Subwavelength Domain Research Articles

A sub-wavelength metamaterial layer for reducing underwater noise radiation of marine structures

Reducing the underwater noise radiated by marine structures is a critical issue for improving submarine stealth or limiting the anthropogenic impact of commercial ships. In this study, we propose a lightweight passive solution based on a metamaterial to efficiently reduce the noise at low frequency (< 1000 Hz) that can be transmitted through the shell plating of a marine vessel. This solution is made of a soft acoustically impervious poro-elastic foam in which inclusions are periodically positioned. Added to a plate, this metamaterial outperforms the mass law in the sub-wavelength domain when separating two air domains. This performance is due to the periodic arrangement of inclusions in the foam, creating a vibro-acoustic bandgap. The aim of this study is therefore to assess this solution where the domain of the transmitted acoustic wave is water. Firstly, numerical results on an unbounded plate are validated alongside the theoretical model of a flexible partition. When the metamaterial layer is added to the plate, it is shown that a vibro-acoustic bandgap still exists in the transmitted wavenumber domain, and that this solution is a good candidate for achieving the objectives. Experimental measurements on a finite structure are then conducted and compared to numerical results

Magnetoelectric Energy in Electrodynamics: Magnetoelectricity, Bi(an)isotropy, and Magnetoelectric Meta‐Atoms

AbstractMagnetoelectricity denotes the relationship between electric polarization and magnetization. In materials with an intrinsic magnetoelectric (ME) effect, the energy density comprises the polarization, magnetization, and ME energy densities. These three components of energy define local (subwavelength) characteristics of electromagnetic (EM) responses in multiferroic materials. In a subwavelength domain, coupling between the electric and magnetic dipole oscillations forms the ME field structures that are characterized by the violation of both spatial and temporal symmetry. Unlike multiferroics, bi(an)isotropic metamaterials are associated with an EM response characterized only by spatial symmetry breaking. This also applies to chiral materials. Since no “intrinsic magnetoelectricity” is assumed in such structures, any concepts about the stored ME energy are not applicable. This clearly points to the effect of nonlocality. That is why the basic concepts of bi(an)isotropy can only be analyzed by the EM far‐field characteristics. In this paper, it is argued that in the implementation of local (subwavelength) ME meta‐atoms and systems for near‐field probing of chirality, the concept on ME energy is crucial. Real ME energy can occur when ME fields in a singular subwavelength domain are characterized by a violation of both the symmetry of time reversal and spatial reflection.

Nanoporous Titanium Oxynitride Nanotube Metamaterials with Deep Subwavelength Heat Dissipation for Perfect Solar Absorption.

We report a quasi-unitary broadband absorption over the ultraviolet-visible-near-infrared range in spaced high aspect ratio, nanoporous titanium oxynitride nanotubes, an ideal platform for several photothermal applications. We explain such an efficient light-heat conversion in terms of localized field distribution and heat dissipation within the nanopores, whose sparsity can be controlled during fabrication. The extremely large heat dissipation could not be explained in terms of effective medium theories, which are typically used to describe small geometrical features associated with relatively large optical structures. A fabrication-process-inspired numerical model was developed to describe a realistic space-dependent electric permittivity distribution within the nanotubes. The resulting abrupt optical discontinuities favor electromagnetic dissipation in the deep sub-wavelength domains generated and can explain the large broadband absorption measured in samples with different porosities. The potential application of porous titanium oxynitride nanotubes as solar absorbers was explored by photothermal experiments under moderately concentrated white light (1-12 Suns). These findings suggest potential interest in realizing solar-thermal devices based on such simple and scalable metamaterials.

Twisting Polarization‐Tunable Subdiffraction‐Limited Magnetization through Vectorial Beam Coupling

All‐optical control of magnetization vectors is considered as a powerful solution that empowers the development of multifunctional integrated optomagnetic devices. The interaction between vectorial light and magnetism has recently experienced a surge of interest due to its prominent ability to steer the magnetic polarization orientations and spatial textures within the subwavelength domain, which, however, usually requires complex wavefront coding optimizers or arduous material fabrication procedures. Herein, an optimization‐free all‐vectorial‐optical strategy for first realizing spatially twist‐controllable and successively polarization‐tunable magnetization at the subdiffraction scale through the inverse Faraday effect is conceived and demonstrated. This facile method relies on the coherent coupling of double crossed azimuthally polarized doughnut‐Gaussian vortex beams in a single optically configured geometry. It is found that both the magnetic twisting manifolds and polarization states, rotating from perpendicular (longitudinal), via hybrid (3D) to in‐plane (transverse) orientations, can be on‐demand mediated by judiciously tuning the crossing angles. It is further unraveled that the orbital angular momenta associated with topological charges of the vortex beams can be viewed as a vital knob to energetically maneuver magnetic twisting and guide magnetization switching. It is believed that the proposed route and presented findings are not only of great theoretical implication in twistronics and optomagnetic topology but also of considerable technological significance in multidimensional high‐density and low‐energy consumption optomagnetic storage and engineering of magnetic topologic materials.

