Circular Polarization States Research Articles

A Wideband Quad-Polarization Reconfigurable Metasurface Antenna

A low-profile wideband metasurface antenna with quad-polarization reconfiguration ability is presented in this paper. The proposed metasurface antenna is composed of a square patch, a metasurface formed by a lattice of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 \times 4$ </tex-math></inline-formula> periodic metal plates, and four switchable feeding probes connected with two designed single-pole double-throws switches. By properly selecting the feeding probes, the polarization of the metasurface antenna can be dynamically reconfigured among four polarization states, i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> -direction linear polarization, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -direction linear polarization, left-hand circular polarization, and right-hand circular polarization. To validate the proposed concept, a prototype operating at 5.6 GHz with a relatively height of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.067~\lambda _{0}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the operating wavelength in free space) is designed, fabricated, and measured. The measured results show that both the 10-dB impedance bandwidths and the 3-dB axial ratio bandwidths are wider than 5.1–6.0 GHz for the linear and circular polarization states. The measured maximum gains are 9.39 dBic and 9.85 dBi for the circular polarization states and the linear polarization states, respectively. The simulation and experimentation verify that the proposed antenna can achieve quad-polarization reconfigurable features in a wide frequency band, which indicate the good performance of the proposed polarization reconfigurable metasurface antenna.

Wideband Quad-Polarization Reconfigurable Antenna Using Switchable Feed Network With Stable Unidirectional Radiation Patterns

A wideband antenna with quad-polarization reconfigurable operation is proposed. It consists of a crossed tri-port feed linearly polarized (LP) radiation structure and a switchable feed network. A dual-port LP element with wideband characteristic and good port isolation is first introduced. The inherent 180° phase difference (PD) between two feed ports vastly simplifies the complexity of the design and improves the efficiency of the feed network. By assembling two grooved LP elements, the crossed tri-port feed radiation structure is formed. Meanwhile, a wideband 90° phase shifter and an unequal Wilkinson power divider are applied in designing the switchable feed network. Through controlling the PIN diodes implanted in the feed line, four working modes with equal amplitude and reconfigurable PD (0° or 90°) are achieved. Based on this, the desired quad-polarization reconfiguration, including two orthogonal linear polarization (LP) states and two orthogonal circular polarization (CP) states, is realized. Measured results show that the proposed antenna has a wide overlapped operation impedance bandwidth of 84.5%, ranging from 1.36 ~ 3.35 GHz, for all polarization states within a compact volume. The measured 3-dB AR bandwidth is 100%, covering the operation impedance frequency range. Radiation patterns exhibit unidirectional and stable quality from the point view of beam slant and split. Besides, a peak efficiency of 75% and a peak gain of 7.8 dBi are achieved.

Review of the Functions of Archimedes' Spiral Metallic Nanostructures.

Here, we have reviewed some typical plasmonic structures based on Archimedes’ spiral (AS) architectures, which can produce polarization-sensitive focusing phenomenon and generate plasmonic vortices (PVs) carrying controllable orbital angular momentum (OAM) because of the relation between the incident polarized states and the chiralities of the spiral structures. These features can be used to analyze different circular polarization states, which has been one of the rapidly developing researching topics in nanophotonics in recent years. Many investigations demonstrate that the multifunctional spiral-based plasmonic structures are excellent choices for chiral selection and generating the transmitted field with well-defined OAM. The circular polarization extinction ratio, as an evaluation criterion for the polarization selectivity of a designed structure, could be effectively improved by properly modulating the parameters of spiral structures. Such functional spiral plasmonic nanostructures are promising for applications in analyzing circular polarization light, full Stokes vector polarimetric sensors, near-field imaging, and so on.

Controlling the Bidirectional Circular Polarization States Using Ultrathin Back-to-Back Quarter-Wave Plates Cavity

Efficiently manipulating the polarization states of electromagnetic waves is of great importance for communication, imaging, and sensing. In this paper, we aim to control the circular polarization states, e.g., left-hand, or right-hand, for the bidirectional radiated waves utilizing a pair of back-to-back quarter-wave plates, which are integrated within an ultrathin cavity. As an example, a bidirectional circularly polarized wave with the same helicity in forward and backward is generated based on numerical analyses, and proved by experiments in microwave region. The proposed ultrathin back-to-back quarter-wave plates cavity can be adopted to higher frequencies, e.g., terahertz and mid-infrared ranges, with lower metallic conductivity. The proposed method exhibits the advantages of compact dimension and low-cost implementation in engineering the bidirectional polarization states of electromagnetic waves.

