Subwavelength Regime Research Articles

Miniaturized Spoof Plasmonic Antennas with Good Impedance Matching.

The ability of spoof surface plasmon polaritons (SSPPs) to confine electromagnetic fields in a subwavelength regime enables the design of miniaturized antennas. However, the impedance matching scheme for miniaturized spoof plasmonic antennas has not been studied systematically. In this paper, we propose a general method in the antenna design based on SSPPs, providing a feasible solution to impedance matching at the feeding point of miniaturized spoof plasmonic antennas. To verify the method, a prototype of a planar spoof plasmonic dipole antenna is simulated, fabricated and measured, of which the dipole arm length is reduced by 35.2% as compared with the traditional dipole antenna. A peak gain level of 4.29 dBi and the radiation efficiency of about 94.5% were measured at 6 GHz. This general method can be extended to solve the impedance matching problem in the design of other spoof plasmonic devices.

Metallic Structures Based on Zinc Oxide Film for Enzyme Biorecognition

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

Generalized homogenization method for subwavelength periodic lattices.

Periodic photonic lattices based on Guided-Mode Resonance (GMR) enable the manipulation of the incident light, making them essential components in a plethora of optical elements including filters, sensors, lasers, and polarizers. The GMR is regarded as a resonance phenomenon in the resonant-subwavelength regime of periodic lattices. We present a method that homogenizes these periodic structures in the subwavelength regime and provides an appropriate analytical interpretation of the resonance effect. Here, we propose a technique based on utilizing the dispersion relation for homogenization, which can be applied to multi-part period lattices under oblique incidence. The effect of asymmetry and emergence of the odd/even modes, not considered in previous methods, will also be taken into account and discussed. As a result of this analytical procedure, resonance lines are obtained, which are useful in designing optical elements such as wideband/narrowband reflectors and polarizers.

Broadband low-frequency bidimensional honeycomb lattice metastructure based on the coupling of subwavelength resonators

The development and application of noise control strategies have been recently the focus of attention by several researchers. In the present study, the authors develop a thin low frequency acoustic metamaterial, composed of critically coupled subwavelength Helmholtz resonators, axially arranged, with a focus on wideband perfect absorption and sound transmission problems, which are here denominated Acoustic Honeycomb Metasurface (AHS). Applying fluid equivalent models to describe the viscous thermal losses through the transfer matrix method and nonlinear optimization, it has been possible to investigate the critically coupling condition in local resonances to enhance quasi-perfect (α>0.8) and perfect sound absorption (α=1). The results demonstrate the proposed absorber system works on a subwavelength regime (λ/52), exhibiting high absorption efficiency (>80%) at low-frequency broadband absorption (approx. 300 Hz to 600 Hz). Finally, the present work opens new possibilities to develop subwavelength opened acoustic metamaterials, with potential application in ventilated systems in different engineering areas.

Plasmonic enhancement in PTB7-Th:PC71BM organic photovoltaics

Organic photovoltaics struggles with limited exciton diffusion rate and low light absorption rates. The trade-off between these deficiencies limits the thickness of the active layer around 100–200 nm range. Plasmonic light management was introduced as one of the solutions to these problems in the last decade. The plasmonic metal nanostructures can manipulate light in subwavelength regime such as thin-film organic photovoltaics. In this work, we introduced a controllable and reproducible method to incorporate quasi-hexagonal arrays of Au nanostructure on the electron transport layer (ETL), which can directly facilitate light management in the active layer. The inverted PTB7-Th:PC71BM OPVs were selected as the model system, and Au nanostructure arrays were introduced in selected interfaces. In order to match the work function of Au nanostructures and the LUMO level of the PC71BM fullerene component of the bulk heterojunction, PEI was utilized to modify the work function of such surfaces. The plasmon-enhanced inverted OPV devices showed up to 22% higher power conversion efficiencies than reference devices. In order to elucidate the underlying reason for such high improvement, finite-difference time-domain simulations of whole OPV devices under AM1.5G solar illumination were performed. The results revealed up to 25 times higher local exciton generation rate in close proximity to the Au nanostructures and ETLs.

