Subwavelength Artificial Structures Research Articles

Subwavelength Artificial Structures: Opening a New Era for Engineering Optics.

In the past centuries, the scale of engineering optics has evolved toward two opposite directions: one is represented by giant telescopes with apertures larger than tens of meters and the other is the rapidly developing micro/nano-optics and nanophotonics. At the nanoscale, subwavelength light-matter interaction is blended with classic and quantum effects in various functional materials such as noble metals, semiconductors, phase-change materials, and 2D materials, which provides unprecedented opportunities to upgrade the performance of classic optical devices and overcome the fundamental and engineering difficulties faced by traditional optical engineers. Here, the research motivations and recent advances in subwavelength artificial structures are summarized, with a particular emphasis on their practical applications in super-resolution and large-aperture imaging systems, as well as highly efficient and spectrally selective absorbers and emitters. The role of dispersion engineering and near-field coupling in the form of catenary optical fields is highlighted, which reveals a methodology to engineer the electromagnetic response of complex subwavelength structures. Challenges and tentative solutions are presented regarding multiscale design, optimization, fabrication, and system integration, with the hope of providing recipes to transform the theoretical and technological breakthroughs on subwavelength hierarchical structures to the next generation of engineering optics, namely Engineering Optics 2.0.

High‐Efficiency and Wide‐Angle Beam Steering Based on Catenary Optical Fields in Ultrathin Metalens

AbstractRecent advances in artificial subwavelength structures promise the realization of ultrathin, lightweight, and flat metalens, providing a potential candidate for traditional bulky and curved lens. Nevertheless, most metalenses are generally suffering from serious off‐axis aberration, resulting in a limited field‐of‐view (FOV). Here, a methodology to extend the FOV of metalens is presented by exploring the local catenary optical fields and symmetry transformation from rotational symmetry to transversal symmetry. As a proof of the concept, a high efficiency (>80% even when the incidence angle is titled by 60°) and ultrathin (≈0.127 λ) metalens consisting of bilayer geometric metasurface is designed, fabricated, and characterized. The wide FOV of metalens benefiting from local catenary fields and symmetry transformation is numerically demonstrated by full‐wave simulation and ray tracing method. Moreover, the wide‐angle beam steering ability beyond ± 60° is experimentally demonstrated, which exhibits obvious advantages compared with previous beam steering transmitarrays. The novelty of the proposed methodology lies at the fact that both transversal and longitudinal local catenary fields are excited in a single component: the transversal catenary field is utilized for high efficiency spin–orbit interaction and phase modulation, while the longitudinal catenary field and symmetry transformation ensure wide‐angle operation.

Theory of microscopic meta-surface waves based on catenary optical fields and dispersion.

Surface waves bounded by subwavelength-structured surfaces have many exotic electromagnetic properties different from those supported by smooth surfaces. However, there is a long-standing misconception, claiming that these waves must propagate along the macroscopic interface. In this paper, we describe in detail the microscopic meta-surface wave (M-wave) in artificial subwavelength structures. It is shown that the waves penetrating macroscopic surfaces share the essence of most surface waves (i.e., they spread along the microscopic interfaces, formed by adjacent constitutive materials). Equivalent circuit theory and transfer matrix method have been adopted to quantitatively describe these M-waves with high accuracy in the form of catenary optical fields and dispersion. Based on these analyses, novel omnidirectional band-stop filters and wide-angle beam deflectors are designed with operational angles up to 88°. We believe these results may provide many new perspectives for both the understanding and design of functional subwavelength structures.

Negative refractive index metasurface in the 20-THz band

Development of continuous wave (CW) sources has recently advanced in various frequencies of the terahertz waveband. However, there are few materials suitable for the terahertz waveband, and attractive materials are eagerly awaited. Metamaterials with artificial sub-wavelength structures would enable designs with an arbitrary refractive index including negative values providing simultaneous control of dielectric and magnetic properties. Here, measurements demonstrate that a simple metasurface, a two-dimensional metamaterial, consisting of asymmetrical metal patches on the front and back of a dielectric substrate has a negative refractive index of −3.03 + j0.29, transmission of 56.4%, and reflection of 3.51% at 1.96 THz. The metasurface would offer attractive optical components such as super lenses, ultrathin collimators, and cloaking.