Merging Bragg and Local Resonance Bandgaps in Perforated Elastic Metamaterials with Embedded Spiral Holes

Perforated elastic metamaterials (EMMs) comprising periodic distribution of holes have recently attracted growing attention due to relatively simple fabrication process and unusual physical properties. Rational design of perforation configuration for practical applications in low-frequency vibration mitigation is challenging and important. In this study, a novel type of perforated EMMs with two bandgap formation mechanisms (i.e., Bragg scattering and local resonance) is proposed for low-frequency and broadband wave attenuation. Four concentric spiral holes are introduced into each matrix material of the conventional EMMs plate with mutually orthogonal rectangular holes. The analysis of bandgap formation mechanism indicates that the coiled waveguide introduced by the spiral holes enhances the wave scattering in the matrix material, and that the wave scattering in the divided matrix material by the orthogonal rectangular holes couples the local resonance modes formed by the spiral holes, where the coupling modes lead to the generation of the coupling bandgap and the improvement of wave attenuation. Both experiments and numerical analyses demonstrate that the proposed metamaterials can create multiple bandgaps in the sub-wavelength frequency domain compared to the metamaterials with single-type holes, which include two Bragg bandgaps with a reduced frequency of 76%, two overlapping bandgaps with local resonance dominating, and two deaf bands.

Extreme anti-reflection enhanced magneto-optic Kerr effect microscopy

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that extreme anti-reflection (EAR) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20° and magnetic domain imaging visibility of 0.47. Especially, EAR-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The EAR technique is promising for optical investigations and applications of nanomagnetic systems.

Controllable two- and three-dimensional atom localization via spontaneously generated coherence

A scheme for two-dimensional (2D) and three-dimensional (3D) atom localization in a four-level N-type atomic system is proposed. The scheme is based on the probe absorption measurement when the atom passes through the orthogonal standing-wave fields. Using the electromagnetically induced transparency, we prove that the probe field absorption is spatially localized in sub-wavelength domain. We also show that the spatial resolution of 2D and 3D atom localization strongly depends on the probe field detuning, the quantum interference parameters induced by spontaneous emission, and the relative phase between the driving fields.

Energy-Based Plasmonicity Index to Characterize Optical Resonances in Nanostructures

Resonances sustained by plasmonic nanoparticles provide extreme electric field confinement and enhancement into the deep subwavelength domain for a plethora of applications. Recent progress in nanofabrication made it even possible to tailor the properties of nanoparticles consisting of only a few hundred atoms. These nanoparticles support both single-particle-like resonances and collective plasmonic charge density oscillations. Prototypical systems sustaining both features are graphene nanoantennas. In pushing the frontier of nanoscience, traditional identification, and classification of such resonances is at stake again. We show that in such nanostructures, the concerted electron cloud oscillation in real space does not necessarily come along with collective dynamics of conduction band electrons in energy space. This unveils an urgent need for a discussion of how a plasmon in nanostructures should be defined. Here, we propose to define it relying on energy space dynamics. The unambiguous identification of the plasmonic nature of a resonance is crucial to find out whether desirable plasmon-assisted features, such as frequency conversion processes, can be expected from a resonance. We elaborate an energy-based figure of merit that classifies the nature of resonances in nanostructures, motivated by tight binding simulations with a toy model consisting of a linear chain of atoms. We apply afterward the proposed figure of merit to a doped hexagonal graphene nanoantenna, which is known to support plasmons in the near infrared and single-particle-like transitions in the visible.

