Deep-subwavelength Features Research Articles

Wavelength-tuned transformation between photonic skyrmion and meron spin textures

Topological spin textures, among which skyrmions and merons are typical examples, have with their swirling vectorial structures triggered enormous interest in physical systems including elementary particles and magnetic materials. Manipulating their symmetry and topology is important for understanding the mechanisms that underlie their topological phase transformation as well as offering tunable degrees of freedom to encode information, which has already been demonstrated in magnetic materials. Recently, the photonic counterparts of skyrmions and merons were constructed in a 2D wave system with deep-subwavelength features promising for optical sensing, imaging, and information decoding. However, their experimental realization relied on stringent excitation conditions that only support a single spin texture type on a specific structure. Here, we demonstrate for the first time the transformation between photonic skyrmion and meron spin lattices on the same metallic meta-surface having a well-designed structural period. We show experimentally the wavelength-tuned symmetry transformation of the photonic spin lattices, which are also found to be robust against disorder in the structure to a certain degree. This work provides new insights into controlling the electromagnetic field symmetry and topology, as well as in developing applications in spin optics and topological photonics.

Spin-Manipulated Photonic Skyrmion-Pair for Pico-Metric Displacement Sensing.

Photonic spin skyrmions with deep-subwavelength features have aroused considerable interest in recent years. However, the manipulation of spin structure in the skyrmions in a desired manner is still a challenge, while this is crucial for developing the skyrmion-based applications. Here, an approach of optical spin manipulation by utilizing the spin-momentum equation is proposed to investigate the spin texture in a photonic skyrmion-pair. With the benefit of the proposed approach, a unique spin texture with spin angular momentum varying linearly along the line connecting the two skyrmion centers is theoretically designed and experimentally verified. The optimized spin texture is then applied in a displacement-sensing system, which is capable of attaining pico-metric sensitivity. Compared with the conventional polarization and phase schemes, the spin-based manipulation mechanism provides a new pathway for optical modulation, which is of great value in nanophotonics from both fundamental and application.

Creation and preservation of superoscillation in a dielectric optical waveguide.

Superoscillation refers to a phenomenon where a band-limited wave locally oscillates faster than its highest Fourier component. Current research on optical superoscillations predominantly lies on the basis of free-space waves. As the optical waveguides play a key role in energy and information transportation, guided waves with precisely controlled deep-subwavelength features offers unprecedented flexibility for applications. In this Letter, we numerically show that, by superimposing eigenmodes of a multimode SiO2 waveguide and forcing the resultant field to pass through a set of predetermined points, superoscillatory fields in various shapes can be formed in preset cross-sectional planes. Furthermore, by padding prescribed intensities in multiple cross sections, we successfully create a persistent superoscillatory saddle.

Light Trapping in Silicon Arrays of Deep Subwavelength Features for Absorption of the Solar Radiation

Light Trapping in Silicon Arrays of Deep Subwavelength Features for Absorption of the Solar Radiation

Anomalous transmission and Anderson localization for alternating propagated and evanescent waves at the deep-subwavelength scale.

Deep-subwavelength features have a minimal impact on wave transport in all dielectric systems; thus the homogenization approach was commonly adopted. Recently, the breakdown of effective medium theory (EMT) for the incident wave near the total reflection (TR) angle was demonstrated in a deep-subwavelength dielectric multilayer. Additionally, anomalous transmission was reported at angles exceeding the TR angle when introducing disorder and was attributed to Anderson localization. Here we firstly demonstrated that the alleged anomalous transmission also occurs in the disorder-free case, illustrating that attributing anomalous transmission to Anderson localization deserves a more in-depth study. To clarify the underlying physics of this asserted anomalous transmission, Anderson localization and broken EMT, the incident angle dependent reflectivity and modes for ordered and disordered deep-subwavelength multilayers were investigated systematically. Actually, the EMT is still convincing and the anomalous transmission is reasonable after a simple correction. However, the anomalous transmission is more accessible and the permittivity correction is more imperative in the disordered system due to the Anderson localization. These findings can be expanded to other wave systems such as acoustic waves and matter waves, providing insight into EMT and deepening our understanding of the intriguing transport phenomena in deep subwavelength systems.