Locally Enhanced Image Quality with Tunable Hybrid Metasurfaces

Metasurfaces represent a new paradigm in artificial subwavelength structures due to their potential to overcome many challenges typically associated with bulk metamaterials. The ability to make very thin structures and change their properties dynamically makes metasurfaces an exceptional meta-optics platform for engineering advanced electromagnetic and photonic metadevices. Here, we suggest and demonstrate experimentally a tunable metasurface capable of enhancing significantly the local image quality in magnetic resonance imaging. We present a design of the hybrid metasurface based on electromagnetically coupled dielectric and metallic elements. We demonstrate how to tailor the spectral characteristics of the metasurface eigenmodes by changing dynamically the effective permittivity of the structure. By maximizing a coupling between metasurface eigenmodes and transmitted and received fields in the magnetic resonance imaging (MRI) system, we enhance the device sensitivity that results in a substantial improvement of the image quality.

Reconfigurable Metasurface for Multifunctional Control of Electromagnetic Waves

AbstractMetasurfaces provide a novel strategy to manipulate electromagnetic (EM) waves by controlling the local phase of subwavelength artificial structures within the wavelength scale. So far, many exciting devices have been developed and most of them are based on passive metasurface, which can only perform a specific functionality. It is still very challenging to simultaneously achieve multiple EM functionalities and real‐time reconfigurability in one design. This study reports a reconfigurable metasurface for multifunctional control of EM waves. By controlling tunable elements, the proposed metasurface can dynamically change its local phase distribution to generate predetermined EM responses. Here, the metasurface can generate beam‐splitting performance that can be used to reduce backward scattering waves, and its scattering reduction frequency is tunable. In addition, such metasurface can also achieve dynamical beam deflection and polarization transformation through reconfigurable design of the phase distribution. The above EM functionalities have been experimentally demonstrated at microwave frequencies, which can be switched in real time by a computer‐controlled voltage source. This study paves the way toward the realization of multifunctional metadevices with real‐time and programmable control, showing great potential in applications of smart materials and devices for EM‐wave manipulation.

Liquid-metal-based metasurface for terahertz absorption material: Frequency-agile and wide-angle

Terahertz metasurface absorption materials, which absorb terahertz wave through subwavelength artificial structures, play a key role in terahertz wave shielding and stealth technology, etc. However, most of the metasurface absorption materials in terahertz suffer from limited tuning range and narrow incident angle characteristics. Here, we demonstrate a liquid-metal-based metasurface through microfluidic technology, which functions as a terahertz absorption material with broadband tunability and wide-angle features. The proposed terahertz metasurface absorption material exhibits an experimental tuning range from 0.246 THz to 0.415 THz (the tuning range of central frequency reaches 51.1%), and the tuning range maintains at high level with wide-angle response up to 60°.

Intrinsic left-handed electromagnetic properties in anisotropic superconductors

Left-handed materials usually are realized in artificial subwavelength structures. Here, we show that some anisotropic superconductors such as Bi2Sr2CaCu2O8+δ, YBa2CuxOy, and La2−xSrxCuO4, are intrinsic left-handed materials. The condition is that the plasma frequency in the c axis, ωc, and in the ab plane, ωab, and the operating angular frequency, ω, satisfy ωc<ω<ωab. In addition, ω should be smaller than the superconducting energy gap to sustain superconductivity. We study the reflection and transmission of electromagnetic waves and reveal negative refraction and the backward wave with the phase velocity opposite to the direction of energy flux propagation. We also discuss possible approaches for improvement, making these properties feasible for experimental validation. Being intrinsic left-hand materials, the anisotropic superconductors are promising for applications in functional electromagnetic devices in the terahertz frequency band.

A Reconfigurable Active Huygens' Metalens.

Metasurfaces enable a new paradigm to control electromagnetic waves by manipulating subwavelength artificial structures within just a fraction of wavelength. Despite the rapid growth, simultaneously achieving low-dimensionality, high transmission efficiency, real-time continuous reconfigurability, and a wide variety of reprogrammable functions is still very challenging, forcing researchers to realize just one or few of the aforementioned features in one design. This study reports a subwavelength reconfigurable Huygens' metasurface realized by loading it with controllable active elements. The proposed design provides a unified solution to the aforementioned challenges of real-time local reconfigurability of efficient Huygens' metasurfaces. As one exemplary demonstration, a reconfigurable metalens at the microwave frequencies is experimentally realized, which, to the best of the knowledge, demonstrates for the first time that multiple and complex focal spots can be controlled simultaneously at distinct spatial positions and reprogrammable in any desired fashion, with fast response time and high efficiency. The presented active Huygens' metalens may offer unprecedented potentials for real-time, fast, and sophisticated electromagnetic wave manipulation such as dynamic holography, focusing, beam shaping/steering, imaging, and active emission control.

