High-Q resonances governed by the quasi-bound states in the continuum in all-dielectric metasurfaces

The realization of high-Q resonances in a silicon metasurface with various broken-symmetry blocks is reported. Theoretical analysis reveals that the sharp resonances in the metasurfaces originate from symmetry-protected bound in the continuum(BIC) and the magnetic dipole dominates these peculiar states. A smaller size of the defect in the broken-symmetry block gives rise to the resonance with a larger Q factor. Importantly, this relationship can be tuned by changing the structural parameter, resulting from the modulation of the topological configuration of BICs. Consequently, a Q factor of more than 3,000 can be easily achieved by optimizing dimensions of the nanostructure. At this sharp resonance, the intensity of the third harmonic generation signal in the patterned structure can be 368 times larger than that of the flat silicon film. The proposed strategy and underlying theory can open up new avenues to realize ultrasharp resonances, which may promote the development of the potential meta-devices for nonlinearity, lasing action, and sensing.

All-dielectric χ^(2) metasurfaces: recent progress

Optical metasurfaces,i.e.arrays of nanoantennas with sub-wavelength size and separation,enable the manipulation of light-matter interactions in miniaturized optical components with no classical counterparts.Six decades after the first ob-servation of the second harmonic generation(SHG)in bulk crystals,these devices are expected to break new ground in the field of nonlinear optics,shifting the focus from the phase matching approach achieved within long propagation dis-tances to that of near-field resonances interplay in leaky nanocavities.Here we review the recent progress in SHG with all-dielectric metasurfaces.We discuss the most used technological platforms which underpinned such advances and analyze different SHG control approaches.We finally compare their performances with other well-established technolo-gies,with the hope to delineate the current state-of-the-art and figure out a few scenarios in which these devices might soon offer unprecedented opportunities.

Design of an all-dielectric long-wave infrared wide-angle metalens

Optical metasurfaces are two-dimensional arrays of nano-scatterers that modify optical wavefronts at subwavelength spatial resolution.They achieve the effect of focusing through phase control under a subwavelength scale,and are called metalenses.They are poised to revolutionize optics by enabling complex low-cost systems.However,there are severe monochromatic aberrations in the metasurfaces.In this paper,the coma of the long-wave infrared optical system is eliminated through a single-layer metasurface.By changing the phase function,this metalens has a numerical aperture of 0.89,a focal length of 150μm and a field of view of 120°(0.4@60 line pairs/mm)that enables diffraction-limited monochromatic imaging along the focal plane at a wavelength of 10.6μm.The designed metasurface maintains a favorable value of the modulation transfer function at different angles.This equipment can be widely used in imaging and industrial processing.

All-dielectric left-handed metamaterial based on dielectric resonator:design,simulation and experiment

Dipoles with Lorentz-type resonant electromagnetic responses can realise negative effective parameters in their negative resonant region. The electric dipole and magnetic dipole can realise, respectively, negative permittivity and negative permeability, so both the field distribution forms of electric and magnetic dipoles are fundamentals in designing left-handed metamaterial. Based on this principle, this paper studies the field distribution in high-permittivity dielectric materials. The field distributions at different resonant modes are analysed based on the dielectric resonator theory. The origination and influence factors of the electric and magnetic dipoles are confirmed. Numerical simulations indicate that by combining dielectric cubes with different sizes, the electric resonance frequency and magnetic resonance frequency can be superposed. Finally, experiments are carried out to verify the feasibility of all-dielectric left-handed metamaterial composed by this means.

Refractive index sensing of double Fano resonance excited by nano-cube array coupled with multilayer all-dielectric film

We propose a hybrid structure of a nano-cube array coupled with multilayer full-dielectric thin films for refractive index sensing.In this structure,discrete states generated by two-dimensional grating and continuous states generated by a photonic crystal were coupled at a specific wavelength to form two Fano resonances.The transmission spectra and electric field distributions of the structure were obtained via the finite-difference time-domain method.We obtained the optimal structural parameters after optimizing the geometrical parameters.Under the optimal parameters,the figure of merit(FOM)values of the two Fano resonances reached 1.7×10^(4)and 3.9×10^(3),respectively.These results indicate that the proposed structure can achieve high FOM refractive index sensing,thus offering extensive application prospects in the biological and chemical fields.

