Dielectric Chiral Metasurfaces for Second-Harmonic Generation with Strong Circular Dichroism

Dielectric chiral metasurfaces can generate harmonic waves with high efficiency and strong circular dichroism (CD), when they are supported by metallic substrates. Numerical results show that the second-harmonic generation (SHG) efficiency of about 10⁻³% for a peak pump intensity of about 5 GW cm⁻² can be achieved in the blue-UV, and the SHG CD reaches up to about 1.8, from a metasurface of Z-shaped lithium niobate nanoantenna array supported by gold substrate. Highly efficient and strong circular dichroic nonlinear responses are attributed to the plasmon-assisted local field enhancement in the dielectric chiral nanoantennae adjacent to the metallic substrate.     

Directional far-field response of a spherical nanoantenna

We study the directional far-field response of a spherical nanoantenna via engineering the plasmonic nanosphere's distance, size, and material. A unified pattern synthesis approach based on the T-matrix method and the particle swarm optimization is proposed for the directional beamforming of the nanoantenna. The angular response of the directional nanoantenna is very sensitive to the material change but is immunized to the random error of the spatial position of each particle. The physical origin of the high directionality is attributed to the coherent near-field distribution with large correlation length. This work provides the fundamental theory and physics for future nanoantenna design.     

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.     

Gradient Chiral Metamirrors for Spin‐Selective Anomalous Reflection

Metasurfaces, the phase‐engineered quasi‐2D interfaces, have attracted intensive interest due to their great capabilities in manipulating the reflection, refraction and transmission of electromagnetic waves. Here, we demonstrate the design and realization of a gradient chiral metamirror tailored for spin‐selective anomalous reflection based on the theory of Pancharatnam‐Berry phase. Asymmetric split ring resonators are employed as the basic meta‐atoms for strong circular dichroism. Dispersionless phase discontinuities are achieved by adjusting the orientation of the meta‐atoms, and spin‐dependent absorption is realized by introducing a chiral resonance. Theoretical results predict both broadband beam deflection and spin‐selective absorption for circularly polarized waves in a designer metamirror. Experimental verification of this bifunctional performance is implemented at microwave frequencies and the measured results agree well with the simulation ones. Such chiral metamirrors could pave an avenue towards spin‐selective modulation of the wavefront and might find promising applications in planar electromagnetic devices.     

Universal method for the synthesis of arbitrary polarization states radiated by a nanoantenna

Optical nanoantennas efficiently convert confined optical energy into free‐space radiation. The polarization of the emitted radiation depends mainly on nanoantenna shape, so it becomes extremely difficult to manipulate it unless the nanostructure is physically altered. Here, a simple way is demonstrated to synthetize the polarization of the radiation emitted by a single nanoantenna so that every point on the Poincaré sphere becomes attainable. The nanoantenna consists of a single scatterer created on a dielectric waveguide and fed from its both sides so that the polarization of the emitted optical radiation is controlled by the amplitude and phase of the feeding signals. The nanoantenna is created on a silicon chip using standard top‐down nanofabrication tools, but the method is universal and can be applied to other materials, wavelengths and technologies. This work will open the way towards the synthesis and control of arbitrary polarization states in nano‐optics.     

Surface Plasmon Mediated Controllable Spin‐Resolved Transmission in Meta‐Hole Structures

Chiral responses are optical responses involving circular polarizations. Controlling the chiral response in a flexible way is very important in optical manipulations. Chiral metamaterials have thus drawn enormous interest due to their flexible designing feature. However, most of the previous studies are mainly realized by designing the structure of the individual meta‐atom. Meanwhile, to enhance the response, complex design and fabrication processes are typically required. Here, by introducing spin‐dependent propagating surface plasmons and spin‐selective interference, giant spin‐resolved transmission is achieved in a simple meta‐hole structure. In this interaction process, spin‐orbital angular momentum conversion plays an essential role. By controlling the phase difference between the interference components, controllable spin‐resolved transmission is achieved. Furthermore, such method can also be applied to realize spin‐resolved excitation of surface plasmons. The proposed controlling strategy offers a versatile platform for a variety of promising applications, such as polarization control, asymmetric transmission, surface plasmon excitation, and on‐chip chiral manipulation.     

