Subwavelength Range Research Articles

Acoustic impurity shielding induced by a pair of metasurfaces respecting PT symmetry

Impurities usually affect the transmission of acoustic signals, and how to effectively suppress their adverse effects is an important content in the research of acoustic materials. In a recent theoretical proposal, it was pointed out that perfect transmission through impurities can be achieved by adding a gain-loss distribution respecting parity-time ($\mathcal{PT}$) symmetry. In this paper, we demonstrate a similar but not identical way to realize the extraordinary physical property of impurity-shielding, which leads to perfect transmission at the so-called exceptional points. By systematically probing into the system composed of an equivalent medium slab sandwiched by a pair of $\mathcal{PT}$-symmetric metasurfaces, we obtain the two complementary solutions of exceptional points corresponding to perfect transmission. Interestingly, the two solutions of exceptional points coalesce into one when the mass density of the slab approaches zero. At such a critical point, we find that the impurity-shielding effect can be perfectly demonstrated, irrespective of embedded impurities of almost any shape and materials, whether they are Hermitian or non-Hermitian. In addition, the proposed system is frequency independent, indicating that the prototype can work well even in the subwavelength range. Our paper shows that exceptional points can be used to eliminate scattering of impurities in a density-near-zero medium, revealing their potential applications in acoustic sensing, directional imaging, and other related wave disciplines.

Body-Centered Double-Square Split-Ring Enclosed Nested Meander-Line-Shaped Metamaterial-Loaded Microstrip-Based Resonator for Sensing Applications.

The strong localization of the electric and magnetic fields in metamaterial-based structures has attracted a new era of radiation fields in the microwave range. In this research work, we represent a double split ring enclosed nested meander-line-shaped metamaterial resonator with a high effective medium ratio layered on a dielectric substrate to enhance the sensitivity for the material characterization. Tailoring a metallic design and periodical arrangement of the split ring resonator in a subwavelength range introduced field enhancement and strong localization of the electromagnetic field. The design methodology is carried out through the optimization technique with different geometric configurations to increase the compactness of the design. The CST microwave studio is utilized for the extraction of the scattering computational value within the defined boundary condition. The effective parameters from the reflection and transmission coefficient are taken into account to observe the radiation characteristics for the interaction with the applied electromagnetic spectrum. The proposed metamaterial-based sensor exhibits high sensitivity for different dielectric materials with low permittivity values. The numerical data of the frequency deviation for the different dielectric constants have shown good agreement using the linear regression analysis where the sensitivity is R2 = 0.9894 and the figure of merit is R2 = 0.9978.

Integral effective medium approach for a metamaterial with radially-inhomogeneous spherical inclusions

In this study, a novel integral effective medium approach (IEMA) to obtain the complex effective permittivity and permeability tensors of 3-D metal-dielectric composite is developed. An infinite isotropic homogenous dielectric medium with periodically imbedded spherical inclusions with spherical inhomogeneity is considered here as an artificial dielectric in subwavelength. Within full-scattering theory, it is shown that such composite belongs to a general class of metamaterials with spherical inclusions.The obtained expressions for the elements of the effective tensors are valid in the entire subwavelength range. In order to test the proposed IEMA, the problem for metallic spherical particles coated by a layer of dielectric is solved analytically in greater detail by using the approach proposed earlier by the first author of the study.We were able to show that a metal-dielectric composite/metamaterial with spherical inhomogeneous inclusions can be considered as a quasi-periodic artificial crystal with a tunable band gap in the subwavelength range. In certain sub-ranges in subwavelength, such crystals can exhibit the properties of homogeneous ferrite material.

Design of low-frequency and broadband acoustic metamaterials with I-shaped antichiral units

