Ultrabroadband optical circular polarizers consisting of double-helical nanowire structures

Single-double-band switchable optical circular polarizers based on surface plasmon resonance

A single-double-band switchable circular polarization filter based on surface plasmon resonance exhibits significant potential for applications in fields such as communication and sensing due to its adjustable, low-cost, and easy integration features. In this study, we propose a bi-layer rod nanostructure and use FEM simulation to study the transmission spectra of the structure. The results demonstrate that the structure exhibits both single- and double-band circular polarization filtering effects, which can be switched by varying geometric parameters such as the distance between the two layers and the width of nanorods. Furthermore, the filtering effects of both single- and double-band are highly dependent on the length of the nanorods, with average extinction rates reaching 486 and 2020/129, respectively; the operating bandwidths (defined as extinction ratio >10) can reach 170 nm and 35 nm/70 nm, respectively. The underlying physical mechanisms are clarified by analyzing the electric dipole, magnetic dipole resonance modes, and induced chiral fields on nanostructures.

Metasurface-based circular polarizer with a controllable phase and its application in holographic imaging

A diatomic circular polarizer based on a single-layered metasurface is proposed. This metasurface circular polarizer carries the controllable phase besides the desired circular dichroism, which is different from the existing circular polarizers. The diatoms contain two nanoholes equivalent to half-wave plates with a specified cross angle and a fixed phase difference. The alternative circular polarization transmission of this circular polarizer depends on the relative angular position of diatoms, and the controllable phase of this circular polarizer can be adjusted through rotating nanoholes. The generation of the optical vortex and holographic imaging verifies the polarization and phase manipulation of the diatomic circular polarizer. The numerical simulations and the experimental measurement give the powerful verification. Simple design, compact structure, and poly-functionality enable the wide applications of circular polarizer in integrated and polarized optics.

Single-Layer Transmissive Chiral Plasma Metasurface with High Circular Polarization Extinction Ratio in Visible Wavelength

Chiral metamaterials are extensively applied in the fields of photoelectric detection, biomedical diagnostics and micro-nano polarization imaging. Currently, single-layer chiral metamaterials are unfortunately limited by several issues, such as a weaker circular polarization extinction ratio and circular polarization transmittance difference. To tackle these issues, a single-layer transmissive chiral plasma metasurface (SCPMs) suitable for visible wavelength is proposed in this paper. Its basic unit is composed of double orthogonal rectangular slots and a spatial π/4 inclined arrangement of the rectangular slot to constitute a chiral structure. Each rectangular slot structure has characteristics that enable the SCPMs to easily achieve a high circular polarization extinction ratio and strong circular polarization transmittance difference. Both the circular polarization extinction ratio and circular polarization transmittance difference of the SCPMs reach over 1000 and 0.28 at a wavelength of 532 nm, respectively. In addition, the SCPMs is fabricated via the thermally evaporated deposition technique and focused ion beam system. This compact structure coupled with a simple process and excellent properties enhances its applicability for the control and detection of polarization, especially during integration with linear polarizers, to achieve the fabrication of a division-of-focal-plane full-Stokes polarimeter.

A Third Angular Momentum of Photons

Photons that acquire orbital angular momentum move in a helical path and are observed as a light ring. During helical motion, if a force is applied perpendicular to the direction of motion, an additional radial angular momentum is introduced, and alternate dark spots appear on the light ring. Here, a third, centrifugal angular momentum has been added by twisting the helical path further according to the three-step hierarchical assembly of helical organic nanowires. Attaining a third angular momentum is the theoretical limit for a photon. The additional angular momentum converts the dimensionless photon to a hollow spherical photon condensate with interactive dark regions. A stream of these photon condensates can interfere like a wave or disintegrate like matter, similar to the behavior of electrons.

Broad-band Chiral Absorbance of Visible Light

The amplification of chiral absorbance and emission is a primary figure of merit for the design of chiral chromophores. However, for dyes to be practically relevant in chiroptical applications, they must also absorb and/or emit chiral light over broad wavelength ranges. We investigate the interplay between molecular symmetry and broad-band chiral absorbance in a series of [6]helicenes. We find that an asymmetric [6]helicene containing two distinct chromophores absorbs chiral light across a much wider wavelength range than the symmetric [6]helicenes investigated here. Chemically reducing the helicenes shifts the absorption edge of the ECD spectra into the near-infrared wavelength range while preserving broad chiral absorption, producing a [6]helicene that absorbs a single handedness of light across the entire visible wavelength range.

