Tailoring magnetic and electric resonances with dielectric nanocubes for broadband and high-efficiency unidirectional scattering

The interference of optically induced electric and magnetic resonances in high-refractive-index dielectric nanoparticles provides a new approach to control and shape the scattering patterns of light in the field of nanophotonics. In this Letter, we spectrally tune the electric and magnetic resonances by varying the geometry of a single isolated lead telluride(Pb Te) dielectric nanocube. Then, we overlap the electric dipole resonance and magnetic dipole resonance to suppress backward scattering and enhance forward scattering in the resonance region.Furthermore, a broadband unidirectional scattering is achieved by structuring the dielectric nanocuboids as a trimer antenna.     

Chirality Driven by Magnetic Dipole Response for Demultiplexing of Surface Waves

Surface electromagnetic waves are characterized by the intrinsic spin‐orbit interaction which results in the fascinating spin‐momentum locking. Therefore, directional coupling of light to surface waves can be achieved through chiral nanoantennas. Here, we show that dielectric nanoantenna provides chiral response with strong spectral dependence due to the interference of electric and magnetic dipole momenta when placed in the vicinity of the metal‐air interface. Remarkably, chiral behaviour in the proposed scheme does not require elliptical polarization of the pump beam or the geometric chirality of the nanoantenna. We show that the proposed ultracompact and simple dielectric nanoantenna allows for both directional launching of surface plasmon polaritons on a thin gold film and their demultiplexing with a high spectral resolution.     

Enhanced light extraction with silicon nanoantenna arrays for white light LED applications

High conversion efficiency and quantum efficiency is essential for the phosphor in an efficient phosphor-based white light LEDs. Here, based on the coherent harmonic and the random independent emitter model, we demonstrate theoretically that the silicon nanoantenna array can dramatically enhance the output power of emitters in a phosphor layer by investigating the far-field radiation enhancement of an electric dipole assisted by silicon nanopillars in a waveguide structure. Compared with the plasmonic silver nanoantenna array, the silicon nanoantenna array can increase the enhancement factor of light extraction efficiency (LEE) over 50% for the dipole source at the wavelength of 620 nm, thus showing potential applications in white light LEDs. The enhanced LEE is ascribed to the low-loss directional light scattering of silicon nanoantennas and the strong guided mode resonances caused by their array. The calculation results also indicate that the far-field radiation can be tailored significantly by changing the aspect ratio of silicon nanopillars while presenting a good directivity. Our research is expected to give more insights into the design and optimization of the solid-state lighting, gaining and lasing systems by integrating silicon-based nanoantennas.     

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.     

基于石墨烯加载的不对称纳米天线对的表面等离激元单向耦合器

本文设计并数值研究了一种石墨烯加载的不对称金属纳米天线对结构.利用石墨烯费米能级的动态调控特性,实现了电控表面等离激元的单向传输.类似于传统的三明治型纳米天线结构,设计的不对称金属纳米天线对结构可以等效为两个共振的磁偶极子,由于磁偶极子辐射电磁波的干涉,将导致单向传输效应.通过计算腔中的电场分布,发现石墨烯的调谐能力与石墨烯区域的电场强度成正比关系.以上现象都可以通过等效电路模型进行理论解释.此外,该结构具有小尺寸、高效率、宽带宽和易于光电集成等优点,在未来的光子集成与光电子学领域将具有重要的应用.     

Circularly polarized unidirectional emission via a coupled plasmonic spiral antenna

In this Letter, we study the emission properties of an electric dipole emitter coupled to a plasmonic spiral structure. The plasmonic spiral structure functions as an optical antenna, coupling the electric dipole emission into circularly polarized unidirectional emission in the far field. Increasing number of turns of the spiral leads to narrower angular width of the emission pattern in the far field. For a spiral antenna with six turns, antenna directivity of 23.5 dB with a directional emission into a narrow angular cone of 4.3° can be achieved. The emitted photons carry spin that is essentially determined by the handedness of the spiral antenna. By reversing the spiral, one can switch the polarization of the emission field between left-hand and right-hand circular polarizations. The spiral antenna may be used as a nanoscale circular polarization source in single molecule sensing, single-photo sources, and integrated photonic circuits.     

Complementary bowtie aperture for localizing and enhancing optical magnetic field

Nanoscale bowtie antenna and bowtie aperture antenna have been shown to generate strongly enhanced and localized electric fields below the diffraction limit in the optical frequency range. According to Babinet's principle, their complements will be efficient for concentrating and enhancing magnetic fields. In this Letter, we discuss the enhancement of magnetic field intensity of nanoscale complementary bowtie aperture as well as complementary bowtie aperture antenna, or diabolo nanoantenna. We show that the complementary bowtie antenna resonates at a smaller wavelength and thus is more suitable for applications near visible wavelengths. The near-field magnetic intensity can be further enhanced by the addition of groove structures that scatter surface plasmon.     