Multitwist M\xf6bius Strips and Twisted Ribbons in the Polarization of Paraxial Light Beams

The polarization of light can exhibit unusual features when singular optical beams are involved. In 3-dimensional polarized random media the polarization orientation around singularities describe 1/2 or 3/2 Möbius strips. It has been predicted that if singular beams intersect non-collinearly in free space, the polarization ellipse rotates forming many-turn Möbius strips or twisted ribbons along closed loops around a central singularity. These polarization features are important because polarization is an aspect of light that mediate strong interactions with matter, with potential for new applications. We examined the non-collinear superposition of two unfocused paraxial light beams when one of them carried an optical vortex and the other one a uniform phase front, both in orthogonal states of circular polarization. It is known that these superpositions in 2-dimensions produce space-variant patterns of polarization. Relying on the symmetry of the problem, we extracted the 3-dimensional patterns from projective measurements, and confirmed the formation of many-turn Möbius strips or twisted ribbons when the topological charge of one of the component beams was odd or even, respectively. The measurements agree well with the modelings and confirmed that these types of patterns occur at macroscopic length scales and in ordinary superposition situations.

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.

Asymptotic theory of circular polarization memory.

We establish a quantitative theory of circular polarization memory, which is the unexpected persistence of the incident circular polarization state in a strongly scattering medium. Using an asymptotic analysis of the three-dimensional vector radiative transfer equation (VRTE) in the limit of strong scattering, we find that circular polarization memory must occur in a boundary layer near the portion of the boundary on which polarized light is incident. The boundary layer solution satisfies a one-dimensional conservative scattering VRTE. Through a spectral analysis of this boundary layer problem, we introduce the dominant mode, which is the slowest-decaying mode in the boundary layer. To observe circular polarization memory for a particular set of optical parameters, we find that this dominant mode must pass three tests: (1)this dominant mode is given by the largest, discrete eigenvalue of a reduced problem that corresponds to Fourier mode k=0 in the azimuthal angle, and depends only on Stokes parameters U and V; (2)the polarization state of this dominant mode is largely circular polarized so that |V|≫|U|; and (3)the circular polarization of this dominant mode is maintained for all directions so that V is sign-definite. By applying these three tests to numerical calculations for monodisperse distributions of Mie scatterers, we determine the values of the size and relative refractive index when circular polarization memory occurs. In addition, we identify a reduced, scalar-like problem that provides an accurate approximation for the dominant mode when circular polarization memory occurs.

Magnetically tunable enhanced absorption of circularly polarized light in graphene-based 1D photonic crystals.

We theoretically investigate the magnetic-field-induced terahertz absorption enhancement of a graphene-based one-dimensional photonic crystal using the 4×4 transfer matrix method for the circular polarization of light. The results show that the magnetically tunable absorption of the structure depends on the circular polarization state, magnetic circular dichroism, and, interestingly, absorption behaviors of right-handed and left-handed circularly polarized light interchange by changing the direction of the magnetic field. These properties can be used to design the circular-polarization-based sensors.

The MOTH II Doppler-Magnetographs and Data Calibration Pipeline

AbstractThe calibration pipeline of the level zero images obtained from the Magneto-Optical filters at Two Heights (MOTH II) instrument is presented. MOTH II consists of two 20 cm aperture instruments, each using a Magneto-Optical Filter (MOF): one at 5896 Å (Na D2-line), the other one at 7700 Å (K I-line). MOTH II instruments thus provide full disk line-of-sight Doppler velocity and magnetic field measurements at two heights in the solar atmosphere. The developed MOTH II pipeline employs a set of standard calibration corrections, a correction for signal leakage, due to the non-ideal behavior of the polarizers, and the geometrical registration between the eight images acquired by four CMOS cameras, relative to two components of the signal in two circular polarization states, in each of the two channels. MOTH II data are used to investigate atmospheric dynamics (e.g., internal gravity waves and magneto-acoustic portals) and Space Weather phenomena. Particularly, flare forecasting algorithms, based on the detection of magnetic active regions (ARs) and associated flare probability estimation, are currently under development. The possible matching of MOTH II data with SDO/HMI and SDO/AIA images into a flux rope model, developed in collaboration between Harvard-Smithsonian CfA and MIT Laboratory for Nuclear Science, is being tested.

Tailoring Metamaterial Microstructures to Realize Broadband Polarization Modulation of Terahertz Waves

We report ultrabroadband, easily tunable, and highly efficient metamaterial-based terahertz wave retarders that are able to convert linear polarization into elliptical and circular polarization states. The functional device consists of a metamaterial microstructure and a grating coupler patterned on each side of fused silica substrates. The dielectric response of the metamaterial microstructure and the angular dependent phase dispersion of the grating coupler allow tuning of the phase differences from –110° to 110° within the range of a few terahertz while keeping the magnitudes of the two orthogonally transmitted waves equal. In particular, a high degree of circular polarization (>0.99) can be achieved from 1.78 to 4.88 THz for a specific dielectric value of spacer material 2.8 and angle of incidence –13°. The experimental results in the accessible frequency range of 0.2–2.3 THz show good agreement with the numerical simulations. Our study opens new opportunities for manipulating the broadband polarization responses of terahertz waves. This facilitates the development of new functional devices based on metamaterials for terahertz imaging and spectroscopy.