Broadband and Omnidirectional Antireflection Surfaces Based on Deep Subwavelength Features for Harvesting of the Solar Energy

Broadband and omnidirectional antireflective surfaces are important to various solar energy technologies. The utilization of deep subwavelength features is reported for decrement in the broadband and omnidirectional reflection of the solar spectrum. The study commences with nanopillar (NP) arrays that are well known for their antireflection properties. Deep subwavelength features to the nanopillar arrays are introduced, which yield further decrease in reflection: quasinanolens (qNL), deep subwavelength sidewall scalloping (DSSS), and decreasing the nanopillar bottom diameter to reshape it into a light funnel (LF). Accordingly, surface silicon arrays are realized in a top−down fabrication process: nanopillar arrays, nanopillar arrays incorporated with DSSS, nanopillar arrays incorporated with qNL, nanopillar arrays incorporated with qNL and DSSS, and nanopillar arrays that are transformed into LF arrays and are complemented with qNL and DSSS. It is experimentally shown that utilization of deep subwavelength features produces a broadband reflection decrement with an almost 80% decrement for normal incidence and 60% decrement for an angle of incidence of 80°, for unpolarized illumination. The introduction of deep subwavelength features concludes a 20% decrement in the broadband diffused reflection compared with undecorated NP arrays. The effect of deep subwavelength features is also explored numerically, and the possible underlying light‐trapping mechanisms are discussed.

On-Chip Metasurface for Optical Directional Rectification

On-chip two-dimensional (2D) metasurfaces create a new degree of freedom of light manipulation within a 2D surface, which is an emerging research area with fascinating possibilities for realization of very dense integration and miniaturization in photonic systems. As a longstanding challenge in optics steering, light rectification, which means to manipulate beams with multiple directional wavevectors to exit to the same outgoing direction, could hardly be made possible based on conventional optics unless with zero-index medium. However, on-chip metasurface could potentially come to rescue with deep-subwavelength features and its engineered phase signature depending on wavevectors. Here, we design and demonstrate a surface-wave rectification on-chip metasurface (SROM) for visible light, which realizes light rectification for oblique incidences within +/-20°. The SROM is formed by cascading two plasmonic on-chip nanowires of semi-elliptical and trapezoidal structures as its unit cell. Through the geometrical change induced by the semi-elliptical and trapezoidal structures and the plasmonic interaction between the two cascaded nanowires, the incident surface waves with multiple wavevector angles would exit and propagate normally along 0° around 500 nm wavelength, thus mimicking a zero-index-like phenomenon. The equal-frequency contour (EFC) of the designed SROM is also numerically retrieved to exhibit flat angular dispersion, which verifies the proof-of-concept light reification phenomenon. The on-chip miniature SROM is of merely ~ 3 μm length, and the light rectification performance remains for the visible-frequency band of 500 - 560 nm. Such new on-chip architecture only involves the mature and conventional nanofabrication techniques to manufacture without any alignment for multilayer metasurfaces. We envision that the created beam-steering freedom of light rectification opens a viable and compatible route towards new invention of photonic integrated devices.

Bloch-type photonic skyrmions in optical chiral multilayers

Magnetic skyrmions are topological quasiparticles in magnetization. Recently, as one of their photonic counterparts, Néel-type photonic skyrmions were discovered in evanescent electromagnetic waves. The deep-subwavelength features of the photonic skyrmions suggest their potential in optical imaging quantum technologies and data storage. Here, by exploiting the photonic quantum spin Hall effect of a plasmonic vortex in a trilayered structure, we predict the existence of photonic twisted-Néel- and Bloch-type skyrmions in chiral materials. Their chirality-dependent features can be considered as additional degrees of freedom for future chiral sensing, information processing, and storage technologies. In particular, our findings enrich the formations of photonic skyrmions and reveal a remarkable resemblance of the feature of chiral materials in two seemingly distant fields: photonic skyrmions and magnetic skyrmions.2 MoreReceived 16 December 2020Accepted 9 April 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.023109Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasChiralityLight-matter interactionNanophotonicsNear-field opticsPlasmonicsSkyrmionsTopological effects in photonic systemsAtomic, Molecular & Optical