Graphene based metasurface with near unity broadband absorption in the terahertz gap

This paper reports a novel graphene metasurface exhibiting near-unity absorption over a wide bandwidth of 9.74 THz ranging from 2.06 to 11.80 THz with a fractional bandwidth of 140.86% with respect to the center frequency of 6.93 THz within the terahertz gap of the electromagnetic spectrum. The proposed structure consists of two layers of graphene separated by silicon dioxide (SiO2). On the top of SiO2, a modified fractal-shaped 1 nm thick graphene pattern is deposited while a square-shaped graphene pattern of identical thickness completely covers the backside of the SiO2 substrate. The absorber was found to be polarization independent under the normal incidence of the approaching electromagnetic wave (EM) toward the top layer of the structure due to its 4-fold structural symmetry. The designed structure offers stable absorption up to 60° incident angle under TE polarization and 40° incident angle under TM polarization. The structure is ~λ/55.9 thin and maintains periodicity of ~λ/21.7; thereby validating the effective homogeneity condition in the sub-wavelength domain. This graphene-based absorber can find several applications including, sensors, detectors, spatial light modulators, and in spectroscopic detectors, and so forth, in the “Terahertz Gap” for next-generation 5G communication system and beyond.

Wavevector mismatch and Doppler-free three-dimensional atom localization

In the regime of the Raman–Nath approximation we show coherence control of three-dimensional atom localization with high precession and resolution via probe absorption when a five-level atomic system is cyclically driven by three orthogonal standing optical wave-fields together with one traveling optical wave-field and two microwave wave-fields in an optical cavity. We apply wavevector mismatch and subsequently the Doppler-free technique to the atom localization system to minimize the imprecision in the atom’s position in the standing wave-field caused by its high velocity in the cavity. The position of the single atom is freed from the two effects by collinear coupling of the driving fields with the atom in the cavity. The atomic position is controlled in the sub-wavelength domain of the standing wave-field with probability and high quality resolution by adjusting the relative phase and amplitude of the driving fields.

Multiresonant plasmonics with spatial mode overlap: overview and outlook

Abstract Plasmonic nanostructures can concentrate light and enhance light-matter interactions in the subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional plasmonic devices and systems are typically optimized for the operation in a single wavelength band and thus are not suitable for multiband nanophotonics applications that either prefer nanoplasmonic enhancement of multiphoton processes in a quantum system at multiple resonant wavelengths or require wavelength-multiplexed operations at nanoscale. To overcome the limitations of “single-resonant plasmonics,” we need to develop the strategies to achieve “multiresonant plasmonics” for nanoplasmonic enhancement of light-matter interactions at the same locations in multiple wavelength bands. In this review, we summarize the recent advances in the study of the multiresonant plasmonic systems with spatial mode overlap. In particular, we explain and emphasize the method of “plasmonic mode hybridization” as a general strategy to design and build multiresonant plasmonic systems with spatial mode overlap. By closely assembling multiple plasmonic building blocks into a composite plasmonic system, multiple nonorthogonal elementary plasmonic modes with spectral and spatial mode overlap can strongly couple with each other to form multiple spatially overlapping new hybridized modes at different resonant energies. Multiresonant plasmonic systems can be generally categorized into three types according to the localization characteristics of elementary modes before mode hybridization, and can be based on the optical coupling between: (1) two or more localized modes, (2) localized and delocalized modes, and (3) two or more delocalized modes. Finally, this review provides a discussion about how multiresonant plasmonics with spatial mode overlap can play a unique and significant role in some current and potential applications, such as (1) multiphoton nonlinear optical and upconversion luminescence nanodevices by enabling a simultaneous enhancement of optical excitation and radiation processes at multiple different wavelengths and (2) multiband multimodal optical nanodevices by achieving wavelength multiplexed optical multimodalities at a nanoscale footprint.

Effect of nearby levels on atom localization in the $\Xi$Ξ atomic system via spatial dependent probe absorption

The present study demonstrates the atom localization in the subwavelength domain by considering a Ξ-type atomic system with nearby upper levels. When an atom passes through standing-wave couple fields, it faces a position dependent probe absorption that is the key factor for the present study. The position corresponding to a large probe absorption refers the localization position in the subwavelength domain. Our numerical calculation based on the density matrix formalism suggests that nearby levels can change the localization pattern significantly in two dimensional (2D) as well as three dimensional (3D) subwavelength domains. In the 2D case, a narrower spike-like localization pattern can be obtained by optimizing the field parameters in the presence of nearby levels. Thus, precise information of the atom’s position can be achieved. Interestingly for the 3D case, the presence of nearby levels enhances the range of probe detuning; consequently, the probability of finding the atom becomes constant in the subwavelength domain. In addition, the maximal detection probability of finding the atom at a position, i.e., unity can be obtained in both the cases, 2D and 3D, by properly adjusting the fields’ parameter in the presence of nearby levels.