Babinet-Inverted Optical Nanoantenna Analogue of Electromagnetically Induced Transparency

A Babinet-inverted optical nanoantenna analogue of electromagnetically induced transparency based on the coupling between two magnetic dipole antennas and a magnetic octupole antenna in a Au film waveguide is demonstrated. Simulation results indicate that a pronounced elimination occurs in the radiating spectrum due to the coupling-induced radiation suppression. A two-oscillator electromagnetically induced transparency model is used to describe the antenna. The coupling coefficient between the magnetic dipole antennas and the magnetic octupole antenna is calculated using the model and is found to decline exponentially with the increase of the distance between them. Such an antenna can be directly integrated with optical waveguides or transmission lines,thus is of fundamental significance for the applications in nano-optics, such as the optical device miniaturizations and photonic circuit integrations.     

Plasmonic nanoloop array antenna

In this Letter, we create an optical nanoantenna array composed of parasitic plasmonic loops that can enhance radiation characteristics and direct the optical energy successfully. Three metallic loops inspired by the concept of the Yagi-Uda antenna are optimized around the region where they feature high scattering performance to control the radiation beam. The loop geometry, when compared to the dipole configuration, has the benefit of using the available aperture in an effective way to provide higher directivity. The angular emission of the nanoloop array antenna is highly directive, and a directivity of 8.2 dB for upward radiation is established.     

Investigation of Euler spiral nanoantenna and its application in absorption enhancement of thin film solar cell

A theoretical study on nanoantenna and its application in enhancing the performance of the thin film solar cell (TFSC) is presented. In this work, a novel design of nanoantenna i.e. Euler spiral nanoantenna (ESNA) is introduced, which has evolved after bending the conventional dipole nanoantenna in the manner of Euler spiral. The bending is performed up to an optimum length so that the antenna can equally respond to the two orthogonally polarized waves. Then the proposed nanoantenna in turnstile manner is examined for the intended application of enhancing the absorption in TFSCs. The antenna is placed on the absorber layer (Si amorphous) of the TFSC with a coating of Zinc Oxide. The simulation results show that the proposed ESNA can significantly increase the absorption in the absorber layer of the TFSC. The performance in terms of absorption and quantum efficiency of the solar cell incorporated with ESNA has been studied. ESNA confines the electric field in a larger area which results in absorption increase. The simulation results show that proposed ESNA can enhance the absorption up to 97.6% in the absorber layer and the photocurrent is enhanced by a factor of 1.39. To the best of our knowledge, this is the first study on Euler spiral nanoantenna and so as in its application with solar cells.     

Nonlocal response of electric and magnetic modes in a silver cuboid dimer

The nonlocal effect on the spontaneous emission of a silver cuboid dimer is investigated using a local analog model. Magnetic as well as electric dipole excitations are introduced to excite different gap modes. The nonlocal response of electric and magnetic modes on various parameters of gap(width and refractive index) are investigated. Unidirectional radiation is achieved by the interaction between electric and magnetic modes in both local and nonlocal models. Compared to local simulations, the resonant wavelength is blue shifted and the spontaneous emission enhancement is weakened in the nonlocal model. The relative shifts of the resonant wavelengths get larger in smaller gaps with a higher refractive index.     

Seismoelectric waves in a borehole excited by an external explosive source

The conversion of energy between seismic and electromagnetic wave fields has been described by Pride’s coupled equations in porous media. In this paper, the seismoelectric field excited by the explosive point source located at the outside of the borehole is studied. The scattering fields inside and outside a borehole are analyzed and deduced under the boundary conditions at the interface between fluid and porous media. The influences of the distance of the point source, multipole components of the eccentric explosive source, and the receiving position along the axis of vertical borehole, on the converted waves inside the borehole are all investigated. When the distance from the acoustic source to the axis of a borehole is far enough, the longitudinal and coseismic longitudinal wave packets dominate the acoustic and electric field, respectively. The three components of both electric field and magnetic field can be detected, and the radial electric field is mainly excited and converted by the dipole component. Owing to the existence of borehole, the electric fields and magnetic fields in the borehole are azimuthal. The distance from the point where the maximum amplitude of the axial components of electric field is recorded, to the origin of coordinate indicates the horizontal distance from the explosive source to the axis of vertical borehole.     

Fano resonances in heterogeneous dimers of silicon and gold nanospheres

We theoretically investigate the optical properties of dimers consisting of a gold nanosphere and a silicon nanosphere. The absorption spectrum of the gold sphere in the dimer can be significantly altered and exhibits a pronounced Fano profile. Analytical Mie theory and numerical simulations show that the Fano profile is induced by constructive and destructive interference between the incident electric field and the electric field of the magnetic dipole mode of the silicon sphere in a narrow wavelength range. The effects of the silicon sphere size, distance between the two spheres, and excitation configuration on the optical responses of the dimers are studied. Our study reveals the coherent feature of the electric fields of magnetic dipole modes in dielectric nanostructures and the strong interactions of the coherent fields with other nanophotonic structures.     