Strong near-field couplings of anapole modes and formation of higher-order electromagnetic modes in stacked all-dielectric nanodisks

We theoretically study the near-field couplings of two stacked all-dielectric nanodisks,where each disk has an electric anapole mode consisting of an electric dipole mode and an electric toroidal dipole(ETD)mode.Strong bonding and anti-bonding hybridizations of the ETD modes of the two disks occur.The bonding hybridized ETD can interfere with the dimer’s electric dipole mode and induce a new electric anapole mode.The anti-bonding hybridization of the ETD modes can induce a magnetic toroidal dipole(MTD)response in the disk dimer.The MTD and magnetic dipole resonances of the dimer form a magnetic anapole mode.Thus,two dips associated with the hybridized modes appear on the scattering spectrum of the dimer.Furthermore,the MTD mode is also accompanied by an electric toroidal quadrupole mode.The hybridizations of the ETD and the induced higher-order modes can be adjusted by varying the geometries of the disks.The strong anapole mode couplings and the corresponding rich higher-order mode responses in simple all-dielectric nanostructures can provide new opportunities for nanoscale optical manipulations.

All-dielectric frequency selective surface design based on dielectric resonator

In this work,we propose an all-dielectric frequency selective surface（FSS） composed of periodically placed highpermittivity dielectric resonators and a three-dimensional（3D） printed supporter.Mie resonances in the dielectric resonators offer strong electric and magnetic dipoles,quadrupoles,and higher order terms.The re-radiated electric and magnetic fields by these multipoles interact with the incident fields,which leads to total reflection or total transmission in some special frequency bands.The measured results of the fabricated FSS demonstrate a stopband fractional bandwidth（FBW）of 22.2%,which is consistent with the simulated result.

Experimental demonstration of ultra-large-scale terahertz all-dielectric metamaterials

All-dielectric metamaterials have emerged as a promising platform for low-loss and highly efficient terahertz devices. However, existing fabrication methods have difficulty in achieving a good balance between precision and cost. Here, inspired by the nano-template-assisted self-assembly method, we develop a micro-templateassisted self-assembly(MTAS) method to prepare large-scale, high-precision, and flexible ceramic microsphere all-dielectric metamaterials with an area exceeding 900 cm × 900 cm. Free from organic solvents, vacuum, and complex equipment, the MTAS method ensures low-cost and environmentally friendly fabrication. The ceramic microsphere resonators can be readily assembled into nearly arbitrary arrangements and complex aggregates, such as dimers, trimers, quadrumers, and chains. Finally, using the heat-shrinkable substrate and dipole coupling effect, a broadband reflector with a bandwidth of 0.15 THz and a reflection of up to 95% is demonstrated.This work provides a versatile and powerful platform for terahertz all-dielectric metamaterials, with potential to be applied in a wide variety of high-efficiency terahertz devices.

Broadband terahertz rotator with an all-dielectric metasurface

Polarization manipulation is essential in developing cutting-edge photonic devices ranging from optical communication displays to solar energy harvesting. Most previous works for efficient polarization control cannot avoid utilizing metallic components that inevitably suffer from large ohmic loss and thus low operational efficiency.Replacing metallic components with Mie resonance-based dielectric resonators will largely suppress the ohmic loss toward high-efficiency metamaterial devices. Here, we propose an efficient approach for broadband, highquality polarization rotation operating in transmission mode with all-dielectric metamaterials in the terahertz regime. By separating the orthogonal polarization components in space, we obtain rotated output waves with a conversion efficiency of 67.5%. The proposed polarization manipulation strategy shows impressive robustness and flexibility in designing metadevices of both linear-and circular-polarization incidences.

Subwavelength on-chip light focusing with bigradient all-dielectric metamaterials for dense photonic integration

Photonic integrated circuits(PICs)provide a promising platform for miniaturized on-chip optical systems for communication,computation,and sensing applications.The dense integration of photonic components is one of the keys to exploit the advantages of PIC.Although light focusing is a fundamental and indispensable function in PICs,focusing light at the micro/nanometer-scale is challenging.Here,a bigradient on-chip metalens(BOML)is proposed to achieve ultrasmall focal lengths and spot sizes at the subwavelength scale for dense PICs.The design of BOML combines gradient geometry and gradient refractive index into one metalens by simultaneously engineering the length and width of subwavelength silicon slots.With a small device footprint of only 168μm,the BOML achieves efficient on-chip focusing with the recordbreaking figure-of-merits,which are the ratio of wavelength to focal length/spot size(0.268 and 2.83)and numerical aperture(1.78).Leveraging on the Fresnel design,the footprint of BOML is further reduced by 55.1%,and the numerical aperture is enhanced to 1.9.The demonstration of mode conversion and beam steering with efficiency over 80%and a tilting range of 7.2°holds the potential for highly dense on-chip photonic systems for optical communication,optical sensing,nonlinear optics,and neural networks for deep learning.

