Extraordinarily transparent compact metallic metamaterials

An Alternating-Electric-Field-Driven Assembly of DNA Nanoparticles into FCC Crystals

Using an alternating electric field is a versatile way to control particle assembly. Programming DNA-AuNP assembly via an electric field remains a significant challenge despite the negative charge of DNA. In DNA-AuNP assembly, a critical percolation state is delicately constructed, where the DNA bond is loosely connected and sensitive to electric fields. In this state, an FCC crystal structure can be successfully constructed by applying a high-frequency electric field to assemble DNA-AuNPs without altering the temperature, which is favorable for temperature-sensitive systems. In addition, the regulation of electric fields can be adjusted through parameters such as the frequency and voltage, which offers more precise control than temperature regulation does. The frequency and voltage can be used to precisely tune the phase structure of DNA-AuNPs from dissolved to disordered or FCC. These findings broaden the potential of DNA-based crystal engineering, revealing new opportunities in electronic nanocomposites and devices.

Flexible sensors with zero Poisson's ratio

Flexible sensors have been developed for the perception of various stimuli. However, complex deformation, usually resulting from forces or strains from multi-axes, can be challenging to measure due to the lack of independent perception of multiaxial stimuli. Herein, flexible sensors based on the metamaterial membrane with zero Poisson's ratio (ZPR) are proposed to achieve independent detection of biaxial stimuli. By deliberately designing the geometric dimensions and arrangement parameters of elements, the Poisson's ratio of an elastomer membrane can be modulated from negative to positive, and the ZPR membrane can maintain a constant transverse dimension under longitudinal stimuli. Due to the accurate monitoring of grasping force by ZPR sensors that are insensitive to curvatures of contact surfaces, rigid robotic manipulators can be guided to safely grasp deformable objects. Meanwhile, the ZPR sensor can also precisely distinguish different states of manipulators. When ZPR sensors are attached to a thermal-actuation soft robot, they can accurately detect the moving distance and direction. This work presents a new strategy for independent biaxial stimuli perception through the design of mechanical metamaterials, and may inspire the future development of advanced flexible sensors for healthcare, human-machine interfaces and robotic tactile sensing.

Hierarchically Doped Plasmonic Nanocrystal Metamaterials

Assembling plasmonic nanocrystals in regular superlattices can produce effective optical properties not found in homogeneous materials. However, the range of these metamaterial properties is limited when a single nanocrystal composition is selected for the constituent meta-atoms. Here, we show how continuously varying doping at two length scales, the atomic and nanocrystal scales, enables tuning of both the frequency and bandwidth of the collective plasmon resonance in nanocrystal-based metasurfaces, while these features are inextricably linked in single-component superlattices. Varying the mixing ratio of indium tin oxide nanocrystals with different dopant concentrations, we use large-scale simulations to predict the emergence of a broad infrared spectral region with near-zero permittivity. Experimentally, tunable reflectance and absorption bands are observed, owing to in- and out-of-plane collective resonances. These spectral features and the predicted strong near-field enhancement establish this multiscale doping strategy as a powerful new approach to designing metamaterials for optical applications.

Fast decomposed method to devise broadband polarization-conversion metasurface

Designing a broadband, wide-angle, and high-efficient polarization converter with a simple geometry remains challenging. This work proposes a simple and computationally inexpensive method for devising broadband polarization conversion metasurfaces. We focus on a cross-shape configuration consisting of two bars of different lengths connected at the center. To design the metasurface, we decompose the system into two parts with two orthogonally polarized responses and calculate the response of each part separately. By selecting the parameters with a proper phase difference in the response between the two parts, we can determine the dimensions of the system. For designing broadband polarization conversion metasurfaces, we define a fitness function to optimize the bandwidth of the linear polarization conversion. Numerical results demonstrate that the proposed method can be used to design a metasurface that achieves a relative bandwidth of [Formula: see text] for converting linearly polarized waves into cross-polarized waves. Additionally, the average polarization conversion ratio of the designed metasurface is greater than [Formula: see text] over the frequency range of 10.9-28.5 GHz. This method significantly reduces the computational expense compared to the traditional method and can be easily extended to other complex structures and configurations.

Plasmonic Polarization Rotation in SERS Spectroscopy

Surface-enhanced Raman optical activity (SEROA) has been extensively investigated due to its ability to directly probe stereochemistry and molecular structure. However, most works have focused on the Raman optical activity (ROA) effect arising from the chirality of the molecules on isotropic surfaces. Here, we propose a strategy for achieving a similar effect: i.e., a surface-enhanced Raman polarization rotation effect arising from the coupling of optically inactive molecules with the chiral plasmonic response of metasurfaces. This effect is due to the optically active response of metallic nanostructures and their interaction with molecules, which could extend the ROA potential to inactive molecules and be used to enhance the sensibility performances of surface-enhanced Raman spectroscopy. More importantly, this technique does not suffer from the heating issue present in traditional plasmonic-enhanced ROA techniques, as it does not rely on the chirality of the molecules.

