal.

Mid-infrared wavelength has unique advantages in revealing the nanostructures and molecular vibrational signatures. However, the mid-infrared subwavelength imaging is also limited by diffraction. Here, we propose a scheme for breaking the limitation in mid-infrared imaging. With the assistance of orientational photorefractive grating established in nematic liquid crystal, evanescent waves are efficiently shifted back into the observation window. The visualized propagation of power spectra in k-space also proves this point. The resolution has an improvement about 3.2 times higher than the linear case, showing potentials in various imaging areas, such as biological tissues imaging and label-free chemical sensing.

Metamaterial-Based LTCC Compressed Luneburg Lens Antenna at 60 GHz for Wireless Communications

In this study, a metamaterial-based LTCC compressed Luneburg lens was designed, manufactured and measured. The lens was designed at 60 GHz to utilize the unlicensed mm-wave spectrum available for short-range high-capacity wireless communication networks. The transformation optics method was applied to ensure the compression of the Luneburg lens antenna and thus maintain a low-profile structure. The two different types of unit cells for low and high permittivity regions were considered. The parametric study of the effect of compression on lens performance was presented. The antenna is implemented with a standard high-permittivity LTCC process, and details of the manufacturing process for the metamaterial lens are discussed. The low-profile lens is thinner than 2 mm and measures 19 mm in diameter. A size reduction of 63.6% in comparison with a spherical lens was achieved. The near-field to far-field mm-wave measurement technique is presented, and the measurement results show a peak antenna gain of 16 dBi at 60 GHz and a beam-scanning capacity with 1 dB scan loss within a 50° field of view.

Enhancing optical contrast of sub-wavelength optical image using graphene oxide coated probe in scanning thermal microscopy

Scanning thermal microscopy (SThM) is a powerful technique for obtaining local thermal information on a sample surface using a nano-fabricated thermometer probe. One important application of SThM is sub-wavelength optical imaging, which captures the photothermal response of the probe to light. In this nano-scale imaging technique, optical contrast and spatial resolution are critical performance indicators. The size and material of the thermometer probe play a significant role in determining image quality, as a smaller radius can improve spatial resolution but reduce optical contrast and sensitivity due to decreased light absorption. To address this challenge, we have developed an approach to improve the photothermal energy conversion efficiency by coating the probe with atomically thin graphene oxide (GO). Our results demonstrate that this GO coating can enhance optical sensitivity by more than 1.5 times with linear optical power dependency. Importantly, the coating significantly improves the root mean square contrast of the image by 2.56 times without compromising spatial resolution. Our study highlights the potential of GO-coated probes to improve the sensitivity and quality of SThM-based sub-wavelength optical imaging.

Elastic Gallium Phosphide Nanowire Optical Waveguides-Versatile Subwavelength Platform for Integrated Photonics.

Emerging technologies for integrated optical circuits demand novel approaches and materials. This includes a search for nanoscale waveguides that should satisfy criteria of high optical density, small cross-section, technological feasibility and structural perfection. All these criteria are met with self-assembled gallium phosphide (GaP) epitaxial nanowires. In this work, the effects of the nanowire geometry on their waveguiding properties are studied both experimentally and numerically. Cut-off wavelength dependence on the nanowire diameter is analyzed to demonstrate the pathways for fabrication of low-loss and subwavelength cross-section waveguides for visible and near-infrared (IR) ranges. Probing the waveguides with a supercontinuum laser unveils the filtering properties of the nanowires due to their resonant action. The nanowires exhibit perfect elasticity allowing fabrication of curved waveguides. It is demonstrated that for the nanowire diameters exceeding the cut-off value, the bending does not sufficiently reduce the field confinement promoting applicability of the approach for the development of nanoscale waveguides with a preassigned geometry. Optical X-coupler made of two GaP nanowires allowing for spectral separation of the signal is fabricated. The results of this work open new ways for the utilization of GaP nanowires as elements of advanced photonic logic circuits and nanoscale interferometers.