Controlling the propagation of low-frequency sound waves on the sub-wavelength scale is still a challenging problem. For the design of low frequency sound reduction materials, this paper introduces the I-shaped antichiral unit to the acoustic metamaterials with Mie resonance, which can manipulate the sub-wavelength acoustic waves and achieve effective low-frequency and broadband sound isolation. By adjusting the width of the ligament, the proportion of the omnidirectional bandgap is 65.6 % in the subwavelength range. Firstly, the designed acoustic metamaterials are constructed by rotating the I-shaped units with mutually orthogonal periodic distribution. The parametric geometric model of metamaterials is established, and the low-frequency and wide bandgap can be effectively realized by adjusting the ligament width. Then, the investigation of sound pressure and phase reveals that the bandgaps attribute to the Mie resonant mode. Furthermore, based on the transfer matrix method, the effective bulk modulus and effective mass density of acoustic metamaterials are derived. The results show that the frequency range of the negative effective parameters obtained is consistent with that of the band gap. Finally, experimental tests are carried out, the region where attenuation occurs is in good agreement with the bandgap range (1158–2500 Hz) predicted by eigenfrequency analysis. The experimental tests verify the effectiveness of the designed antichiral acoustic metamaterials for low frequency sound reduction. The introduction of the antichiral unit provides a broad space for the application of acoustic metamaterials in low-frequency sound reduction.The low-frequency and broadband can be effectively realized by adjusting the ligament width.The low-frequency can be effectively realized by adjusting the ligament width.The experimental tests verify the effectiveness of the designed antichiral unit.

Revisiting Defect-Induced Light Field Enhancement in Optical Thin Films.

Based on a finite-difference time-domain method, we revisited the light field intensification in optical films due to defects with different geometries. It was found that defect can induce the local light intensification in optical films and the spherical defects resulted in the highest light intensification among the defect types investigated. Light intensification can increase with defect diameter and the relative refractive index between the defect and the film layer. The shallow defects tended to have the highest light intensification. Finally, the extinction coefficient of the defect had a significant effect on light intensification. Our investigations revealed that the light field intensification induced by a nano-defect is mainly attributed to the interference enhancement of incident light and diffracted or reflected light by defects when the size of the defect is in the subwavelength range.

Vibration Energy Harvesting from the Subwavelength Interface State of a Topological Metamaterial Beam.

Topological metamaterial has been a research hotpot in both physics and engineering due to its unique ability of wave manipulation. The topological interface state, which can efficiently and robustly centralize the elastic wave energy, is promising to attain high-performance energy harvesting. Since most of environmental vibration energy is in low frequency range, the interface state is required to be designed at subwavelength range. To this end, this paper developed a topological metamaterial beam with local resonators and studied its energy-harvesting performance. First, the unit cell of this topological metamaterial beam consists of a host beam with two pairs of parasitic beams with tip mass. Then, the band structure and topological features are determined. It is revealed that by tuning the distance between these two pairs of parasitic beams, band inversion where topological features inverse can be obtained. Then, two sub-chains, their design based on two topologically distinct unit cells, are assembled together with a piezoelectric transducer placed at the conjunction, yielding the locally resonant, topological, metamaterial, beam-based piezoelectric energy harvester. After that, its transmittance property and output power were obtained by using the frequency domain analysis of COMSOL Multiphysics. It is clear that the subwavelength interface state is obtained at the band-folding bandgap. Meanwhile, in the interface state, elastic wave energy is successfully centralized at the conjunction. From the response distribution, it is found that the maximum response takes place on the parasitic beam rather than the host beam. Therefore, the piezoelectric transducer is recommended to be placed on the parasitic beam rather than host beam. Finally, the robustness of the topological interface state and its potential advantages on energy harvesting were studied by introducing a local defect. It is clear that in the interface state, the maximum response is always located at the conjunction regardless of the defect degree and location. In other words, the piezoelectric transducer placed at the conjunction can maintain a stable and high-efficiency output power in the interface state, which makes the whole system very reliable in practical implementation.

Enhancing low frequency sound radiation of electrodynamic loudspeakers with acoustic metamaterials

Since the radiation resistance of a vibrating membrane in the air is proportional to frequency in the subwavelength range, low frequency (<100Hz) sound radiating devices generally require a large vibrating diaphragm and a bulky enclosure to be efficient and loud enough for practical use. The tapped horn topology, where the front radiation of an electrodynamic driver adds up with the back radiation after propagating through the internal volume of a speaker box, has been widely used in custom audio to provide low-frequency enhancement to the bass unit. Recently, people have demonstrated the application of acoustic metamaterials based on multiple Helmholtz resonators (HRs) in sound absorption, sound insulation, directivity control, impedance matching, etc. In this presentation, we show that by branching the sound path of a tapped horn speaker box with multiple Helmholtz resonators and fine-tuning the geometry of each individual element, we can produce high and relatively constant sound pressure levels at sub-wavelength frequencies, making our methodology a good candidate for compact subwoofer design. Genetic algorithm (GA) is used to bring forth optimal performance within the geometric framework. Both transfer matrix method and numerical simulation are employed to derive the sound output of a particular subwoofer.

Compact magnetic field sensor based on plasmonic fiber-tip.