Ultra-directional forward scattering by a high refractive index dielectric T-shaped nanoantenna in the visible

In this work, we design and numerically demonstrate a touching dielectric nanoantenna with high directionality. This antenna consists of a dielectric cuboid dimer with different heights, and there are no gaps between the subunits of the dimer. Superior unidirectional scattering is achieved when the electric and magnetic dipolar modes inside the antenna satisfy the first Kerker condition. This unidirectional scattering is much more prominent than its components (i.e., the dielectric cuboid nanoantennas with different heights) in the considered spectral region. Furthermore, the radiation angle can be tailored in a 10-degree range by properly rotating the antenna along the out of axis. The off-normal scattering is due to the interference between one induced magnetic dipole and two electric dipoles inside the nanoantenna. Furthermore, we also demonstrate that similar unidirectional scattering effect can also be maintained when the antenna is close to an electric (or magnetic) dipole source, and the forward emission direction can be efficiently controlled by the relative position of the dipole source. Finally, we show that it is possible to further enhance the unidirectionality by arranging the antenna in an array and the main lobe angular beam width of the 2D far-field pattern can be reduced to 28 degree.     

Tailoring of the spectral–directional characteristics of rare-earth fluorescence by metal–dielectric planar structures

It is demonstrated that the spectrum, direction and polarization of rare-earth fluorescence can be tailored by embedding the impurity ions into a planar metal–dielectric structure (MDS). The latter was designed by spin coating a rare-earth-doped oxide film (TiO₂:Sm³⁺) onto a gold-covered glass substrate. For spectral–directional investigations of Sm³⁺ fluorescence, the MDS was attached to a semi-cylindrical prism and excited by UV light from the flat side. An angular scan revealed a strongly polarized and directional emission of Sm³⁺ from the convex side of the prism. The tuning of TiO₂ film thickness in the MDS allows a control of the polarization and direction of the emission bands. A theoretical modeling of the reflectivity of the MDS suggests that the observed angular resonances in the fluorescence emission are caused by its effective coupling with surface plasmons on the gold–dielectric interface or coupling with leaky modes in sufficiently thick dielectric films working as a waveguides.     

Characteristics of Surface Spin Waves in a Metal–Dielectric–Ferrite–Dielectric–Metal Structure

The dispersion and isofrequency characteristics of surface spin waves in a tangentially magnetized metal–dielectric–ferrite–dielectric–metal structure are investigated. A loop-like change in the isofrequency dependences for spin waves is observed in a certain range of frequencies, and the origin of the loop is always located at wave number value k → 0.     

Polarization-Independent Directional Beaming of Light by a Subwavelength Metal Slit

We present the directional beaming effect of light at the terahertz frequency by using a subwavelength slit in the metal film. The metal is dressed with anisotropic dielectric so that both the transverse electric （TE） and transverse magnetic （TM） polarized waves can be well guided on the metal surface and reach the phase matching. By using a periodical array of dielectric ridges and grooves around the slit, the guided waves can be scattered out of the slit and interfere with the transmitted light directly through the slit. The results performed by finite-difference at time-domain computations indicate that the directional beaming of light can be obtained simultaneously for both the TE and TM polarized waves after optimizing the geometric parameters. The structure may find great applications in polarization-independent optical devices such as couplers, connectors, beam collimator, and etc.     

Huygens optical elements and Yagi—Uda nanoantennas based on dielectric nanoparticles

A new class of optical nanoantennas based on dielectric nanoparticles has been proposed and their main characteristics have been analyzed. It has been shown that one dielectric nanoparticle can have the properties of a Huygens element in the optical wavelength range. A Yagi-Uda nanoantenna based on dielectric nano-particles has been studied analytically and numerically.     

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.     

Light scattering from bulk and surface spin waves in opaque ferromagnets; The influence of quadratic coupling terms

We calculate the light scattering spectrum from spin waves, for backscattering from an opaque ferromagnet with magnetization parallel to the surface. The spectra contain features arising from both bulk and surface magnons. Including the quadratic terms in the spin density in the fluctuating part of the dielectric tensor leads to an asymmetry in the Stokes/anti-Stokes ratio for scattering for both bulk and surface waves. Our calculations give a good account of the data reported by Grunberg and Metawe, for scattering from EuO.     

Design of efficient coupling configuration with L-shaped plasmoic waveguide and nanoantenna

Optical Review - A novel design for an efficient coupling configuration, which consists of a L-shaped dielectric loaded surface plasmon polaritons (DLSPPs) waveguide and L-shaped nanoantenna, is...     