Dual non-diffractive terahertz beam generators based on all-dielectric metasurface

The applications of terahertz(THz)technology can be greatly extended using non-diffractive beams with unique field distributions and non-diffiactive transmission characteristics.Here,we design and experimentally demonstrate a set of dual non-diffractive THz beam generators based on an all-dielectric metasurface.Two kinds of non-diffractive beams with dramatically opposite focusing properties,Bessel beam and abruptly autofocus-ing(AAF)beam,are considered.A Bessel beam with longdistance non-diffractive characteristics and an AAF beam with low energy during transmission and abruptly increased energy near the focus are generated for x-and^-polarized incident waves,respectively.These two kinds of beams are characterized and the results agree well with simulations.In addition,we show numerically that these two kinds of beams can also carry orbital angular momentum by further imposing proper angular phases in the design.We believe that these metasurface-based beam generators have great potential use in THz imaging,communications,non-destructive evaluation,and many other fields.

High-efficiency all-dielectric transmission metasurface for linearly polarized light in the visible region

We propose and numerically investigate an efficient transmission-mode metasurface that consists of quasicontinuous trapezoid-shaped crystalline silicon nanoantennas on a quartz substrate. This metasurface provides a linear phase gradient and realizes both full 2π phase shift and high transmission efficiency in the operating wavelength range from 740 to 780 nm. At the central wavelength around 751 nm, the total transmission efficiency is up to 88.0% and the section of the desired anomalous refraction is 80.4%. The anomalous refraction angle is 29.62°, and larger refraction angle can be achieved by changing the period of the super cell. We demonstrate a refraction angle as large as 38.59°, and the anomalous transmission efficiency reaches 76.6% at wavelength of 741 nm. It is worth mentioning that the structure is much simpler than conventional metasurfaces based on arrays of discrete nanoantennas. Our research may pave the way for designing efficient all-dielectric phase-gradient metasurfaces and applying them in integrated optical devices for wavefront control.

All-dielectric metamaterial frequency selective surface based on spatialarrangement ceramic resonators

In this paper,we demonstrate a method of designing all-dielectric metamaterial frequency selective surface(FSS)with ceramic resonators in spatial arrangement.Compared with the traditional way,spatial arrangement provides a flexible way to handle the permutation and combination of different ceramic resonators.With this method,the resonance response can be adjusted easily to achieve pass/stop band effects.As an example,a stop band spatial arrangement all-dielectric metamaterial FSS is designed.Its working band is in 11.65–12.23 GHz.By adjusting permittivity and geometrical parameters of ceramic resonators,we can easily modulate the resonances,band pass or band stop characteristic,as well as the working band.

All-dielectric metamaterial frequency selective surface

Frequency selective surface(FSS)has been extensively studied due to its potential applications in radomes,antenna reflectors,high-impedance surfaces and absorbers.Recently,a new principle of designing FSS has been proposed and mainly studied in two levels.In the level of materials,dielectric materials instead of metallic patterns are capable of achieving more functional performance in FSS design.Moreover,FSSs made of dielectric materials can be used in different extreme environments,depending on their electrical,thermal or mechanical properties.In the level of design principle,the theory of metamaterial can be used to design FSS in a convenient and concise way.In this review paper,we provide a brief summary about the recent progress in all-dielectric metamaterial frequency selective surface(ADM-FSS).The basic principle of designing ADM-FSS is summarized.As significant tools,Mie theory and dielectric resonator(DR)theory are given which illustrate clearly how they are used in the FSS design.Then,several design cases including dielectric particle-based ADM-FSS and dielectric network-based ADM-FSS are introduced and reviewed.After a discussion of these two types of ADM-FSSs,we reviewed the existing fabrication techniques that are used in building the experiment samples.Finally,issues and challenges regarding the rapid fabrication techniques and further development aspects are discussed.

Polarization insensitive metamaterial absorber based on E-shaped all-dielectric structure

In this paper,we designed a metamaterial absorber performed in microwave frequency band.This absorber is composed of E-shaped dielectrics which are arranged along different directions.The E-shaped all-dielectric structure is made of microwave ceramics with high permittivity and low loss.Within about 1 GHz frequency band,more than 86%absorption efficiency was observed for this metamaterial absorber.This absorber is polarization insensitive and is stable for incident angles.It is figured out that the polarization insensitive absorption is caused by the nearly located varied resonant modes which are excited by the E-shaped all-dielectric resonators with the same size but in the different direction.The E-shaped dielectric absorber contains intensive resonant points.Our research work paves a way for designing all-dielectric absorber.