Maximum gain enhancement in wireless power transfer using anisotropic metamaterials

We present an analysis for metamaterial (MM) enhanced wireless power transfer (WPT) that includes new results revealing the impact of magnetostatic surface waves and their degradation of WPT efficiency. Our analysis shows that the commonly used fixed loss model used by previous works leads to the incorrect conclusion regarding the highest efficeincy MM configuration. Specifically, we show that the “perfect lens” configuration provides lower WPT efficiency enhancement in comparison to many other MM configurations and operating conditions. To understand why, we introduce a model for quantifying loss in MM-enhanced WPT and introduce a new figure of merit on efficiency enhancement, G_{rho }. Using both simulation and experimental prototypes, we show that while the “perfect-lens” MM achieves a field enhancement of four times the other configurations considered, its internal loss due to magnetostatic waves significantly reduces its efficiency-enhancement. Surprisingly, all the MM configurations analyzed other than the “perfect-lens” achieved higher efficiency enhancement in simulation and in experiment than the perfect lens.

Metamaterial lenses for monostatic and bistatic 77 GHz radar systems

AbstractMetamaterial lenses are appealing thin alternatives to conventional dielectric lenses. In this contribution we describe their design and application in two exemplary 77 GHz automotive radar sensors operating in monostatic and bistatic modes. The frontends are built around commercially available MMICs and small feed antennas are used in coordination with the six-layer printed circuit board-based realizations of metamaterial lenses. The design follows the principles of dielectric lenses, while the required local phase shift is obtained from a set of metasurface bandpasses. Their design uses a novel interpolation procedure to extract layout parameters for a given phase shift. Despite the small structural sizes due to the high frequency, the fabricated frequency-selective surfaces show very good performance in the required frequency range. Verification measurements were conducted on single bandpasses as well as on metamaterial lenses mounted on the radar frontend. The results agree very well with the simulation and confirm the applicability of thin lenses operating at mm-wave frequencies for automotive radar applications.

Broadband subwavelength imaging of flexural elastic waves in flat phononic crystal lenses

Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range.

Generation of Photonic Nanojet Using Gold Film Dielectric Microdisk Structure.

Due to their narrow beam waist size, high intensity, and long propagation distance, photonic nanojets (PNJs) can be used in various fields such as nanoparticle sensing, optical subwavelength detection, and optical data storage. In this paper, we report a strategy to realize an SPP-PNJ by exciting a surface plasmon polariton (SPP) on a gold-film dielectric microdisk. In detail, an SPP is excited by the grating-coupling method, then it irradiates the dielectric microdisk to form an SPP-PNJ. The characteristics of the SPP-PNJ, including maximum intensity, full width at half maximum (FWHM), and propagation distance, are studied by using finite difference time domain (FDTD) numerical solutions. The results demonstrate that the proposed structure can produce a high-quality SPP-PNJ, the maximum quality factor of which is 62.20, and the propagation distance of the SPP-PNJ is 3.08 λ. Furthermore, the properties of the SPP-PNJ can be modified flexibly by changing the thickness and refractive index of the dielectric microdisk.

A Novel L-Shaped Metalens for Ultra-Wide Band (UWB) Antenna Gain Improvement

In this work, a novel metamaterial lens (metalens) is designed and optimized to improve the radiation performance of an antipodal Vivaldi antenna for wideband applications. The metalens is integrated into the antenna substrate, and placed close to the tapered slot in the end-fire direction, allowing the preservation of the lightweight and compactness of the antenna. The prototype has been fabricated and characterized, demonstrating good agreement with the simulations. The insertion of the metalens allows, with respect to the pristine Vivaldi, a measured maximum gain of Gmax=14.2 dB, increased by about ΔGmax=4.8 dB; an operating bandwidth of f=3÷14.7 GHz, increased by Δf=1.2 GHz; and a radiation pattern with a maximum reduction in half-power beamwidth of ΔHPBWmax=31.3°, more symmetrical in the E and H planes.

Acoustic beam splitter based on acoustic metamaterial Luneburg lens

This paper proposes an acoustic beam splitter based on acoustic Luneburg lens. A two-dimensional acoustic Luneburg lens with gradient refractive index is achieved to provide a practical realization for the acoustic beam splitter. The acoustic wave propagation through the beam splitter was explored theoretically, numerically and experimentally. Results show that three output channels can be achieved using a point source on the edge of the beam splitter in a broadband frequency range. Furthermore, the output channel directions can be tuned via changing the design parameter.