Nonradiative subdiffraction near-field patterns using metagratings

We present a synthesis scheme to mold periodic nonradiative field patterns in transmission using the recent concept of metagratings (MGs). To this end, we utilize our previously developed analytical model to analyze the interaction of an incoming plane wave with these sparse periodic arrangements of polarizable particles (meta-atoms). As the model reliably predicts coupling to all scattered Floquet–Bloch modes, both propagating and evanescent, desired reactive near-field profiles with deep subwavelength features can be generated. This approach forms an appealing alternative to previously proposed near-field plates based on metasurfaces, where abstract homogenization introduces uncertainties regarding utilization of highly evanescent spectrum, and meta-atom realization incurs full-wave optimization. In contrast, the outlined MG-based methodology, verified via full-wave simulations, directly yields fabrication-ready printed-circuit-board configurations, enabling versatile control of reactive near fields with no interfering radiative components, with potential uses in sensing, selective microwave heating, and wireless power transfer.

Resolving deep sub-wavelength scattering of nanoscale sidewalls using parametric microscopy

The quantitative optical measurement of deep sub-wavelength features with sub-nanometer sensitivity addresses the measurement challenge in the semiconductor fabrication process. Optical scatterings from the sidewalls of patterned devices reveal abundant structural and material information. We demonstrated a parametric indirect microscopic imaging (PIMI) technique that enables recovery of the profile of wavelength-scale objects with deep sub-wavelength resolution, based on measuring and filtering the variations of far-field scattering intensities when the illumination was modulated. The finite-difference time-domain (FDTD) numerical simulation was performed, and the experimental results were compared with atomic force microscopic (AFM) images to verify the resolution improvement achieved with PIMI. This work may provide a new approach to exploring the detailed structure and material properties of sidewalls and edges in semiconductor-patterned devices with enhanced contrast and resolution, compared with using the conventional optical microscopy, while retaining its advantage of a wide field of view and relatively low cost.

Photonic hyperinterfaces for light manipulations

Optical metamaterials are building blocks for the control of light behaviors and designs of photonic devices, where the inner interfaces in deep-subwavelength features are expected to have little impact on light transport, based on the concept of homogenization. Here we theoretically and experimentally study a new type of photonic interface (namely a hyperinterface) inside an optical metamaterial made of a zigzag alternating multilayer structure [namely structured metamaterials (SMMs)] in the deep-subwavelength regime. It is found that the subwavelength hyperinterfaces play a great role in the optical properties of such SMMs, and the electromagnetic properties of the hyperinterfaces can be effectively manipulated in a feasible way. In particular, the absorption of the SMMs strongly depends not only on the intrinsic absorption of the SMMs’ unit cells, but also on the structural absorption that is induced by the hyperinterfaces inside the SMMs and their period p , even for the long wavelength limitation. These outcomes are attributed to the dispersion relations of the hyperinterfaces, that is, the interplay of the angle formed by the asymptotes of the iso-frequency contour (hyperbola) of the SMMs’ unit cells and the geometric rotating angle of zigzag structures. Such interplay leads to an effect of clipping and the recombination of energy flow distributions at the hyperinterfaces. Our findings may pave the way to the manipulation of a light field to enhance the conversion efficiency of optoelectronic devices, e.g., solar cells and photodetectors.