A Rationale for Mesoscopic Domain Formation in Biomembranes.

Cell plasma membranes display a dramatically rich structural complexity characterized by functional sub-wavelength domains with specific lipid and protein composition. Under favorable experimental conditions, patterned morphologies can also be observed in vitro on model systems such as supported membranes or lipid vesicles. Lipid mixtures separating in liquid-ordered and liquid-disordered phases below a demixing temperature play a pivotal role in this context. Protein-protein and protein-lipid interactions also contribute to membrane shaping by promoting small domains or clusters. Such phase separations displaying characteristic length-scales falling in-between the nanoscopic, molecular scale on the one hand and the macroscopic scale on the other hand, are named mesophases in soft condensed matter physics. In this review, we propose a classification of the diverse mechanisms leading to mesophase separation in biomembranes. We distinguish between mechanisms relying upon equilibrium thermodynamics and those involving out-of-equilibrium mechanisms, notably active membrane recycling. In equilibrium, we especially focus on the many mechanisms that dwell on an up-down symmetry breaking between the upper and lower bilayer leaflets. Symmetry breaking is an ubiquitous mechanism in condensed matter physics at the heart of several important phenomena. In the present case, it can be either spontaneous (domain buckling) or explicit, i.e., due to an external cause (global or local vesicle bending properties). Whenever possible, theoretical predictions and simulation results are confronted to experiments on model systems or living cells, which enables us to identify the most realistic mechanisms from a biological perspective.

Characteristics of transient processes in a silver nanowire with optically dynamic shell

Fundamental theoretical understanding of the process of the time domain dynamics of the surface plasmon excitation in the structure, consisting of a metal nanowire coupled with the optically dynamic shell is presented. Details are described of the dynamics in the metal nanowire by means of meticulous and exact mathematical calculations that found from the semi-analytic solution into the Laplace transform domain and its accurate inversion into transient domain by virtue of residues assessment at singular points that correspond to the excited plasmons of the structure.This problem is a 2D model of nanolaser and has applications in the most growing areas of nanophotonics that require technologies, which can stimulate coherent plasmon fields in subwavelength domain.

Nano-optical imaging of monolayer MoSe2-WSe2 lateral heterostructure with subwavelength domains

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are the materials of recent interest to study the spatial confinement of charge carriers, photons, and phonons. Heterostructures based on TMD monolayers, especially composed of Mo and W, form type-II band alignment, and hence, the optically excited carriers can be easily separated for applications pertaining to optoelectronics. Mapping the spatially confined carriers or photons in lateral heterostructures with nanoscale resolution as well as their recombination behavior at the heterointerfaces is necessary for the effective use of 2D materials in optoelectronic devices. Near-field (NF) optical microscopy has been used as a viable route to understand the nanoscale material properties below the diffraction limit. The authors performed tip-enhanced photoluminescence (TEPL) imaging with a spatial resolution of 40 nm of multijunction monolayer MoSe2-WSe2 lateral heterostructures with subwavelength domains grown by chemical vapor deposition. Monolayer MoSe2 and WSe2 domains were identified by atomic force microscopy (AFM) through the topography and phase mapping. Far-field (FF) and NF techniques were used for the optical imaging of the WSe2 ↔ MoSe2 multijunction heterostructure correlated with AFM phase imaging. Near-field TEPL imaging was able to successfully distinguish the presence of distinct crystalline boundaries across the WSe2 ↔ MoSe2 interfaces in 2D lateral heterostructures with a higher spatial resolution, as compared to the far-field imaging, which failed to resolve the interfaces on one of the crystal sides due to the asymmetric FF excitation.

Multiresonant Composite Optical Nanoantennas by Out-of-plane Plasmonic Engineering.