A plasmonic fiber-tip based on the metallic metasurface and the multimode fiber (MMF) alleviates the limitation of the inevitable large sensing size caused by fiber side wall functionalization. Localized surface plasmon resonance (LSPR) based on metasurface on the fiber-tip provides a promising way to manipulate and interrogate the transmitted and reflection light in sub-wavelength range. Combining the advantages of plasmonic fiber-tip and magnetic fluid, a compact magnetic field fiber-optic sensor is proposed and verified by experiments. The developed fiber-optic magnetic field sensor has linear response and high magnetic strength sensitivity of 0.532 nm/mT over a range of 0-20 mT. In addition, the results also prove the feasibility of pseudo-vector magnetic field sensing.

Towards structured SPP manipulation of light at the nanoscale

Surface plasmon photonics is a rapidly developing area of physics, optics, and nanotechnology. The unique ability of meso- and nano-structures to manipulate light in the subwavelength range down to nanoscale volumes stimulated their use in a vast research endeavours. The investigations are driven by interests in both fundamental and practical applications aspects where plasmonic light concentrators elegantly interface mesoscale dielectric structure with thin metal films. The effects of a photonic nanojet and a photonic hook, discovered by Minins, have been studied in sufficient detail in the literature, but only recently have they been able to be confirmed experimentally for low-dimensional systems – in-plane surface plasmon waves. The nature of these phenomenas lies in the dispersion of the phase velocity of waves inside the dielectric structure, which leads to constructive interference of the transmitted, diffracted, and near-field waves. Our results set the grounds for in-plane plasmonic wavelength scaled optics with unprecedented control of the energy flow at the nanoscale, and shown a way toward realizing the densely packed optical elements needed for future plasmonic and optical devices.

Refractive Index Sensing Based on Multiple Fano Resonances in a Split-Ring Cavity-Coupled MIM Waveguide

A metal–insulator–metal (MIM) waveguide consisting of a circular split-ring resonance cavity (CSRRC) and a double symmetric rectangular stub waveguide (DSRSW) is designed, which can excite quadruple Fano resonances. The finite element method (FEM) is used to investigate influences of geometric parameters on the transmission characteristics of the structure. The results show that Fano resonances are excited by the interference between the DSRSW and the CSRRC. Among them, the resonance wavelengths of the Fano resonances are tuned by the narrow-band discrete state excited by the CSRRC, and the resonance line transmittance and profiles are tuned by the wide-band continuous state excited by the DSRSW. The sensitivity (S) can be up to 1328.8 nm/RIU, and the figure of merit (FOM) can be up to 4.80 × 104. Based on these advantages, the structure has potential applications in sensing in the sub-wavelength range.

Dual-frequency multi-function switchable metasurface

Most metasurfaces can manipulate electromagnetic waves in the deep sub-wavelength range, but often have the disadvantages of low modulation depth and single-frequency operation. Here, a vanadium dioxide composite aluminum antenna (VCAA) for dual-frequency switchable function is proposed. After the vanadium dioxide (VO2) phase transition, the VCAA can achieve a high reflection coefficient (about 0.78) and 2π phase coverage independently at different frequencies under circular polarization (CP) incidence by rotating double C-type resonator and double D-type resonator. In order to better understand this characteristic, the surface electric field distribution of the metasurface is calculated and analyzed. Three temperature-switchable metasurfaces with beam splitting, focusing (divergence) and vortex beams functions at different frequencies are designed by encoding different arrangements of particles and convolution operations. And these functions are not influenced by the temperature at 0.52 THz, but have the effect of temperature-switchable at 0.98 THz. The functions of these three metasurfaces are demonstrated and verified from the perspective of electric field and phase. This work provides a new method for the dynamic regulation of terahertz waves and the design of multi-frequency devices, and accelerates the development of terahertz communications and information encryption.