Super-resolution acoustic image montage via a biaxial metamaterial lens

Ever since the Victorian era, montage, the process of pictorial composition made by juxtaposing or superimposing photographs, has been a very popular post-editing imaging technique. Despite showing a strong power in demonstrating complex wave field effects, this technique has neither been fully explored in acoustic imaging nor been realized in real-time systems with the capability beyond diffraction limits. On the other hand, the recent prospect of metamaterials has shown their great potentials in super-resolution acoustic imaging. However, the miracle jigsaw of more advanced functional modulation of acoustic wave fields at deep subwavelength scale still remains elusive. Here we report the experimental implementation of super-resolution acoustic image montage through a judiciously designed biaxial metamaterial lens. Based on the non-diffraction birefringence in the biaxial metamaterials, we realized various montage functionalities such as duplication, composition, and decomposition of sound images with distinctive deep subwavelength features. Our work represents an important step in developing versatile functional acoustic metamaterial devices for imaging purposes, as it provides on-demand editing of sound field patterns beyond diffraction limits.

An efficient and deterministic photon management using deep subwavelength features

Light trapping and the broadband absorption of the solar radiation are significant to a plethora of absorption-based photonic devices. Specifically, efficient broadband absorption was recently demonstrated with arrays of subwavelength structures. The current study examines both numerically and experimentally light trapping driven by deep sidewall subwavelength structures (DSSS) in silicon nanopillar (NP) arrays (DSSS arrays). Particularly, the focus is on DSSS geometries that are an inherent outcome of the top-down dry etch approach used in arrays of high aspect ratio NPs due to the periodical operationality of the Bosch dry etch method. ~10% enhancement in the broadband absorption of DSSS arrays compared with NP arrays is demonstrated numerically, as well as the generation of near-IR absorptivity peaks of ~25% for DSSS arrays. Importantly, it is shown that the introduction of DSSS systematically blue-shifts the absorptivity peaks of the NP arrays and in this manner a deterministic light trapping is possible. Finally, decrements of ~40% in direct reflectivity and ~7% in diffused reflectivity in DSSS arrays realized on silicon wafers is demonstrated experimentally.

Full-coloration based on metallic nanostructures through phase discontinuity in Fabry-Perot resonators.

We demonstrate a flexible full-color plate using Fabry-Perot (FP) resonators with two different types of silver nanostructures, a uniform nanofilm and a layer of nanoislands, for transmissive color elements. Two different nanostructures with deep-subwavelength features are selectively generated according to the layer thickness during vacuum deposition with no patterning process. In the nanofilm case, the primary optical mode accountable for generating the color shifts to blue from the original FP resonance while in the nanoislands case, it shifts to red so that a wide spectrum in the visible range is available through the phase discontinuity in the FP resonators. The peaks in the FP resonance shifted toward the opposite directions are attributed to the opposite signs of the phase retardations by a nanofilm and nanoislands. This approach paves a new way of constructing full-color elements for a variety of display devices and image storage systems.

Efficient mode converter to deep-subwavelength region with photonic-crystal waveguide platform for terahertz applications.

Metallic deep-subwavelength features can aid in integration of microscopic components or strong light-matter interaction with a low-loss dielectric waveguide platform. A mode converter or coupler is required to integrate the devices. However, there is a vast difference in the physical scale and modal distribution between the deep-subwavelength structures and the dielectric waveguide platform. Here, we employ a tapered-slot mode converter to facilitate the electromagnetic wave transition from a gap width smaller than 1/100 of a wavelength (λ) to a larger-scale mode that is amenable to a terahertz (THz) silicon photonic-crystal waveguide. The mode converter is metallic, and fabricated on top of indium phosphide substrate, leading to incongruity with the modal field distribution of the silicon photonic-crystal waveguide. To mitigate this, a sandwiched structure is developed to match the symmetry of the mode of photonic-crystal waveguide, thereby facilitating efficient transfer of energy. For a proof of concept, we integrate a resonant tunneling diode (< 2 µm) as a THz detector in a photonic-crystal waveguide platform in the 0.3-THz band (λ ∼ 1 mm). The coupling efficiency is close to unity (∼90%) with broadband operation (∼50 GHz) in experiments. Thereafter, we employ the developed integrated device as a receiver in a THz communication experiment. In this manner, we successfully achieve real-time error-free data transmission at 32 Gbit/s, and demonstrate wireless transmission of uncompressed 4K high-definition video.