Metasurface circular polarizer based on rotational symmetric nanoholes

A circular polarizer is proposed based on a single layered metasurface. This metasurface circular polarizer is composed of L-shaped nanoholes etched on the silver film. The L-shaped nanoholes are rotational symmetric, and the special symmetric structure determines the polarization selection transmission of the metasurface. The theoretical analysis elaborates the design process of the metasurface circular polarizer. The proposed metasurface circular polarizers have good polarization selective transmittance, and more interestingly, they take on the opposite circular dichroism at different wavebands. The numerical simulations and experiment measurement confirm the circular dichroism of the proposed circular polarizers. The compact design, ultrathin thickness and available performance can expand the applications of the metasurface circular polarizers in the integrated optics.

Constructing Metastructures with Broadband Electromagnetic Functionality

Electromagnetic metastructures stand for the artificial structures with a characteristic size smaller than the wavelength, which may efficiently manipulate the states of light. However, their applications are often restricted by the bandwidth of the electromagnetic response of the metastructures. It is therefore essential to reassert the principles in constructing broadband electromagnetic metastructures. Herein, after summarizing the conventional approaches for achieving broadband electromagnetic functionality, some recent developments in realizing broadband electromagnetic response by dispersion compensation, nonresonant effects, and several trade-off approaches are reviewed, followed by some perspectives for the future development of broadband metamaterials. It is anticipated that broadband metastructures will have even more substantial applications in optoelectronics, energy harvesting, and information technology.

Imaging three-dimensional magnetic systems with x-rays

Recent progress in nanofabrication and additive manufacturing have facilitated the building of nanometer-scale three-dimensional (3D) structures, that promise to lead to an emergence of new functionalities within a number of fields, compared to state-of-the-art two dimensional systems. In magnetism, the move to 3D systems offers the possibility for novel magnetic properties not available in planar systems, as well as enhanced performance, both of which are key for the development of new technological applications. In this review paper we will focus our attention on 3D magnetic systems and how their magnetic configuration can be retrieved using x-ray magnetic nanotomography. We will start with an introduction to magnetic materials, and their relevance to our everyday life, along with the growing impact that they will have in the coming years in, for example, reducing energy consumption. We will then briefly introduce common methods used to study magnetic materials, such as electron holography, neutron and x-ray imaging. In particular, we will focus on x-ray magnetic circular dichroism (XMCD) and how it can be used to image magnetic moment configurations. As a next step we will introduce tomography for 3D imaging, and how it can be adapted to study magnetic materials. Particular attention will be given to explaining the reconstruction algorithms that can be used to retrieve the magnetic moment configuration from the experimental data, as these represent one of the main challenges so far, as well as the different experimental geometries that are available. Recent experimental results will be used as specific examples to guide the reader through each step in order to make sure that the paper will be accessible for those interested in the topic that do not have a specialized background on magnetic imaging. Finally, we will describe the future prospects of such studies, identifying the current challenges facing the field, and how these can be tackled. In particular we will highlight the exciting possibilities offered by the next generation of synchrotron sources which will deliver diffraction limited beams, as well as with the extension of well-established methodologies currently implemented for the study of two-dimensional magnetic materials to achieve higher dimensional investigations.

Broadband circular polarizer based on twisted plasmonic nano-disks

The broadband circular polarizer based on the surface plasmon of a metal nanostructure has important practical application due to its adjustability, low cost, and easy integration. In this article, a twisted multilayer double semi-disc structure is designed to allow for a broadband circular polarizer, whose transmission spectra are simulated using the finite element method under left-handed circularly polarized and right-handed circularly polarized extinction. The results show a large extinction ratio up to 900 for properly designed geometry and a broad bandwidth (defined as extinction ratio >10) of 50-160 nm. Additionally, the broadband position, bandwidth, and extinction rate are all sensitive to some geometric parameters, such as rotation angle, disk radius, and more, and the mechanism of this phenomenon is clarified by exploring the plasmon electromagnetic resonance on nanostructures.

Tunable broadband circular polarizer based on ultrahigh-order surface plasmonic resonance

Circular polarizers have potential applications in optical communication and liquid crystal display. In this paper, a multilayer twisted nanoring-rod nanostructure is designed. The finite element method is used to explore the surface plasmon of the structure under the excitation of left-handed circularly polarized light and right-handed circularly polarized light. The results show that the structure can be used as a polarizer in the tunable operating bandwidth of 400-1290 nm, and tunability is achieved by changing the filling medium near the structure instead of the geometric parameters of the nanostructures. We qualitatively reveal the physical mechanism of this phenomenon from the perspective of plasmon resonance coupling by plotting the charge distribution at several specific wavelengths.