Far-field pattern analysis of 94 GHz extended hemispherical lens monopulse antennas

In this paper, the far-field patterns are investigated for double-slot antenna array placed on extended hemispherical lenses. The radiation patterns of the extended hemispherical lens fed by single double-slot antenna or double-slot antenna array are computed using ray-tracing inside the dielectric lens and electric and magnetic field integration on the spherical dielectric surface. The computation results show that the sharp null of the difference pattern is below -37dB.     

Investigation of a slot nanoantenna in optical frequency range

Following the analogy of radio frequency slot antenna and its complementary dipole, we propose the implementation of a slot nanoantenna (SNA) in the optical frequency range. Using finite-difference time-domain (FDTD) method, we investigate the electromagnetic (EM) properties of a SNA formed in a thin gold film and compare the results with the properties of a gold dipole nanoantenna (DNA) of the same dimension as the slot. It is found that the response of the SNA is very similar to the DNA, like their counterparts in the radio frequency (RF) range. The SNA can enhance the near field intensity of incident field which strongly depends on its feedgap dimension. The resonance of the SNA is influenced by its slot length; for the increasing slot length, resonant frequency decreases whereas the sharpness of resonance increases. Besides, the resonance of the SNA is found sensitive to the thickness of metal film, when the latter is smaller than the skin depth. The effect of polarization of incident field on the EM response of the SNA was examined; the field enhancement is optimum when polarization is parallel to the feedgap. Finally, we calculate the radiation patterns of the DNA and SNA and compare them with those of the RF dipole antenna. The radiation pattern of the SNA is found to be independent of its slot length when excited at resonant frequency. To the best of our knowledge, this is the first study on a slot antenna in the optical frequency.     

组合振子天线磁偶特性实验研究

通过对组合振子的实验研究,验证了磁偶极子回路所具有的优势:能够极大地改善天线低频传输以及辐射特性,具有明显的低频补偿能力。此外,通过设置磁偶回路周长比,比较了不同磁偶回路状态下的天线特性,发现双磁偶回路对于低频特性的改善要优于单磁偶回路,后者的优势主要体现在高频。根据这一现象,采用单磁偶回路结构设计工作频带为300MHz~5GHz组合振子接收天线。     

Strong enhancement of magnetic dipole emission in a multilevel electronic system

The Purcell effect is commonly used to increase light emission by enhancing the radiative decay of electric dipole transitions. In this Letter, we demonstrate that the opposite effect, namely, the inhibition of electric dipole transitions, can be used to strongly enhance light emission via magnetic dipole transitions. Specifically, by exploiting the differing symmetries of competitive electric and magnetic dipole transitions in trivalent europium, we demonstrate a fourfold enhancement of the far-field emission from the (5)D(0)→(7)F(1) magnetic dipole transition in trivalent europium. We show that this strong enhancement is well predicted by a three-level model that couples the individual Purcell enhancement factors of competitive transitions from the same excited state.     

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.     

Polarization control over electric and magnetic dipole resonances of dielectric nanoparticles on metallic films

We reveal unusually strong polarization sensitivity of electric and magnetic dipole resonances of high‐index dielectric nanoparticles placed on a metallic film. By employing dark‐field spectroscopy, we observe the polarization‐controlled transformation from high‐Q magnetic‐dipole scattering to broadband suppression of scattering associated with the electric dipole mode, and show numerically that it is accompanied by a strong enhancement of the respective fields by the nanoparticle. Our experimental data for silicon nanospheres are in an excellent agreement with both analytical calculations based on Green's function approach and the full‐wave numerical simulations. Our findings further substantiate dielectric nanoparticles as strong candidates for many applications in enhanced sensing, spectroscopy and nonlinear processes at the nanoscale.

     

基于介质谐振器原理的左手材料设计

本文通过对高介电常数介质基于介质谐振器理论进行分析,明确了利用高介电常数介质产生负介电常数或负磁导率的途径在于介质中产生具有Lorentz谐振形式的电磁响应的电偶极子或磁偶极子,指出了这种偶极子的产生来源于电磁波在介质中形成的驻波,而左手通带的形成正是由于电偶极子和磁偶极子之间的相互影响,破坏了驻波形成的条件所实现的. 模拟结果表明,通过将尺寸相同,介电常数不同的介质进行组合,使二者电谐振和磁谐振的频率点重合从而实现左手通带,最后利用高介电常数,低损耗的陶瓷进行样品制作并测试,测试结果证实了基于这一原理实 关键词： 全介质左手材料 介质谐振器 磁偶极子 电偶极子