Deeply subwavelength giant monopole elastodynamic metacluster resonators.

The giant monopole resonance is a well-known phenomenon, employed to tune the dynamic response of composite materials comprising voids in an elastic matrix which has a bulk modulus much greater than its shear modulus, e.g. elastomers. This low frequency resonance (e.g. for standard elastomers, where and are the compressional wavelength and void radius, respectively) has motivated acoustic material design over many decades, exploiting the subwavelength regime. Despite this widespread use, the manner by which the resonance arising from voids in close proximity is affected by their interaction is not understood. Here, we illustrate that for planar elastodynamics (circular cylindrical voids), coupling due to near-field shear significantly modifies the monopole (compressional) resonant response. We show that by modifying the number and configuration of voids in a metacluster, the directionality, scattering amplitude and resonant frequency can be tailored and tuned. Perhaps most notably, metaclusters deliver a lower frequency resonance than a single void. For example, two touching voids deliver a reduction in resonant frequency of almost 16% compared with a single void of the same volume. Combined with other resonators, such metaclusters can be used as meta-atoms in the design of elastic materials with exotic dynamic material properties.

Subwavelength Su-Schrieffer-Heeger topological modes in acoustic waveguides.

Topological systems furnish a powerful way of localizing wave energy at edges of a structured material. Usually, this relies on Bragg scattering to obtain bandgaps with nontrivial topological structures. However, this limits their applicability to low frequencies because that would require very large structures. A standard approach to address the problem is to add resonating elements inside the material to open gaps in the subwavelength regime. Unfortunately, generally, one has no precise control on the properties of the obtained topological modes, such as their frequency or localization length. In this work, a unique construction is proposed to couple acoustic resonators such that acoustic modes are mapped exactly to the eigenmodes of the Su-Schrieffer-Heeger (SSH) model. The relation between energy in the lattice model and the acoustic frequency is controlled by the characteristics of the resonators. In this way, SSH topological modes are obtained at any given frequency, for instance, in the subwavelength regime. The construction is also generalized to obtain well-controlled topological edge modes in alternative tunable configurations.

Underwater metamaterial absorber with impedance-matched composite.

By using a structured tungsten-polyurethane composite that is impedance matched to water while simultaneously having a much slower longitudinal sound speed, we have theoretically designed and experimentally realized an underwater acoustic absorber exhibiting high absorption from 4 to 20 kHz, measured in a 5.6 m by 3.6 m water pool with the time-domain approach. The broadband functionality is achieved by optimally engineering the distribution of the Fabry-Perot resonances, based on an integration scheme, to attain impedance matching over a broad frequency range. The average thickness of the integrated absorber, 8.9 mm, is in the deep subwavelength regime (~λ/42 at 4 kHz) and close to the causal minimum thickness of 8.2 mm that is evaluated from the simulated absorption spectrum. The structured composite represents a new type of acoustic metamaterials that has high acoustic energy density and promises broad underwater applications.

Vector optical field manipulation via structural functional materials: Tutorial

Vector optical field (VOF) manipulation greatly extended the boundaries of traditional scalar optics over the past decades. Meanwhile, the newly emerging techniques enabled by structural functional optical materials have driven the research domain into the subwavelength regime, where abundant new physical phenomena and technologies have been discovered and exploited for practical applications. In this Tutorial, we outline the basic principles, methodologies, and applications of VOF via structural functional materials. Among various technical routes, we focus on the metasurface-based approaches, which show obvious advantages regarding the design flexibility, the compactness of systems, and the overall performances. Both forward and inverse design methods based on the rigorous solution of Maxwell's equations are presented, which provide a valuable basis for future researchers. Finally, we discuss the generalized optical laws and conventions based on VOF manipulation. The applications in optical imaging, communications, precision measurement, laser fabrication, etc. are highlighted.