Chiroptical response of a single plasmonic nanohelix

We investigate the chiroptical response of a single plasmonic nanohelix interacting with a weakly focused circularly polarized Gaussian beam. The optical scattering at the fundamental resonance is characterized experimentally and numerically. The angularly resolved scattering of the excited nanohelix is verified experimentally and it validates the numerical results. We employ a multipole decomposition analysis to study the fundamental and first higher-order resonance of the nanohelix, explaining their chiral properties in terms of the formation of chiral dipoles.

Chiroptically Active Metallic Nanohelices with Helical Anisotropy

Manipulation of circular polarization states of light in the ultraviolet-visible-near infrared range, which cannot be operated using natural materials, is of fundamental interest derived from a wide range of chirality-associated optical, chemical and biological applications. To achieve this objective, chiral metamaterials made of metallic nanohelices (NHs) have been artificially designed and generated. The circular polarization manipulation is as a result of helical anisotropy and localized surface plasmon resonance of metallic NHs, and engineering helical structures and metallic composition gives rise to flexible tailoring of the circular polarization manipulation. Herein, we review the latest development of metallic NHs in terms of fabrication, fundamentally optical and magnetic properties, and chiroptical applications, followed by envisioning prospective applications closely related to molecular chirality. We strongly believe that the helicity-induced anisotropy of metallic NHs will provide us a wide range of opportunities to tackle some prominent problems and challenges intrinsically associated with the natural homochirality.

All-dielectric metasurface circular dichroism waveplate

We propose and experimentally demonstrate a high efficient circularly polarizing dichroism waveplate (CPDW) using a Si-based all-dielectric 2Dchiral metasurface. We demonstrate that the CPDW exhibits a unique dichroism in that it functions as a transmissive quarter waveplate for one of either left-or right-handed circularly polarized incident lightand a reflective mirror for the opposite polarization. The circular polarization dichroism (CPD = I_RCP − I_LCP) in transmission at wavelength ~1.5 μm reaches 97% and the extinction ratio (ER = I_RCP/I_LCP) is as high as 345:1. Experimental fabrications and measurements of the proposed all-dielectric metasurface are implemented and found to be in excellent agreement with the simulations. The proposed all-dielectric chiral metasurface is of advantages of high-dichroism, easy-fabrication and standard semiconductor fabrication techniques compatible, which could lead to enhanced security in fiber and free-space communications, as well as imaging and sensing applications for circularly polarized light with a highly integrated photonic platform.

Broadband circularly polarizing dichroism with high efficient plasmonic helical surface

We propose and experimentally demonstrate a broadband and high efficient circularly polarizing dichroism using a simple single-cycle and single-helical plasmonic surface array arranged in square lattice. Two types of helical surface structures (partially or completely covered with a gold film) are investigated. It is shown that the circular polarization dichroism in the mid-IR range (3µm - 5µm) can reach 80% (when the surface is partially covered with gold) or 65% (when the surface is completely covered with gold) with a single-cycle and single-helical surface. Experimental fabrications of the proposed helical plasmonic surface are implemented with direct 3D laser writing followed by electron beam evaporation deposition of gold. The experimental evaluations of the circular polarization dichroism are in excellent agreement with the simulation. The proposed helical surface structure is of advantages of easy-fabrication, high-dichroism and scalable to other frequencies as a high efficient broadband circular polarizer.

Tailoring chiro-optical effects by helical nanowire arrangement

In this work, we experimentally investigate the chiro-optical properties of 3D metallic helical systems at optical frequencies. Both single and triple-nanowire geometries have been studied. In particular, we found that in single-helical nanostructures, the enhancement of chiro-optical effects achievable by geometrical design is limited, especially with respect to the operation wavelength and the circular polarization conversion purity. Conversely, in the triple-helical nanowire configuration, the dominant interaction is the coupling among the intertwined coaxial helices which is driven by a symmetric spatial arrangement. Consequently, a general improvement in the g-factor, extinction ratio and signal-to-noise-ratio is achieved in a broad spectral range. Moreover, while in single-helical nanowires a mixed linear and circular birefringence results in an optical activity strongly dependent on the sample orientation and wavelength, in the triple-helical nanowire configuration, the obtained purely circular birefringence leads to a large optical activity up to 8°, independent of the sample angle, and extending in a broad band of 500 nm in the visible range. These results demonstrate a strong correlation between the configurational internal interactions and the chiral feature designation, which can be effectively exploited for nanoscale chiral device engineering.