An Ultra-Broadband Chirality Selective Metastructure Absorber using High Impedance Surface

In this paper, an ultra-wideband chirality selective metastructure absorber is proposed that enables differential absorption and reflection of circularly polarized waves in the terahertz (THz) range. The structure achieves circular dichroism (CD) by using asymmetrically split metal rings as fundamental meta-atoms. Most critically, the high impedance surface and air-resonant cavities are inserted separately in the meta-atoms and dielectric substrate to enhance CD and broaden the bandwidth of absorption. The metastructure absorber can achieve more than 90% absorption of right circularly polarized waves at 0.675–1.244 THz, and it can maintain more than 90% reflection of left circularly polarized waves at 0.607–1.229 THz without changing the direction of rotation. Besides, its CD can reach more than 80% at 0.687–1.213 THz with a relative bandwidth of 55.3%. Spin-selective absorption, which is closely related to breaking chiral symmetry, is investigated through power loss distribution, wide-angle incidence, and scan parameter optimization. The proposed strategy is further validated in the THz band, and the polarization selection and manipulation techniques can be applied to chiral sensing/radio-thermometry, circular polarization detectors/lasers, and molecular spectroscopy.     

Influence of polymer dielectric surface energy on thin‐film transistor performance of solution‐processed triethylsilylethynyl anthradithiophene (TES‐ADT)

This study investigates the correlation between surface energy of polymer dielectrics and the film morphology, microstructure, and thin‐film transistor performance of solution‐processed 5,11‐bis(triethylsilylethynyl) anthradithiophene (TES‐ADT) films. The low surface energy polyimide (PI) dielectric induced large grains with strong X‐ray reflections for spin‐cast TES‐ADT films in comparison to high surface en‐ ergy poly(4‐vinyl phenol) (PVP) dielectric. Furthermore, thin‐film transistors based on spin‐cast TES‐ADT films on PI dielectric exhibited enhanced electrical performance, small hysteresis, and high stability under bias stress with carrier mobility as high as 0.43 cm²/Vs and a current on/off ratio of 10⁷. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A spectral invariant representation of spectral reflectance

Spectral image acquisition as well as color image is affected by several illumination factors such as shading, gloss, and specular highlight. Spectral invariant representations for these factors were proposed for the standard dichromatic reflection model of inhomogeneous dielectric materials. However, these representations are inadequate for other characteristic materials like metal. This paper proposes a more general spectral invariant representation for obtaining reliable spectral reflectance images. Our invariant representation is derived from the standard dichromatic reflection model for dielectric materials and the extended dichromatic reflection model for metals. We proof that the invariant formulas for spectral images of natural objects preserve spectral information and are invariant to highlights, shading, surface geometry, and illumination intensity. It is proved that the conventional spectral invariant technique can be applied to metals in addition to dielectric objects. Experimental results show that the proposed spectral invariant representation is effective for image segmentation.     

Dual-band asymmetric electromagnetic wave transmission for dual polarizations in chiral metamaterial structure

In this paper, we propose a chiral metamaterial structure that enables dual-band asymmetric transmission effect for different linearly polarized electromagnetic waves. The metamaterial is composed of metallic spirals with two split-ring resonators sandwiching a dielectric slab and connecting with via hole. Strong one-way transmission of two orthogonally polarized waves at different frequency bands has been confirmed through both full-wave simulation and test on fabricated prototype at the microwave band. Analysis also shows such asymmetric transmission can be attributed to the induced asymmetric current distributions in the spiral that support strong polarization conversion and cross-polarization transmission. By scaling down the metamaterial structure, the concept could also be utilized at other frequency bands, such as submillimeter or even terahertz band and find applications in designing one-way electromagnetic wave devices or polarization spectral filters.     

Scattering of Circularly Polarized Terahertz Waves on a Graphene Nanoantenna

We present a surface current method to model the graphene rectangular nanoantenna scattering in the terahertz band with Comsol.Compared with the equivalent thin slab method,the results obtained by the surface current method are more accurate and efficient.Then the electromagnetic scattering of circularly polarized terahertz waves on graphene nanoantennas is numerically analyzed by utilizing the surface current method.The dependences of the antenna resonant frequency with the circularly polarized wave on width and length are consistent with those for the linear polarized waves.These results are proved to be useful to design efficient nanoantennas in terahertz wireless communications.     

V型纳米金球阵列天线侧向散射方向性的数值模拟

纳米天线侧向散射的方向性在生物感测以及表面增强光谱学等领域都有重要研究价值。侧向散射方向性的产生需要破坏纳米天线相对于平面波极化方向的镜像对称,由纳米粒子阵列构成的纳米天线排列灵活,易于实现非镜像对称结构。采用T-matrix方法对V型纳米金球阵列天线进行数值模拟,研究了其侧向散射的方向性,分别讨论了不同阵元数目构成的不同臂长、不同臂间开角、不同球形阵元半径、不同阵元球心间距对V型天线侧向散射方向性的影响。研究表明V型纳米阵列天线具有侧向散射方向性的可调性。