The effect of dopant on light trapping characteristics in random silicon nanowires for solar cell applications

The Light management is a fundamental study to improve the solar cell and photoelectrical devices conversion efficiency. Here, the spectral responses of reflection and absorption characteristics of random silicon nanowires (Si-NWs) arrays of different lengths and doping type has been investigated. The Si-NWs have been synthesized by metal induced etching (MIE) approach have dimensions of Sub-wavelength regime. These Si-NWs arrays show efficient light trapping and low reflectance than bare Si due to suppression of Fresnel reflection substantially over a wide spectral range. The comparative study of light trapping in Si-NWs, fabrication by using n-type and p-type Si wafer have been explored to add new dimension in the field of photovoltaic application. Here, diffuse reflection spectra have been used to analyzed the light-scattering properties of Si-NWs, which reveal that the diffuse scattering decreases as the pore depth increases. The field distribution of Si-NWs is simulated using finite differential time domain (FDTD) method, results shows that reflectance could be modified due to light absorption driven by multiple scattering inside the structure reaching high values of apparent absorbance. Current-voltage (I–V) curves have been measured under illumination at standard testing conditions (STC), i.e., 300 K, 1000 W/m2 and 1.5 a.m. global by using solar simulator. These measurement studies indicate that dopant (n-type and p-type) have been effectively influenced the photovoltaic parameters. I–V curve indicated that the power conversion efficiency (PCE) of n-type and p-type doped Si-NWs exhibited PCE of 3.94% and 4.01% respectively.

Impact of non-Hermitian mode interaction on inter-cavity light transfer

Understanding inter-site mutual mode interaction in coupled physical systems is essential to comprehend large compound systems, as this local interaction determines the successive multiple inter-site energy transfer efficiencies. In the present study, we demonstrate that only the non-Hermitian coupling can correctly account for the light transfer between two coupled optical cavities. We also reveal that the non-Hermitian coupling effect becomes crucial as the system dimension decreases. Our results provide important insight for handling general-coupled devices in the subwavelength regime.

Higher-order topological states in locally resonant elastic metamaterials

Higher-order topological insulators (HOTIs), capable of hosting topological states over multiple dimensionalities, have received considerable attention recently, providing unprecedented platforms for robust wave manipulation. Aiming at applications of HOTIs for integrated sensing, energy harvesting, or control of structural vibration propagation, challenges remain in achieving topological states at low frequencies with ample flexibility and tunability. Here, we report the theoretical modeling and experimental realization of HOTIs in elastic locally resonant metamaterials (LRMs). By exploring the interplay between local resonance couplings and nontrivial band topology, a variety of higher-order topological corner states (TCSs) are constructed in deep sub-wavelength regime with high efficiency in energy confinement. More importantly, we reveal that the TCSs are dependent on localization mechanisms of interacting sites at interfaces, which endows our HOTIs with unique frequency-selective and dimension-switching abilities. We further design complex domain walls to demonstrate the TCSs can be selectively switched on at desired frequencies or geometric corners. Our findings not only offer effective routes for the design of deep sub-wavelength topological devices but also enrich the understandings of higher-order topological physics that can be extended to other classic systems.

How to determine the capacitance of a nanocomposite capacitor

The theory of the effective dielectric function of a nanocomposite dielectric disposed between the metallic electrodes in a capacitor is developed from first principles. Following the Maxwell Garnett approach, the spherical nanosized inclusions in the dielectric are modeled by point dipoles and the electromagnetic field of the induced dipoles reflected from the electrodes is taken into account using the dyadic Green’s function. The developed theory substitutes the Maxwell Garnett approximation for nanocomposites in the subwavelength regime, which is realized in electrical engineering.

Responsive photonic nanopixels with hybrid scatterers.