Infrared Plasmonic Metamaterials Based on Transparent Nanoparticle Films of In2O3:Sn for Solar-Thermal Shielding Applications

Three-dimensional nanoparticle (NP) assemblies show interesting optical responses that differ from naturally occurring materials, such as metals, oxides, and semiconductors. In this study, we investigate the optical response of thin films comprising Sn:In₂O₃ NPs (ITO NP films) based on the correlation between complex permittivity and infrared (IR) reflectance for solar-thermal shielding applications. IR ellipsometry measurements are conducted to clarify the presence of Lorentz resonances in plasmonic metamaterials. The Lorentz resonances are correlated to the electric field strength at interparticle gaps by varying the Sn dopant concentration, as confirmed using finite-difference time-domain (FDTD) simulations. High solar-thermal shielding performance was obtained owing to selective near-IR reflection based on strong Lorentz resonances as the ITO NP films were electrically polarizable but magnetically inactive. Thermal shielding efficiency was demonstrated via a comparison of the air temperature change in a simulated box used as a model house. Additionally, we demonstrate the significance of NP packing density on the enhancement of the near-IR reflectance. The role of interparticle spacing for high near-IR reflectance was revealed by comparing effective medium approximation analyses and FDTD simulations. This relationship was also demonstrated by the reduction of solar-thermal shielding performance when using aggregated ITO NPs. Our work confirmed that the control of complex permittivity in plasmonic metamaterials must be considered in the structural design of transparent and reflective materials for solar-thermal shielding applications.

Review of Metasurfaces and Metadevices: Advantages of Different Materials and Fabrications

Flat optics, metasurfaces, metalenses, and related materials promise novel on-demand light modulation within ultrathin layers at wavelength scale, enabling a plethora of next-generation optical devices, also known as metadevices. Metadevices designed with different materials have been proposed and demonstrated for different applications, and the mass production of metadevices is necessary for metadevices to enter the consumer electronics market. However, metadevice manufacturing processes are mainly based on electron beam lithography, which exhibits low productivity and high costs for mass production. Therefore, processes compatible with standard complementary metal–oxide–semiconductor manufacturing techniques that feature high productivity, such as i-line stepper and nanoimprint lithography, have received considerable attention. This paper provides a review of current metasurfaces and metadevices with a focus on materials and manufacturing processes. We also provide an analysis of the relationship between the aspect ratio and efficiency of different materials.

Microscopies Enabled by Photonic Metamaterials

In recent years, the biosensor research community has made rapid progress in the development of nanostructured materials capable of amplifying the interaction between light and biological matter. A common objective is to concentrate the electromagnetic energy associated with light into nanometer-scale volumes that, in many cases, can extend below the conventional Abbé diffraction limit. Dating back to the first application of surface plasmon resonance (SPR) for label-free detection of biomolecular interactions, resonant optical structures, including waveguides, ring resonators, and photonic crystals, have proven to be effective conduits for a wide range of optical enhancement effects that include enhanced excitation of photon emitters (such as quantum dots, organic dyes, and fluorescent proteins), enhanced extraction from photon emitters, enhanced optical absorption, and enhanced optical scattering (such as from Raman-scatterers and nanoparticles). The application of photonic metamaterials as a means for enhancing contrast in microscopy is a recent technological development. Through their ability to generate surface-localized and resonantly enhanced electromagnetic fields, photonic metamaterials are an effective surface for magnifying absorption, photon emission, and scattering associated with biological materials while an imaging system records spatial and temporal patterns. By replacing the conventional glass microscope slide with a photonic metamaterial, new forms of contrast and enhanced signal-to-noise are obtained for applications that include cancer diagnostics, infectious disease diagnostics, cell membrane imaging, biomolecular interaction analysis, and drug discovery. This paper will review the current state of the art in which photonic metamaterial surfaces are utilized in the context of microscopy.

Near-Perfect Absorbing Copper Metamaterial for Solar Fuel Generation

Metamaterials are a new class of artificial materials that can achieve electromagnetic properties that do not occur naturally, and as such they can also be a new class of photocatalytic structures. We show that metal-based catalysts can achieve electromagnetic field amplification and broadband absorption by decoupling optical properties from the material composition as exemplified with a ZnO/Cu metamaterial surface comprising periodically arranged nanocubes. Through refractive index engineering close to the index of air, the metamaterial exhibits near-perfect 98% absorption. The combination of plasmonics and broadband absorption elevates the weak electric field intensities across the nonplasmonic absorption range. This feedback between optical excitation and plasmonic excitation dramatically enhances light-to-dark catalytic rates by up to a factor of 181 times, compared to a 3 times photoenhancement of ZnO/Cu nanoparticles or films, and with angular invariance. These results show that metamaterial catalysts can act as a singular light harvesting device that substantially enhances photocatalysis of important reactions.