Laguerre-Gaussian quasi-modal q-plates from nanostructured glasses

A quasi-modal version of the recently introduced Laguerre-Gaussian modal q-plates [Rafayelyan and Brasselet, Opt. Lett. 42, 1966–1969 (2017)] is proposed and implemented using femtosecond direct laser writing of space-variant nanogratings in the bulk of silica glass. The corresponding design consists of linear azimuthal modulation of the optical axis orientation and polynomial radial modulation of the retardance profile. Experimental demonstration is made for Laguerre-Gaussian modes with azimuthal indices l =(1, 2, 3) and radial index p = 0. Such quasi-modal q-plates overcome previous limitations regarding the robustness of modality against the handedness of the incident circular polarization state.

Design of a Wideband Quad-Polarization Reconfigurable Patch Antenna Array Using a Stacked Structure

In this paper, a wideband stacked patch antenna array composed of four linearly polarized antenna elements is proposed for the implementation of four polarization reconfigurable states, i.e., vertical linear polarization (LP), horizontal LP, left-hand circular polarization, and right-hand circular polarization. A double-layer structure, including a driven substrate integrated waveguide (SIW) cavity and four parasitic radiation patches, is used to develop the wideband antenna element. The parasitic radiation patches are excited by etching a ring slot on the top surface of the SIW cavity, which has two diagonal ports fed by grounded coplanar waveguide lines. A 180° phase-shifting is achieved for polarization reconfigurable applications by individually feeding the antenna element with the two diagonal ports. A prototype operating at 5.7 GHz is fabricated and tested for the demonstration of the polarization reconfigurable capability. The measured results show that both the 10-dB impedance bandwidths and the 3-dB axial ratio bandwidths are wider than 5.19–6.1 GHz for the linear and circular polarization states. The measured maximum gains of the antenna array are 9.85 dBic and 10.1 dBi for the circular polarization states and the LP states, respectively. Experimental results show good agreement with simulated results, which indicate a good performance of the proposed patch antenna array.

Magneto-optical double resonance driven by fictitious fields.

Light-shift caused by the interaction between atoms and detuned light can be equivalent to the phenomena that atoms are coupled to an external magnetic or electric field. A fictitious magnetic or electric field is used to describe the effect of light-shift. Whether fictitious magnetic or electric field should be used depends on the polarization state of the detuned light. The fictitious fields can shift Zeeman sublevels and excite transitions between different sublevels. We study the magneto-optical double resonance of optically polarized 4He atoms driven by the detuned light with either circular or linear polarization states, model the light-shift with effective Hamiltonian, and the polarized 4He metastable state with density matrix calculations. Our proposed theoretical explanations based on the fictitious fields show good agreement with experimental results, especially for amplitude and frequency shifts of the signals at Larmor frequency and double Larmor frequency driven by the fictitious electric field.

Monstar disclinations in the polarization of singular optical beams.

The control of spatial and polarization modes of optical beams enables the production of topological singularities encoded on the polarization of the light. This allows the study of topological disclinations not easily found in the natural setting. In this article we report on the observation of new features in disclinations realized with singular optical beams. They were prepared using three spatial modes bearing optical vortices in non-separable superpositions with circular polarization states. The disclinations involve asymmetric rotational dislocations, whose optical counterparts in the optical far field are known as C-points, and which are classified as monstars. They have been known to have a singularity index that can be positive, or negative as reported by us recently. Here we report on monstars with an index of zero. Monstars are characterized by having sectors bound by radial lines that involve curved lines radiating from the singularity. We found that kinks in otherwise smooth line patterns of asymmetric disclinations are scars of a separate but related pattern of line-slope discontinuities, carried optically by C-lines in the far field. These scars are indicative of the underlying structure or symmetry of the pattern. We present a general formalism to understand and generate monstars, along with measurements: the experimental results are in excellent agreement with theoretical modelings.

Revisiting the mechanism of nitrogen fluorescence emission induced by femtosecond filament in air

The backward propagating and side emitted fluorescence during the femtosecond filamentation in air is experimentally investigated in this paper. By comparing the fluorescence emission in the circular and linear polarization states, we find that in the shorter focal length case, the direct ionization of N2 greatly affects the fluorescence emission behaviors: the fluorescence from N2+ and N2 is always stronger in the linear and circular polarization cases, respectively. Based on the observation, the emission mechanism of nitrogen fluorescence emission induced by a femtosecond filament is discussed.