Design studies of a far-field plasmonic superlens with an enlarged field of view

Based on our previous studies, we numerically demonstrate imaging capability of a cascaded plasmonic superlens with an enlarged field of view for directly imaging subwavelength objects with magnification in the far field at a visible wavelength. The superlens consists of two plasmonic slabs. One is a plasmonic cavity lens for near-field coupling, and the other one is a planar plasmonic lens for phase retardation and thus image magnification is enabled. First, we investigate the superlens with an inverted plasmonic cavity structure (with respect to our previous design), which simplifies the nano-fabrication process. Then we design the planar plasmonic lens with an increased aperture size. Simulation results show that the superlens with the inverted structure can still achieve subwavelength imaging. Furthermore, a lateral resolution of 180 nm at the wavelength of 640 nm is manifested by the superlens with a larger field of view. When the aperture size of the planar plasmonic lens is doubled from 2.1 μm to 4.2 μm, the field of view of the superlens is increased by a factor of 1.4. The results provide a profound physical insight into the design of a cascaded plasmonic superlens system and pave the way for further experimental investigations.

A tunable sub-wavelength acoustic imaging planar metalens

Acoustic metamaterials can be used to fabricate metalenses that can achieve sub-wavelength imaging resolution, but the reported devices are often passive and cannot adapt to changes in working frequency. To this end, we propose a dynamic tunable acoustic imaging planar metalens by introducing a telescopic tube between two sets of perforated Fabry–Pérot resonant metamaterial lenses to adjust the combined length of the imaging hole. Since the working frequency of the Fabry–Pérot resonant structure is directly related to the thickness of the metalens, continuous adjustment of the working frequency can be achieved by continuously changing the length of the imaging hole. This Fabry–Pérot resonant perforated structure belongs to an acoustic metamaterial unit, which can be resonantly coupled with evanescent waves to enhance the evanescent waves carrying detailed information of objects, thereby realizing high-resolution imaging. Relying on the resonance tunneling effect of the imaging hole, high acoustic transmittance can be obtained at the working frequency, ensuring excellent imaging quality. Through numerical calculation and experimental measurement results, the imaging performance and tunability of the tunable sub-wavelength acoustic imaging metalens are demonstrated. The proposed tunable acoustic planar metalens overcomes the shortcomings of the fixed working frequency of passive metalenses and has potential applications in the fields of ultrasound imaging and medical diagnosis.

Photochemical Imaging of Near‐Field and Dissymmetry Factor in Chiral Nanostructures

AbstractChiral nanostructures interact differently with right‐ or left‐handed circularly polarized light. Due to the lack of experimental approaches able to unveil the optical near‐field around chiral nanostructures, their optical characterization generally relies either on far‐field techniques or on numerical simulations. Here, a subwavelength imaging approach based on the interaction between the enhanced optical near‐field around chiral nanoparticles and an azobenzene molecular probe is reported. Under the action of light, the azobenzene molecules undergo isomerization cycles resulting in a measurable displacement of matter. The resulting imprint of the optical near‐field can then be imaged using atomic force microscopy. Using two shifted gold nanorods separated by a nanogap as a canonical chiral structure, a dissymmetry in the near‐field of the chiral structures under left and right circular polarizations is experimentally evidenced. Notably, it is observed that the gap mode is excited only when the handedness of the exciting light matches the handedness of the nanostructure, evidencing a chiral hot spot inside the gap. Moreover, it is shown that it is possible to experimentally map the near‐field dissymmetry factor, a quantity related to the local circular dichroism of the near‐field intensity.

Low-cost laser cutting fabricated all-metallic metamaterial near-field focusing lens

In this work, a low-cost all-metal metamaterial near-field lens based on laser cutting technology is proposed. A novel spiral-slot structure is proposed to achieve miniaturized unit cells with an adjustable 360-degree phase shift at a length smaller than 0.2 times the operating wavelength. Since the unit is entirely constructed of stainless steel, it is resistant to high temperatures and high pressures compared to existing results. Moreover, a four-layer structure is used to increase the transmission coefficient. The final L-band near-field lens is constructed of 20 × 20 units. Simulation and measured results show that the half-power beamwidth of the focus is less than 211 mm from 1.52 GHz to 1.68 GHz at the focal spot observation plane of 500 mm from the lens. Since numerically controlled machine tools and three-dimensional printing are prohibitively expensive for machining large metal components, a low-cost all-metallic lens was manufactured using laser cutting technology. The measured results are in agreement with the simulation results.

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.