Subwavelength electromagnetics below the diffraction limit

As a fundamental property of waves, diffraction plays an important role in many physical problems. However, diffraction makes waves in free space unable to be focused into an arbitrarily small space, setting a fundamental limit (the so-called diffraction limit) to applications such as imaging, lithography, optical recording and waveguiding, etc. Although the diffraction effect can be suppressed by increasing the refractive index of the surrounding medium in which the electromagnetic and optical waves propagate, such a technology is restricted by the fact that natural medium has a limited refractive index. In the past decades, surface plasmon polaritons (SPPs) have received special attention, owing to its ability to break through the diffraction limit by shrinking the effective wavelength in the form of collective excitation of free electrons. By combining the short wavelength property of SPPs and subwavelength structure in the two-dimensional space, many exotic optical effects, such as extraordinary light transmission and optical spin Hall effect have been discovered and utilized to realize functionalities that control the electromagnetic characteristics (amplitudes, phases, and polarizations etc.) on demand. Based on SPPs and artificial subwavelength structures, a new discipline called subwavelength electromagnetics emerged in recent years, thus opening a door for the next-generation integrated and miniaturized electromagnetic and optical devices and systems. In this paper, we review the theories and methods used to break through the diffraction limit by briefly introducing the history from the viewpoint of electromagnetic optics. It is shown that by constructing plasmonic metamaterials and metasurfaces on a subwavelength scale, one can realize the localized phase modulation and broadband dispersion engineering, which could surpass many limits of traditional theory and lay the basis of high-performance electromagnetic and optical functional devices. For instance, by constructing gradient phase on the metasurfaces, the traditional laws of reflection and refraction can be rewritten, while the electromagnetic and geometric shapes could be decoupled, both of which are essential for realizing the planar and conformal lenses and other functional devices. At the end of this paper, we discuss the future development trends of subwavelength electromagnetics. Based on the fact that different concepts, such as plasmonics, metamaterials and photonic crystals, are closely related to each other on a subwavelength scale, we think, the future advancements and even revolutions in subwavelength electromagnetics may rise from the in-depth intersection of physical, chemical and even biological areas. Additionally, we envision that the material genome initiative can be borrowed to promote the information exchange between different engineering and scientific teams and to enable the fast designing and implementing of subwavelength structured materials.

Gate-Tunable Conducting Oxide Metasurfaces.

Metasurfaces composed of planar arrays of subwavelength artificial structures show promise for extraordinary light manipulation. They have yielded novel ultrathin optical components such as flat lenses, wave plates, holographic surfaces, and orbital angular momentum manipulation and detection over a broad range of the electromagnetic spectrum. However, the optical properties of metasurfaces developed to date do not allow for versatile tunability of reflected or transmitted wave amplitude and phase after their fabrication, thus limiting their use in a wide range of applications. Here, we experimentally demonstrate a gate-tunable metasurface that enables dynamic electrical control of the phase and amplitude of the plane wave reflected from the metasurface. Tunability arises from field-effect modulation of the complex refractive index of conducting oxide layers incorporated into metasurface antenna elements which are configured in reflectarray geometry. We measure a phase shift of 180° and ∼30% change in the reflectance by applying 2.5 V gate bias. Additionally, we demonstrate modulation at frequencies exceeding 10 MHz and electrical switching of ±1 order diffracted beams by electrical control over subgroups of metasurface elements, a basic requirement for electrically tunable beam-steering phased array metasurfaces. In principle, electrically gated phase and amplitude control allows for electrical addressability of individual metasurface elements and opens the path to applications in ultrathin optical components for imaging and sensing technologies, such as reconfigurable beam steering devices, dynamic holograms, tunable ultrathin lenses, nanoprojectors, and nanoscale spatial light modulators.

Electromagnetic metamaterial simulations using a GPU-accelerated FDTD method

Metamaterials composed of artificial subwavelength structures exhibit extraordinary properties that cannot be found in nature. Designing artificial structures having exceptional properties plays a pivotal role in current metamaterial research. We present a new numerical simulation scheme for metamaterial research. The scheme is based on a graphic processing unit (GPU)-accelerated finite-difference time-domain (FDTD) method. The FDTD computation can be significantly accelerated when GPUs are used instead of only central processing units (CPUs). We explain how the fast FDTD simulation of large-scale metamaterials can be achieved through communication optimization in a heterogeneous CPU/GPU-based computer cluster. Our method also includes various advanced FDTD techniques: the non-uniform grid technique, the total-field/scattered-field (TFSF) technique, the auxiliary field technique for dispersive materials, the running discrete Fourier transform, and the complex structure setting. We demonstrate the power of our new FDTD simulation scheme by simulating the negative refraction of light in a coaxial waveguide metamaterial.