Analysis of triple-band binary metamaterial absorber based on low-permittivity all-dielectric resonance surface

Different from the conventional metamaterial absorbers(MAs),which used metal resonance surface and ternary structure(metal–dielectric-backplane),as an alternative route,the all-dielectric resonance surface(ADRS)made of single low-permittivity dielectric is proposed to design binary(dielectric backplane)metamaterial absorber(BMA).As an illustration,a triple-band BMA composed of ADRS with a metallic backplane is designed and fabricated,where the ADRS incorporates two dielectric layers with different hole-array structures.The absorbing mechanisms of this kind of absorber are analyzed via analyzing configurations of power loss density,electric field and magnetic field,as well as investigating dependences of absorbing properties on structure dimensions.The study indicates that the structural design of ADRS leads to reverse magnetic field rings distributed inside the dielectric,forming the strong couplings at the resonance frequencies.The proposed BMA relies on low-permittivity ADRS,with the rapid preparation and low cost greatly simplifying the design of MAs.The current concept is also suitable to design multi-band and broadband MAs worked at other bands,by changing the structural design of ADRS.

Reconfigurable and polarization-dependent optical filtering for transflective full-color generation utilizing low-loss phase-change materials

All-dielectric metasurface, which features low optical absorptance and high resolution, is becoming a promising candidate for full-color generation. However, the optical response of current metamaterials is fixed and lacks active tuning. In this work, we demonstrate a reconfigurable and polarization-dependent active color generation technique by incorporating low-loss phase change materials(PCMs) and CaF_2 all-dielectric substrate. Based on the strong Mie resonance effect and low optical absorption structure, a transflective, full-color with high color purity and gamut value is achieved. The spectrum can be dynamically manipulated by changing either the polarization of incident light or the PCM state. High transmittance and reflectance can be simultaneously achieved by using low-loss PCMs and substrate. The novel active metasurfaces can bring new inspiration in the areas of optical encryption, anti-counterfeiting, and display technologies.

全绝缘自支撑电缆系统设计(英文)

All-dielectric self-supporting(ADSS) cables are installed along with transmission line for the purpose of communication.During installation the outer layer of the cable is hydrophobic and is not prone to dry band arcing. These cables become less hydrophobic over time and become vulnerable to dry band arcing.This loss in hydrophobicity is because of the contamination formed on the outer layer due to pollution.This is one of the reasons which cause cable failure.Considerable amount of losses will be incurred on the occurrence of a cable failure as the cables are also leased to other companies.An improved equivalent circuit is used to calculate the voltage and current distribution of the double circuit line.A three-phase single circuit line and a three-phase double circuit line are used to calculate their corresponding voltage distribution and current distribution.The results could be used to predict dry band arcing on similar models.The method used considers sag,span and pollution on ADSS cable.

Cylindrical vector beams reveal radiationless anapole condition in a resonant state

Nonscattering optical anapole condition is corresponding to the excitation of radiationless field distributions in open resonators,which offers new degrees of freedom for tailoring light-matter interaction.Conventional mechanisms for achieving such a condition relies on sophisticated manipulation of electromagnetic multipolar moments of all orders to guarantee superpositions of suppressed moment strengths at the same wavelength.In contrast,here we report on the excitation of optical radiationless anapole hidden in a resonant state of a Si nanoparticle utilizing a tightly focused radially polarized(RP)beam.The coexistence of magnetic resonant state and anapole condition at the same wavelength further enables the triggering of resonant state by a tightly focused azimuthally polarized(AP)beam whose corresponding electric multipole coefficient could be zero.As a result,high contrast inter-transition between radiationless anapole condition and ideal magnetic resonant scattering can be achieved experimentally in visible spectrum.The proposed mechanism is general which can be realized in different types of nanostructures.Our results showcase that the unique combination of structured light and structured Mie resonances could provide new degrees of freedom for tailoring light-matter interaction,which might shed new light on functional meta-optics.

Achieving a multi-band metamaterial perfect absorber via a hexagonal ring dielectric resonator

A multi-band absorber composed of high-permittivity hexagonal ring dielectric resonators and a metallic ground plate is designed in the microwave band. Near-unity absorptions around 9.785 GHz, 11.525 GHz, and 12.37 GHz are observed for this metamaterial absorber. The dielectric hexagonal ring resonator is made of microwave ceramics with high permittivity and low loss. The mechanism for the near-unity absorption is investigated via the dielectric resonator theory. It is found that the absorption results from electric and magnetic resonances where enhanced electromagnetic fields are excited inside the dielectric resonator. In addition, the resonance modes of the hexagonal resonator are similar to those of standard rectangle resonators and can be used for analyzing hexagonal absorbers. Our work provides a new research method as well as a solid foundation for designing and analyzing dielectric metamaterial absorbers with complex shapes.