Asymmetric all-dielectric active metasurface for efficient dual reflection modulation

Active metasurface provides an efficient way to achieve optical response in the subwavelength range, dielectric metasurface has attracted much attention due to its low-loss mode and excitable electric/magnetic resonance mode, resulting in a far wider range of applications. However, the resonance spectrum is relatively broad due to the strong radiative loss of the symmetric dielectric metasurface, which limits its application in large modulation extinction ratio. Hence, an active metasurface integrated with the phase change material Ge2Sb2Te5 (GST) is proposed. The active metasurface can support the symmetry-protected quasi bound states in the continuum (QBIC) and excite resonance modes with extremely high quality-factors. As an active medium, the GST layer undergoes a transition from the amorphous state to the crystalline state when the temperature increases, leading to a change in the amplitude/phase of the reflection spectrum. It is demonstrated that dual reflection modulation and the figure-of-merit (FoM) can reach up to 92.0% at the wavelength of 1.45 μm and 92.5% at the wavelength of 1.52 μm as the GST layer is in the middle of nanodisks. An extremely high FoM of 98.5% is also realized when the surface of silicon nanodisks is coated with the GST layer. In addition, the modulation mechanism of the optical response of the active metasurface has been investigated, which is of great significance to the design of the active metasurface. The proposed active all-dielectric asymmetric metasurface has a huge potential in tunable nonlinear optical devices.

Ultrahigh-Q system of a few coaxial disks

Abstract Resonant modes of high contrast dielectric disk have finite Q-factors in the subwavelength range due to radiation leakage into the surrounding space. That leakage can be reduced considerably (a few times) by exploiting of the mechanism of destructive interference of two modes for avoided crossing of resonances (ACR) (Rybin et al. M. V. Rybin, K. L. Koshelev, Z. F. Sadrieva, et al., “High-Q Supercavity Modes in Subwavelength Dielectric Resonators,” Phys. Rev. Lett., vol. 119, p. 243901, 2017.). In the present paper we report suppression of radiation leakage by a few orders in magnitude via the ACR in the structure of three and four different coaxial disks. For fine multi-scale tuning of disks we reveal the ultrahigh-Q resonances of order 105 for the case of three disks and of order 106 for the case of four coaxial disks of equal radii.

Bidirectional deep-subwavelength band gap induced by negative stiffness

Owing to its tremendous potential application in wave manipulation, metamaterials have attracted significant interest since their appearance at the start of this century. The resonant mechanism breaks the dependency of the lattice constant on the operation frequency, successfully achieving wave manipulation in the subwavelength range. However, it fails to simultaneously control two types of wave propagations in the deep-subwavelength range due to the specific configuration of the resonator. In this study, we introduce a novel negative-stiffness mechanism into the resonator to open a bidirectional deep-subwavelength band gap. Both the translational and torsional stiffness of the resonator can be neutralised by the proposed negative-stiffness mechanism. More importantly, adjusting the parameters of the negative-stiffness mechanism provides a convenient way to tune the bidirectional stiffness characteristic of the resonator, which is beneficial for manipulating the elastic wave in a large frequency range. The fundamental principle of opening the bidirectional deep-subwavelength band gap is examined through mechanical analysis, dispersion relation, and wave propagation features. By cascading the presented resonator in a host structure, this study realises the suppression of the propagations of two types of elastic waves within the deep-subwavelength frequency range as well as the filtration of one of these two types of elastic waves in a particular frequency range.

Shaping light in 3d space by counter-propagation

We extend the established transverse customization of light, in particular, amplitude, phase, and polarization modulation of the light field, and its analysis by the third, longitudinal spatial dimension, enabling the visualization of longitudinal structures in sub-wavelength (nm) range. To achieve this high-precision and three-dimensional beam shaping and detection, we propose an approach based on precise variation of indices in the superposition of higher-order Laguerre-Gaussian beams and cylindrical vector beams in a counter-propagation scheme. The superposition is analyzed experimentally by digital, holographic counter-propagation leading to stable, reversible and precise scanning of the light volume. Our findings show tailored amplitude, phase and polarization structures, adaptable in 3D space by mode indices, including sub-wavelength structural changes upon propagation, which will be of interest for advanced material machining and optical trapping.

Guest Editorial Introduction to JSTQE Issue on Nanobiophotonics

The papers in this special section focus on nanobiophotonics. This is an emerging field of modern science and biomedical nanotechnology. This new field continues to vastly expand with state-of-the-art developments across the entire spectrum of biomedical applications ranging from fundamental studies of light-nanobiomaterial interactions to clinical diagnostics and therapeutics. In nanobiophotonics research areas, there has been a great impetus recently for the development of noninvasive imaging and sensing of intracellular structures and cellular functions as well as obtaining quantitative information for light-tissue interactions at the cellular, intracellular, and molecular level. Existing photonics technologies provide unique capabilities for investigating these scientific questions in terms of using non-ionizing optical radiation for providing noninvasive intracellular, cellular, and tissue imaging, and in-vivo sensing with ultrahigh resolution in the subwavelength nanoscale range (smaller than 100 nm). Furthermore, recent technology development efforts leverage the advanced features of photonics to move beyond the diffraction limit, providing sub-nanometer resolution into cellular processes.