Appraising the Extensibility of Optics-Based Metrology for Emerging Materials

To advance computational capabilities beyond conventional scaling limitations, novel device architectures enabled by emerging materials may be required. Optics-based methodologies, central to modern-day process control, will be pursued by the nanoelectronics industry to interrogate these devices as optics are inexpensive, nondestructive, and fast. As geometrical and material complexity define new metrology requirements, these should be considered relative to the broader challenge of perpetuating optical methods for deep-subwavelength features. Using examples from our group and from others, the tailoring of the illumination conditions, sample, collection path, and data analysis are emphasized for model-based quantitative measurements. The successful fitting of structures comprised from these emerging materials will require the accurate determination of material optical constants, which may be both thickness dependent and anisotropic. Atomistic models such as tight-binding calculations or density-functional theory are potential approaches for understanding the dielectric function of these materials.

Deep-subwavelength features of photonic skyrmions in a confined electromagnetic field with orbital angular momentum

In magnetic materials, skyrmions are nanoscale regions where the orientation of electron spin changes in a vortex-type manner. Here we show that spin-orbit coupling in a focused vector beam results in a skyrmion-like photonic spin distribution of the excited waveguided fields. While diffraction limits the spatial size of intensity distributions, the direction of the field, defining photonic spin, is not subject to this limitation. We demonstrate that the skyrmion spin structure varies on the deep-subwavelength scales down to 1/60 of light wavelength, which corresponds to about 10 nanometre lengthscale. The application of photonic skyrmions may range from high-resolution imaging and precision metrology to quantum technologies and data storage where the spin structure of the field, not its intensity, can be applied to achieve deep-subwavelength optical patterns.

Broadband Antireflection Coatings Employing Multiresonant Dielectric Metasurfaces

The energy efficiency of optoelectronic components and devices is critically dependent on minimizing undesired reflections from interfaces between materials with differing optical properties. Antireflection coatings based on metamaterials with deep-subwavelength features offer superior performance over their homogeneous counterparts as they afford subtle tuning of the refractive index and gradients therein. Recent work also showed that arrays of larger-sized (250 nm diameter), high-index nanostructures placed on semiconductor surfaces reduce the reflectivity by capitalizing on optical Mie resonances. Here, we start by demonstrating that a judiciously designed, single Mie resonator can enable perfect, local antireflection at its resonance frequency. This insight opens the door to the development of entirely new, multiresonant antireflection coating (ARC) designs in which differently sized Mie resonators manage antireflection at different wavelengths. We demonstrate the value of such multiresonant ARCs for ...

Tunable and reconfigurable metasurfaces and metadevices

Metasurfaces, two-dimensional equivalents of metamaterials, are engineered surfaces consisting of deep subwavelength features that have full control of the electromagnetic waves. Metasurfaces are not only being applied to the current devices throughout the electromagnetic spectrum from microwave to optics but also inspiring many new thrilling applications such as programmable on-demand optics and photonics in future. In order to overcome the limits imposed by passive metasurfaces, extensive researches have been put on utilizing different materials and mechanisms to design active metasurfaces. In this paper, we review the recent progress in tunable and reconfigurable metasurfaces and metadevices through the different active materials deployed together with the different control mechanisms including electrical, thermal, optical, mechanical, and magnetic, and provide the perspective for their future development for applications.

Analytical description of inverse filter emulating the plasmon injection loss compensation scheme and implementation for ultrahigh-resolution hyperlens

An inverse filter full analytical description and implementation of a recently proposed plasmon injection scheme for improving the resolution of a hyperlens is presented. Different types of loss mechanisms existing in the hyperlens imaging system are identified and studied in detail. It is shown that the plasmon injection scheme and its inverse filter analog can compensate all the major loss mechanisms. As a result, an object with deep subwavelength features, otherwise unresolvable with a hyperlens alone, is fully reconstructed.