Triple-helical nanowires by tomographic rotatory growth for chiral photonics

Three dimensional (3D) helical chiral metamaterials resulted effective in manipulating circularly polarized light in the visible-infrared for advanced nano-photonics. Their potentialities are severely limited by the lack of full rotational symmetry preventing broadband operation, high signal-to-noise ratio, and inducing high optical activity sensitivity to structure orientation. Complex intertwined 3D structures like Multiple-Helical Nanowires could overcome these limitations, allowing the achievement of several chiro-optical effects combining chirality and isotropy.

Here we report 3D triple-helical nanowires, engineered by the innovative Tomographic Rotatory Growth, based on Focused Ion Beam Induced Deposition. These three dimensional nanostructures show up to 37% of circular dichroism in a broad range (500-1000 nm), with a high signal-to-noise ratio (up to 24 dB). Optical activity up to 8° only due to the circular birefringence is also shown, tracing the way towards chiral photonic devices which can be integrated in optical nanocircuits to modulate the visible light polarization.

N-single-helix photonic-metamaterial based broadband optical range circular polarizer by induced phase lags between helices

In this work, we have designed a photonic-metamaterial based broadband circular polarizer using N=4 phase-lagged aluminum single helices arranged in a square array as a unit cell. The effect of phase differences between the helices in an array on the optical performance of the structure is studied, and a comparative study is done with that of multi-intertwined helices. It is observed that the proposed metamaterial structure shows circular polarization sensitivity over a broad optical wavelength range (≈450-900  nm), with improved optical performance in average extinction ratio and broad positive circular dichroism in comparison to multiple intertwined helices. The induced phase lag between the helices in a square-array based unit cell reduces the linear birefringence and leads to the recovery of circular space symmetry in the structure.

Infrared broadband quarter-wave and half-wave plates synthesized from anisotropic Bézier metasurfaces

In this work a new technique for synthesizing metamaterials using Bézier surfaces is introduced. First, the computational efficiency for the optimization of a reconfigurable Bézier quarter-wave plate metasurface is compared to the popular technique of optimizing pixelized surfaces via a binary Genetic Algorithm (GA). For the presented design methodology, a real valued optimization technique is employed which is based on the Covariance Matrix Adaptation Evolutionary Strategy (CMA-ES). When compared to the GA, the optimizations of Bézier surfaces using CMA-ES are shown to consistently arrive at better solutions with an order of magnitude reduction in the required number of function evaluations. Additionally, more examples of Bézier metasurfaces are presented in the form of broadband quarter-wave and half-wave plate designs operating at optical wavelengths, subsequently exhibiting bandwidths which outperform metasurface designs found in the current literature.

Tapered N-helical metamaterials with three-fold rotational symmetry as improved circular polarizers

Chiral helix-based metamaterials can potentially serve as compact and broadband circular polarizers. We have recently shown that the physics of structures composed of multiple intertwined helices, so called N-helices with N being an integer multiple of 4, is distinct from that of structures made of single circular helices (N = 1). In particular, undesired circular polarization conversion is strictly eliminated for N = 4 helices arranged on a square lattice. However, the fabrication of such structures for infrared/visible operation wavelengths still poses very significant challenges. Thus, we here revisit the possibility of reducing N from 4 to 3, which would ease micro-fabrication considerably. We show analytically that N = 3 helices arranged on a hexagonal lattice exhibit strictly vanishing circular polarization conversion. N = 3 is the smallest option as N = 2 obviously leads to linear birefringence. To additionally improve the circular-polarizer operation bandwidth and the extinction ratio while maintaining high transmission for the wanted polarization and zero conversion, we also investigate by numerical calculations N = 3 helices with tapered diameter along the helix axis. We find operation bandwidths as large as 2.4 octaves.

High performance bianisotropic metasurfaces: asymmetric transmission of light

It is experimentally shown that bianisotropic metasurfaces allow for extreme polarization control of light with high performance. A metasurface providing asymmetric transmission (i.e., polarization conversion) of circularly polarized light is reported at a wavelength of 1.5  μm. The experimental transmittance and extinction ratio are 50% and 20:1, which represents an order of magnitude improvement over previous optical structures exhibiting asymmetric transmission. The metasurface consists of patterned gold sheets that are spaced at a subwavelength distance from each other. The same design and fabrication processes can be used in the future to completely control the phase, amplitude, and polarization of light.