Targets and Tools: Nucleic Acids for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) merges nanotechnology with conventional Raman spectroscopy to produce an ultrasensitive and highly specific analytical tool that has been exploited as the optical signal read-out in a variety of advanced applications. In this feature article, we delineate the main features of the intertwined relationship between SERS and nucleic acids (NAs). In particular, we report representative examples of the implementation of SERS in biosensing platforms for NA detection, the integration of DNA as the biorecognition element onto plasmonic materials for SERS analysis of different classes of analytes (from metal ions to microorgniasms) and, finally, the use of structural DNA nanotechnology for the precise engineering of SERS-active nanomaterials.

Paper-based plasmonic substrates as surface-enhanced Raman scattering spectroscopy platforms for cell culture applications

The engineering of advanced materials capable of mimicking the cellular micro-environment while providing cells with physicochemical cues is central for cell culture applications. In this regard, paper meets key requirements in terms of biocompatibility, hydrophilicity, porosity, mechanical strength, ease of physicochemical modifications, cost, and ease of large-scale production, to be used as a scaffold material for biomedical applications. Most notably, paper has demonstrated the potential to become an attractive alternative to conventional biomaterials for creating two-dimensional (2D) and three-dimensional (3D) biomimetic cell culture models that mimic the features of in vivo tissue environments for improving our understanding of cell behavior (e.g. growth, cell migration, proliferation, differentiation and tumor metastasis) in their natural state. On the other hand, integration of plasmonic nanomaterials (e.g. gold nanoparticles) within the fibrous structure of paper opens the possibility to generate multifunctional scaffolds equipped with biosensing tools for monitoring different cell cues through physicochemical signals. Among different plasmonic based detection techniques, surface-enhanced Raman scattering (SERS) spectroscopy emerged as a highly specific and sensitive optical tool for its extraordinary sensitivity and the ability for multidimensional and accurate molecular identification. Thus, paper-based plasmonic substrates in combination with SERS optical detection represent a powerful future platform for monitoring cell cues during cell culture processes. To this end, in this review, we will describe the different methods for fabricating hybrid paper-plasmonic nanoparticle substrates and their use in combination with SERS spectroscopy for biosensing and, more specifically, in cell culture applications.

Surface-Enhanced Raman Scattering and Surface-Enhanced Infrared Absorption by Plasmon Polaritons in Three-Dimensional Nanoparticle Supercrystals

Surface-enhanced vibrational spectroscopy strongly increases the cross section of Raman scattering and infrared absorption, overcoming the limited sensitivity and resolution of these two powerful analytic tools. While surface-enhanced setups with maximum enhancement have been studied widely in recent years, substrates with reproducible, uniform enhancement have received less attention although they are required in many applications. Here, we show that plasmonic supercrystals are an excellent platform for enhanced spectroscopy because they possess a high density of hotspots in the electric field. We describe the near field inside the supercrystal within the framework of plasmon polaritons that form due to strong light-matter interaction. From the polariton resonances we predict resonances in the far-field enhancement for Raman scattering and infrared absorption. We verify our predictions by measuring the vibrations of polystyrene molecules embedded in supercrystals of gold nanoparticles. The intensity of surface-enhanced Raman scattering is uniform within 10% across the crystal with a peak integrated enhancement of up to 300 and a peak hotspot enhancement of 10⁵. The supercrystal polaritons induce pairs of incoming and outgoing resonances in the enhanced cross section as we demonstrate experimentally by measuring surface-enhanced Raman scattering with multiple laser wavelengths across the polariton resonance. The infrared absorption of polystyrene is likewise enhanced inside the supercrystals with a maximum enhancement of 400%. We show with a coupled oscillator model that the increase originates from the combined effects of hotspot formation and the excitation of standing polariton waves. Our work clearly relates the structural and optical properties of plasmonic supercrystals and shows that such crystals are excellent hosts and substrates for the uniform and predictable enhancement of vibrational spectra.

Exploiting Colloidal Metamaterials for Achieving Unnatural Optical Refractions

The scaling down of meta-atoms or metamolecules (collectively denoted as metaunits) is a long-lasting issue from the time when the concept of metamaterials was first suggested. According to the effective medium theory, which is the foundational concept of metamaterials, the structural sizes of meta-units should be much smaller than the working wavelengths (e.g., << 1/5 wavelength). At relatively low frequency regimes (e.g., microwave and terahertz), the conventional monolithic lithography can readily address the materialization of metamaterials. However, it is still challenging to fabricate optical metamaterials (metamaterials working at optical frequencies such as the visible and near-infrared regimes) through the lithographic approaches. This serves as the rationale for using colloidal self-assembly as a strategy for the realization of optical metamaterials. Colloidal self-assembly can address various critical issues associated with the materialization of optical metamaterials, such as achieving nanogaps over a large area, increasing true 3D structural complexities, and cost-effective processing, which all are difficult to attain through monolithic lithography. Nevertheless, colloidal self-assembly is still a toolset underutilized by optical engineers. Here, the design principle of the colloidally self-assembled optical metamaterials exhibiting unnatural refractions, the practical challenge of relevant experiments, and the future opportunities are critically reviewed.