Metallic and dielectric nanoscatterers are optical pigments that offer rich resonating coloration in the subwavelength regime with prolonged material consistency. Recent advances in responsive materials, whose mechanical shapes and optical properties can change in response to stimuli, expand the scope of scattering-based colorations from static to active. Thus, active color-changing pixels are achieved with extremely high spatial resolution, in conjunction with various responsive polymers and phase-change materials. This review discusses recent progress in developing such responsive photonic nanopixels, ranging from electrochromic to other color-changing concepts. We describe what parameters permit modulation of the scattering colors and highlight superior functional devices. Potential fields of application focusing on imaging devices, including active full-color printing and flexible displays, information encryption, anticounterfeiting, and active holograms, are also discussed.

Mueller Matrix Ellipsometric Approach on the Imaging of Sub-Wavelength Nanostructures

Conventional spectroscopic ellipsometry is a powerful tool in optical metrology. However, when it comes to the characterization of non-periodic nanostructures or structured fields that are much smaller than the illumination spot size, it is not well suited as it integrates the results over the whole illuminated area. Instead, imaging ellipsometry can be applied. Especially imaging Mueller matrix ellipsometry is highly useful in nanostructure characterization and defect inspection, as it is capable to measure the complete Mueller matrix for each pixel in a microscope image of the sample. It has been shown that these so-called Mueller matrix images can help to distinguish geometrical features of nanostructures in the sub-wavelength regime due to visible differences in off-diagonal matrix elements. To further investigate the sensitivity of imaging Mueller matrix ellipsometry for sub-wavelength sized features, we designed and fabricated a sample containing geometrical nanostructures with lateral dimensions ranging from 50 to 5,000 nm. The structures consist of square and circular shapes with varying sizes and corner rounding. For the characterization of their Mueller matrix images, we constructed an in-house Mueller matrix microscope capable of measuring the full Mueller matrix for each pixel of a CCD camera, using an imaging system and a dual-rotating compensator configuration for the ellipsometric system. The samples are illuminated at 455 nm wavelength and the measurements can be performed in both transmission and reflection. Using this setup, we systematically examine the sensitivity of Mueller matrix images to small features of the designed nanostructures. Within this contribution, the results are compared with traceable atomic force microscopy measurements and the suitability of this measurement technique in optical nanometrology is discussed. AFM measurements confirm that the fabricated samples closely match their design and are suitable for nanometrological test measurements. Mueller matrix images of the structures show close resemblance to numerical simulations and significant influence of sub-wavelength features to off-diagonal matrix elements.

Ultrafast imaging of terahertz electric waveforms using quantum dots

Microscopic electric fields govern the majority of elementary excitations in condensed matter and drive electronics at frequencies approaching the Terahertz (THz) regime. However, only few imaging schemes are able to resolve sub-wavelength fields in the THz range, such as scanning-probe techniques, electro-optic sampling, and ultrafast electron microscopy. Still, intrinsic constraints on sample geometry, acquisition speed and field strength limit their applicability. Here, we harness the quantum-confined Stark-effect to encode ultrafast electric near-fields into colloidal quantum dot luminescence. Our approach, termed Quantum-probe Field Microscopy (QFIM), combines far-field imaging of visible photons with phase-resolved sampling of electric waveforms. By capturing ultrafast movies, we spatio-temporally resolve a Terahertz resonance inside a bowtie antenna and unveil the propagation of a Terahertz waveguide excitation deeply in the sub-wavelength regime. The demonstrated QFIM approach is compatible with strong-field excitation and sub-micrometer resolution—introducing a direct route towards ultrafast field imaging of complex nanodevices in-operando.

Non-degenerate two-wave mixing in the subwavelength periodic motion regime

We experimentally study non-degenerate two-wave mixing where reflection from a moving mirror is used to create frequency shifts and show that both the subwavelength motion of the mirror and periodic reversal of the intensity grating direction significantly affect the frequency dependence of the two-wave mixing signals detected both in ruby and colloidal media. It is shown that the Doppler shifts are suppressed when the reflecting moving mirror excursions are small compared to a wavelength and that the periodic reversal of the mirror motion results in a response that does not produce the expected maximum at the grating relaxation time. An approximate theoretical model agrees well with the results of the experiments and elucidates the transition from the observed response to the previously described idealized Doppler shift models.