Implications of the causality principle for ultra chiral metamaterials.

Chiral metamaterials – artificial subwavelength structures with broken mirror symmetry – demonstrate outstanding degree of optical chirality that exhibits sophisticated spectral behavior and can eventually reach extreme values. Based on the fundamental causality principle we show how one can unambiguously relate the metamaterial circular dichroism and optical activity by the generalized Kramers-Kronig relations. Contrary to the conventional relations, the generalized ones provide a unique opportunity of extracting information on material-dependent zeroes of transmission coefficient in the upper half plane of complex frequency. We illustrate the merit of the formulated relations by applying them to the observed ultra chiral optical transmission spectra of subwavelength arrays of chiral holes in silver films. Apart from the possibility of precise verification of experimental data, the relations enable resolving complex eigenfrequencies of metamaterial intrinsic modes and resonances.

Finite element study of the Lorentz model in metamaterials

Metamaterials are materials with electromagnetic properties that originate from artificial sub-wavelength structures. Although the Lorentz metamaterial model has been used by physicists and electrical engineers, its mathematical properties have not been investigated yet. Also more robust and efficient algorithms for time-domain simulations are needed. In this paper, we initiate our work in this respect by developing some finite element schemes for solving the Lorentz model. Stability and error analysis are proved for these schemes, and 2-D numerical results justifying our theoretical results are presented.

Molding the light flow at desire

Prof. Shi-ning ZHU How to manipulate light at will, either in space or in time, is one of the central problems in the optics community. In the past decade, researchers have demonstrated the ability to control light in unprecedented ways with the aid of artificially engineered materials, among which metamaterials have attracted world-wide interest. Metamaterials are composites of subwavelength artificial structures. From effective medium theory, a metamaterial can be characterized as a homogeneous material, whose EM properties (i.e., electric permittivity and magnetic permeability) mainly rely on the internal structures rather than the chemical elements of the metamaterial. Some unusual material properties unavailable in nature, such as optical magnetism and negative refractive index, have been realized by metamaterials. With the full freedom offered by metamaterials, researchers have demonstrated negative refraction, reversed Cherenkov radiation, super lenses, and recent invisibility cloaks, which bring revolutionary impacts on modern optics. Metamaterials have become a very active research area involving many other disciplines. For instance, at the early stage of metamaterial research, negative refraction and superlensing were concurrently explored in photonic crystals. Nowadays, as metamaterials enter the optical regime, the plasmonic effect of metals can play an important role. Indeed, the investigation on coupling between metallic metamaterial elements and biosensing by metamaterials has been rapidly developing. It is expected that the interplay between the communities of photonic crystals, metamaterials, and plasmonics will continue to stimulate exciting ideas and discoveries. Frontiers of Physics in China gathers together experts in these three communities for this special issue. Ten excellent papers present recent progress and a future outlook in artificial wave-functional materials. On the subject of photonic crystals, Y. Liu et al. present ultrafast optical switching from Kerr nonlinearity; J. F. Liu and X. H. Wang demonstrate the manipulation of spontaneous emission by photonic crystal or metallic micro-/nanostructures; while recent research on surface mode optical microcavities in photonic crystals is reviewed by J. Wang and M. Qiu. T. R. Zhai et al. introduce the holographic method to fabricate photonic crystals with low refractive index materials and apply it to the design of microlasers. In terms of metamaterials, H. Liu et al. present a review on the hybridization effect due to the coupling between individual metamaterial elements, which is beyond the traditional effective medium theory. J. M. Hao et al. show how to manipulate light polarizations by microwave and optical metamaterials. Two papers focus on transformation optics. One is by Y. Lai et al., who present the remote illusion device that can transform the stereoscopic image of an object into the illusion of another different object; while in the other paper, D. Bao et al. propose a broadband nonmagnetic dielectric cloak. Sub-diffraction-limited far-field imaging in infrared is investigated by P. Chaturvedi and N. X. Fang. Finally, in plasmonics, A. Garcia-Etxarri et al. reports chemical sensing based on plasmonic nanoparticle aggregation. This special issue summarizes some of the exciting results obtained in the past few years. Without any doubt, we envision that this field will grow, and many more will emerge in the near future.