Chemical Funneling of Colloidal Gold Nanoparticles on Printed Arrays of End-Grafted Polymers for Plasmonic Applications

Spatially defined assembly of colloidal metallic nanoparticles is necessary for fabrication of plasmonic devices. In this study, we demonstrate high-resolution additive jet printing of end-functional polymers to serve as templates for directed self-assembly of nanoparticles into architectures with substantial plasmonic activity. The intriguing aspect of this work is the ability to form patterns of end-grafted poly(ethylene glycol) through printing on a hydrophobic layer that consists of fluoroalkylsilanes. The simultaneous dewetting of the underlying hydrophobic layer together with grafting of the printed polymer during thermal annealing enables fabrication of spatially defined binding sites for assembly of nanoparticles. The employment of electrohydrodynamic jet printing and aqueous inks together with reduction of the feature size during thermal annealing are critically important in achieving high chemical contrast patterns as small as ∼250 nm. Gold nanospheres of varying diameters selectively bind and assemble into nanostructures with reduced interparticle distances on the hydrophilic patterns of poly(ethylene glycol) surrounded with a hydrophobic background. The resulting plasmonic arrays exhibit intense and pattern-specific signals in surface-enhanced Raman scattering (SERS) spectroscopy. The localized seed-mediated growth of metallic nanostructures over the patterned gold nanospheres presents further routes for expanding the composition of the plasmonic arrays. A representative application in SERS-based surface encoding is demonstrated through large-area patterning of plasmonic structures and multiplex deposition of taggant molecules, all enabled by printing.

Soft Plasmonic Assemblies Exhibiting Unnaturally High Refractive Index

The increases in refractive indices (n) of materials are crucial for transformative optical technologies. With the progress of monolithic lithography, large advances have been achieved with several semiconductors, including silicon, germanium, and gallium arsenide, which generally provide higher n of ∼4.0 compared to those of other elements. Nevertheless, above this upper limit of naturally available n, the range of light-matter interactions could be unprecedentedly expanded, which in turn enriches the possible applications. Here, we present a soft self-assembly of polyhedral Au colloids as a promising method to achieve unnaturally high n values. The interfacial assembly of Au nanocubes provides n of 6.4 at the resonant wavelength (near-infrared) and 4.5 in the off-resonant regimes (mid-infrared), which have not been previously reached. The soft self-assembly of polyhedral Au colloids can be a versatile and highly effective route for the fabrication of optical metamaterials with unnaturally high n values.

Soft optical metamaterials

Optical metamaterials consist of artificially engineered structures exhibiting unprecedented optical properties beyond natural materials. Optical metamaterials offer many novel functionalities, such as super-resolution imaging, negative refraction and invisibility cloaking. However, most optical metamaterials are comprised of rigid materials that lack tunability and flexibility, which hinder their practical applications. This limitation can be overcome by integrating soft matters within the metamaterials or designing responsive metamaterial structures. In addition, soft metamaterials can be reconfigured via optical, electrical, thermal and mechanical stimuli, thus enabling new optical properties and functionalities. This paper reviews different types of soft and reconfigurable optical metamaterials and their fabrication methods, highlighting their exotic properties. Future directions to employ soft optical metamaterials in next-generation metamaterial devices are identified.

Cancer Diagnosis through SERS and Other Related Techniques

Cancer heterogeneity increasingly requires ultrasensitive techniques that allow early diagnosis for personalized treatment. In addition, they should preferably be non-invasive tools that do not damage surrounding tissues or contribute to body toxicity. In this context, liquid biopsy of biological samples such as urine, blood, or saliva represents an ideal approximation of what is happening in real time in the affected tissues. Plasmonic nanoparticles are emerging as an alternative or complement to current diagnostic techniques, being able to detect and quantify novel biomarkers such as specific peptides and proteins, microRNA, circulating tumor DNA and cells, and exosomes. Here, we review the latest ideas focusing on the use of plasmonic nanoparticles in coded and label-free surface-enhanced Raman scattering (SERS) spectroscopy. Moreover, surface plasmon resonance (SPR) spectroscopy, colorimetric assays, dynamic light scattering (DLS) spectroscopy, mass spectrometry or total internal reflection fluorescence (TIRF) microscopy among others are briefly examined in order to highlight the potential and versatility of plasmonics.