Locally Resonant Effective Phononic Crystals for Subwavelength Vibration Control of Torsional Cylindrical Waves

Abstract Locally resonant materials allow for wave propagation control in the subwavelength regime. Even though these materials do not need periodicity, they are usually designed as periodic systems since this allows for the application of the Bloch theorem and analysis of the entire system based on a single unit cell. However, geometries that are invariant to translation result in equations of motion with periodic coefficients only if we assume plane wave propagation. When wave fronts are cylindrical or spherical, a system realized through tessellation of a unit cell does not result in periodic coefficients and the Bloch theorem cannot be applied. Therefore, most studies of periodic locally resonant systems are limited to plane wave propagation. In this article, we address this limitation by introducing a locally resonant effective phononic crystal composed of a radially varying matrix with attached torsional resonators. This material is not geometrically periodic but exhibits effective periodicity, i.e., its equations of motion are invariant to radial translations, allowing the Bloch theorem to be applied to radially propagating torsional waves. We show that this material can be analyzed under the already developed framework for metamaterials. To show the importance of using an effectively periodic system, we compare its behavior to a system that is not effectively periodic but has geometric periodicity. We show considerable differences in transmission as well as in the negative effective properties of these two systems. Locally resonant effective phononic crystals open possibilities for subwavelength elastic wave control in the near field of sources.

Bound states in the continuum in resonant nanostructures: an overview of engineered materials for tailored applications

Abstract From theoretical model to experimental realization, the bound state in the continuum (BIC) is an emerging area of research interest in the last decade. In the initial years, well-established theoretical frameworks explained the underlying physics for optical BIC modes excited in various symmetrical configurations. Eventually, in the last couple of years, optical-BICs were exploited as a promising tool for experimental realization with advanced nanofabrication techniques for numerous breakthrough applications. Here, we present a review of the evolution of BIC modes in various symmetry and functioning mediums along with their application. More specifically, depending upon the nature of the interacting medium, the excitations of BIC modes are classified into the pure dielectric and lossy plasmonic BICs. The dielectric constituents are again classified as photonic crystal functioning in the subwavelength regime, influenced by the diffraction modes and metasurfaces for interactions far from the diffraction regime. More importantly, engineered functional materials evolved with the pure dielectric medium are explored for hybrid-quasi-BIC modes with huge-quality factors, exhibiting a promising approach to trigger the nanoscale phenomena more efficiently. Similarly, hybrid modes instigated by the photonic and plasmonic constituents can replace the high dissipative losses of metallic components, sustaining the high localization of field and high figure of merit. Further, the discussions are based on the applications of the localized BIC modes and high-quality quasi-BIC resonance traits in the nonlinear harmonic generation, refractometric sensing, imaging, lasing, nanocavities, low loss on-chip communication, and as a photodetector. The topology-controlled beam steering and, chiral sensing has also been briefly discussed.

Spontaneous Emission Enhancement of CdSe Quantum Dots Embedded in a Two-dimensional Photonic Crystal L3 Nanocavity

Two-dimensional photonic crystal nanocavities were designed to tailor cavity quantum electrodynamics. Enhancing the spontaneous emission of low-quality factor nanocavity with embedded CdSe quantum dots (QDs) emitters is the aim of this study. Low concentration layer of CdSe QDs was sandwiched between two layers of Si2 N3 membrane using plasma-enhanced chemical vapor deposition. The modification rate in spontaneous emission of L3 nanocavity up to 2.3-fold has been observed at 629.5 nm in compare to bare cavities. High field confinement in the sub-wavelength regime became an interest field for quantum electrodynamics applications and good platform to study light matter interactions.