Dynamic Tunable Plasma-Induced Transparency of Periodic Images Of Graphene Nanoribbons

Metamaterials are synthetic special structural units, usually in the wavelength or subwavelength range. When the incident light is coupled with the structural element, the free electrons on the surface can generate surface plasmons under the periodic drive of electromagnetic wave. In this paper, we mainly study the transparency of dynamically adjustable plasma-induced periodic graphical graphene nanoribbons. This paper discusses the geometric parameters that affect the variation of PIT. The gap distance G and the disc moving distance S can change the coupling strength of the light and dark modes, and the variation of coupling strength can adjust the transparency and frequency band width of PIT window. The Fermi energy of graphene can be changed by adjusting the electrical conductivity of graphene, and the change of Fermi energy can also regulate the PIT effect.

Nano-structure made of donor-acceptor quantum dots embedded in three dimensional photonic crystals for all optical tunable quantum memory/switch applications

A robust, energy efficient, tunable as well as self-directed quantum structure made of two different sized QDs is proposed for switching and memory applications in quantum photonics. Three dimensional photonic crystal (PC) made of nonlinear polystyrene spheres is considered as the substrate to carry the closely located dots. Theoretical model based on density matrix approach is introduced to evaluate the physical features of the proposed system by taking in to consideration of the near field optical energy transfers whose spatial nature is characterized by a Yukawa-type potential. Furthermore, all couplings of excitons to thermal bath besides of the possible intra-dot relaxations are considered in modellings. Probabilities of the quantum states and switchings in between the states are studied systematically over time by exploring the populations of 8 possible configurations. In order to better understand the potential of the suggested structure to be utilized as a tunable quantum element, dependence of the possible absorption/dispersions and the population of each quantum state, to the structure temperature and features of the illuminating lights are discussed in detail. Results clearly reflect that energy transfer in subwavelength range is possible using the suggested system via combined effects of near field interactions and relaxations of energy levels. Then this study provides insight into design and fabrication of structures based on network of QDs embedded in PC that are both self-tunable as well as being controllable using external factors and thus meet the goals of integrated photonic circuits.

Surface metallization from ab initio theory and subwavelength coupling via surface plasmon polaritons in Cu-doped lithium niobate/tantalate owing to charge accumulation

Surface metallization in Z-cut Cu-doped lithium niobate and its isomorphous lithium tantalate slabs was achieved by exhibiting metal optical properties in the visible regime using ab initio calculation. The giant opposite surface potentials were measured with sKPFM, indicating large amount of surface charges accumulated. The metallization was firstly evidenced from reflectivity enhancement under single laser beam irradiation on a Cu-doped LT slab, which is compared with the calculated reflectivity from a metallic film, seemingly amplified by energy coupling from visible surface plasmon polaritons supported by the metallic film. Since the very first reflection (VFR) is dictated by what happens in the subwavelength range, it leads us a direct way to monitoring SPPs excitation in the metallic layer. The VFR varies from 17% to 86% under two counterpropagating beams illumination, pointing clearly to a subwavelength energy coupling via SPPs. In addition, several intriguing phenomena were observed and aligned well with the SPPs picture. This work should be universal to all n-type ferroelectric oxides with photorefractive and photovoltaic effects, unveiling a missing important aspect in the past and opening an avenue towards designing novel photonic circuits, devices and active plasmonics applications in a truly nanometric scale.

Dual band visible metamaterial absorbers based on four identical ring patches

Abstract This paper proposes a dual-frequency tunable ideal visible light metamaterial absorber based on the sub-wavelength range, which achieves perfect absorption in the visible light band. The absorber consists of an Au reflector and a nodal (SiO2) layer, and a top Au absorber layer. The absorption structure on the top of the absorber adopts the style of ancient Chinese coins and forms a square arrays structure. As a result, the absorber's two absorption peaks are perfect absorption peaks, and the absorption rate can reach 99.9%. In addition, we also researched the influence of adjusting the absorber's geometric parameters on the absorber, and analyzed the absorption mechanism under different circumstances. Through research, it can be known that the absorber has near-perfect dual-band absorption characteristics that do not depend on polarization angle and incident angle insensitivity. Therefore, the absorber designed by us will have a broad application prospect in the filter, solar cell, and so on.