Broadband infrared quarter wave plate realized through perpendicular-to-helical-axis wave propagation in a helix array

The ability of helix arrays to filter circularly polarized light efficiently when the light propagates parallel to the helical axis has been demonstrated recently. In this Letter, we present a broadband linear-to-circular polarization transformer composed of metal microhelices. The device provides significant transformation performance combined with high transmittance over a broad infrared wave band. High performance is achieved through fine adjustment of a finite-element electromagnetic model. The array design assumes wave propagation perpendicular to the helical axis, which distinguishes it from well-studied analogous designs that filter light propagating parallel to the helical axis. To the best of our knowledge, this is the first time this scheme has been realized in the infrared range.

Cholesteric liquid crystals with a broad light reflection band

The cholesteric-liquid-crystalline structure, which concerns the organization of chromatin, collagen, chitin, or cellulose, is omnipresent in living matter. In technology, it is found in temperature and pressure sensors, supertwisted nematic liquid crystal displays, optical filters, reflective devices, or cosmetics. A cholesteric liquid crystal reflects light because of its helical structure. The reflection is selective - the bandwidth is limited to a few tens of nanometers and the reflectance is equal to at most 50% for unpolarized incident light, which is a consequence of the polarization-selectivity rule. These limits must be exceeded for innovative applications like polarizer-free reflective displays, broadband polarizers, optical data storage media, polarization-independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. Novel cholesteric-liquid-crystalline architectures with the related fabrication procedures must therefore be developed. This article reviews solutions found in living matter and laboratories to broaden the bandwidth around a central reflection wavelength, do without the polarization-selectivity rule and go beyond the reflectance limit.

Engineering of micro- and nanostructured surfaces with anisotropic geometries and properties

Widespread approaches to fabricate surfaces with robust micro- and nanostructured topographies have been stimulated by opportunities to enhance interface performance by combining physical and chemical effects. In particular, arrays of asymmetric surface features, such as arrays of grooves, inclined pillars, and helical protrusions, have been shown to impart unique anisotropy in properties including wetting, adhesion, thermal and/or electrical conductivity, optical activity, and capability to direct cell growth. These properties are of wide interest for applications including energy conversion, microelectronics, chemical and biological sensing, and bioengineering. However, fabrication of asymmetric surface features often pushes the limits of traditional etching and deposition techniques, making it challenging to produce the desired surfaces in a scalable and cost-effective manner. We review and classify approaches to fabricate arrays of asymmetric 2D and 3D surface features, in polymers, metals, and ceramics. Analytical and empirical relationships among geometries, materials, and surface properties are discussed, especially in the context of the applications mentioned above. Further, opportunities for new fabrication methods that combine lithography with principles of self-assembly are identified, aiming to establish design principles for fabrication of arbitrary 3D surface textures over large areas.

Topological activity in Bragg elliptical twisted fibers

We have theoretically shown that Bragg twisted elliptical fibers manifest, in certain spectral regions, the property of topological activity--the ability to change in the reflected field the topological charge of incoming optical vortices and fundamental modes by two units. This property could be used for narrowband generation of optical vortices from Gaussian beams and for changing the topological charge of incoming optical vortices.

Polarization characteristics of the metallic structure with elliptically helical metamaterials

In the last few years, there has been growing interest in the research of the polarizing optics consisting of sub-wavelength metamaterials due to the advantages of broad wavelength ranges, high temperature durability, and compact structures. So far, the metallic structure with the sub-wavelength metamaterials has been proved to achieve the linearly and the circularly polarized light. Therefore, there should be one question raised easily: Is it possible for the metallic structure with sub-wavelength metamaterials to generate the elliptically polarized light? To answer this question, we proposed a metallic structure with elliptically helical nanowires, and analyzed the polarization states of the transmitted light using FDTD method. It is confirmed that this metallic structure does have a giant elliptical dichroism. Furthermore, we also compared the distinct optical performances of elliptical single-, double-, three-, and four-helixes, and made a qualitative explanation for them.

Higher extinction ratio circular polarizers with hetero-structured double-helical metamaterials

We have recently shown circular polarizers with the homo-structured double-helical metamaterials, which have broader operation bands than those of the single-helical structures [Opt. Lett. 35, 2588 (2010)]. However, trying to get more operation bands deteriorates the extinction ratio. In this paper, we proposed circular polarizers with hetero-structured double-helical metamaterials. The extinction ratios of these circular polarizers are two times higher than those with homo-structured double-helical metamaterials. Furthermore, we qualitatively explained the phenomenon of the higher extinction ratio from the viewpoint of the interaction between the two helix-wires in a double-helical unit.