Optical nanoantennas can concentrate light and enhance light-matter interactions in subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional optical nanoantennas operating at a single wavelength band are not suitable for multiband applications. Here, we propose and exploit an out-of-plane plasmonic engineering strategy to design and create composite optical nanoantennas that can support multiple nanolocalized modes at different resonant wavelengths. These multiresonant composite nanoantennas are composed of vertically stacked building blocks of metal-insulator-metal loop nanoantennas. Studies of multiresonant composite nanoantennas demonstrate that the number of supported modes depends on the number of vertically stacked building blocks and the resonant wavelengths of individual modes are tunable by controlling the out-of-plane geometries of their building blocks. In addition, numerical studies show that the resonant wavelengths of individual modes in composite nanoantennas can deviate from the optical response of building blocks due to hybridization of magnetic modes in neighboring building blocks. Using Au nanohole arrays as deposition masks to fabricate arrays of multilayered composite nanoantennas, we experimentally demonstrate their multiresonant optical properties in good agreement with theory predictions. These studies show that out-of-plane engineered multiresonant composite nanoantennas can provide new opportunities for fundamental nanophotonics research and practical applications involving optical multiband operations, such as multiphoton process, broadband solar energy conversion, and wavelength-multiplexed optical system.

Gap-mode-assisted light-induced switching of sub-wavelength magnetic domains

Creating sub-micron hotspots for applications such as heat-assisted magnetic recording (HAMR) is a challenging task. The most common approach relies on a surface-plasmon resonator (SPR), whose design dictates the size of the hotspot to always be larger than its critical dimension. Here, we present an approach which circumvents known geometrical restrictions by resorting to electric field confinement via excitation of a gap-mode (GM) between a comparatively large Gold (Au) nano-sphere (radius of 100 nm) and the magnetic medium in a grazing-incidence configuration. Operating a λ=785 nm laser, sub-200 nm hot spots have been generated and successfully used for GM-assisted magnetic switching on commercial CoCrPt perpendicular magnetic recording media at laser powers and pulse durations comparable to SPR-based HAMR. Lumerical electric field modelling confirmed that operating in the near-infrared regime presents a suitable working point where most of the light's energy is deposited in the magnetic layer, rather than in the nano-particle. Further, modelling is used for predicting the limits of our method which, in theory, can yield sub-30 nm hotspots for Au nano-sphere radii of 25–50 nm for efficient heating of FePt recording media with a gap of 5 nm.

Resonance fluorescence microscopy via three-dimensional atom localization

A scheme is proposed to realize three-dimensional (3D) atom localization in a driven two-level atomic system via resonance fluorescence. The field arrangement for the atom localization involves the application of three mutually orthogonal standing-wave fields and an additional traveling-wave coupling field. We have shown the efficacy of such field arrangement in tuning the spatially modulated resonance in all directions. Under different parametric conditions, the 3D localization patterns originate with various shapes such as sphere, sheets, disk, bowling pin, snake flute, flower vase. High-precision localization is achieved when the radiation field detuning equals twice the combined Rabi frequencies of the standing-wave fields. Application of a traveling-wave field of suitable amplitude at optimum radiation field detuning under symmetric standing-wave configuration leads to 100% detection probability even in sub-wavelength domain. Asymmetric field configuration is also taken into consideration to exhibit atom localization with appreciable precision compared to that of the symmetric case. The momentum distribution of the localized atoms is found to follow the Heisenberg uncertainty principle under the validity of Raman–Nath approximation. The proposed field configuration is suitable for application in the study of atom localization in an optical lattice arrangement.

Dielectric V-ridge gratings: transition from antireflection to retroreflection.

We present a systematic study of the optical properties of dielectric gratings with symmetric V-shaped ridges having a 90 degree apex angle and refractive index n. Such structures exhibit completely different optical properties if the dimensions are scaled with respect to the vacuum wavelength λ0 of light. In the subwavelength domain, where the grating period d is less than λ0/n, the grating behaves as an antireflection layer. In the large-period domain d≫λ0 (with normal incidence from the dielectric side), the grating turns into a micro-retroreflector array. The transition between these well-known domains is studied using rigorous diffraction theory. The results are verified experimentally by fabricating and characterizing V-profile gratings with the aid of wet etching of silicon using a process that defines a 90 degree apex angle and replication into a polymer.

Sub-half-wavelength atom localization via two standing waves

We present a simple scheme of high-efficiency one-dimensional (1D) atom localization via manipulation of excited state population in a four-level inverted-Y atomic system. Because of the joint quantum interference induced by the two standing-wave fields, the 100% detecting probability of the atom in the subwavelength domain appears when the corresponding conditions are satisfied. The proposed scheme may open a promising way to achieve high-precision and high-efficiency 1D atom localization, which provides some potential applications to spatially selective single-qubit phase gate, entangling gates, and quantum error correction for quantum information processing.