Flat polarization-controlled cylindrical lens based on the Pancharatnam–Berry geometric phase

The working principle of ordinary refractive lenses can be explained in terms of the space-variant optical phase retardations they introduce, which reshape the optical wavefront curvature and hence affect the subsequent light propagation. These phases, in turn, are due to the varying optical path length followed by light at different transverse positions relative to the lens center. A similar lensing behavior can, however, be obtained when the optical phases are introduced by an entirely different mechanism. Here, we consider the ‘geometric phases’ that arise from the polarization transformations occurring in anisotropic optical media, named after Pancharatnam and Berry. The medium anisotropy axis is taken to be space-variant in the transverse plane and the resulting varying geometric phases give rise to the wavefront reshaping and lensing effect, which however also depends on the input polarization. We describe the realization and characterization of a cylindrical geometric-phase lens that is converging for a given input circular-polarization state and diverging for the orthogonal one, which provides one of the simplest possible examples of optical elements based on geometric phases. The demonstrated lens is flat and only a few microns thick (not including the supporting substrates); moreover, its working wavelength can be tuned and the lensing can be switched on and off by the action of an external control electric field. Other kinds of lenses or more general phase elements inducing different wavefront distortions can be obtained by a similar approach. Besides their potential for optoelectronic technology, these devices offer good opportunities for introducing college-level students to an advanced topic of modern physics, such as the Berry phase, with the help of interesting optical demonstrations.

Organic photodiodes from homochiral l-proline derived squaraine compounds with strong circular dichroism.

We suggest and explore a novel route towards organic photodetectors sensitive to the circular polarization state of light. For this, we insert fullerene-blended thin films of homochiral squaraine compounds acting as a highly circular dichroic active layer into conventional bulk hetero-junction photodiodes. Initially, we discuss steady-state characterization of photodiodes with unpolarized light. The homochiral, l-proline derived squaraine compounds are obtained via a chiral pool synthesis in sizable quantities. The aggregation behavior of the two compounds with varying side chain length is complex. They exhibit H-type spectral signatures only in colloidal solution, and both H- and J-type features with large splitting in neat and fullerene-blended thin films. We probe strong excitonic circular dichroism for both aggregate species, showing the most competitive dissymmetry factors up to -0.055 in fullerene-blended thin films. We vary the blend ratio and layer thickness of such active layers in the photodiodes. The device performance is in all cases limited by a low fill factor which is accompanied by a voltage-dependent photocurrent. Current-voltage measurements show light intensity dependent characteristics, which are S-shaped, contrary to our expectations, only for thin active layers independent of the blend ratio. The external quantum efficiency is in some cases extraordinarily high, exceeding 100 percent in the blue spectral range under modest reverse bias voltages for thin, fullerene-rich devices. However, the most promising are the devices with thick, donor-rich layers defined by a spectral overlap of the strongest photocurrent response and the maximum circular dichroism within the green spectral range. Thus, we demonstrate the feasibility of combining photodiode functionality and strong circular dichroism as intrinsic material properties.

Оценка анизотропных свойств метеообразований на основе использования кругового деполяризационного отношения

In the paper, a question about an estimation possibility of the hydrometeors’ anisotropic properties by using of circle depolarization ratio is considered and discussed. It is known that the factors limiting the efficiency of the circular depolarization ratio method include the pronounced dependence of the method upon the differential phase shift. The last one is introduced by a propagation medium between orthogonal components of electromagnetic wave. On this basis, a determination of the circular depolarization ratio estimation error is carried out. Such an error appears with an additional phase shift between orthogonal components of an electromagnetic wave. When receiving signals with circular polarization states of the right and left rotation direction, a special attention is given to the evaluation of its phase difference variability. Also, an interpretation of given results stands apart as another important question to discuss. The calculation data are presented and analyzed. In particular, an applicability of the signals’ phase difference estimation procedure is illustrated. It is needed for an implementation of probed meteorological formations identification in case of ambiguous situations. It is found that appearance of the phase shift, introduced by the propagation medium, leads to a significant distinction between the measured circular depolarization ratio value and the polarization anisotropy true value of the probed meteorological formations DOI 10.14258/izvasu(2017)4-08

Chiral optical response of planar and symmetric nanotrimers enabled by heteromaterial selection.

Chirality is an intriguing property of certain molecules, materials or artificial nanostructures, which allows them to interact with the spin angular momentum of the impinging light field. Due to their chiral geometry, they can distinguish between left- and right-hand circular polarization states or convert them into each other. Here we introduce an approach towards optical chirality, which is observed in individual two-dimensional and geometrically mirror-symmetric nanostructures. In this scheme, the chiral optical response is induced by the chosen heterogeneous material composition of a particle assembly and the corresponding resonance behaviour of the constituents it is built from, which breaks the symmetry of the system. As a proof of principle, we investigate such a structure composed of individual silicon and gold nanoparticles both experimentally, as well as numerically. Our proposed concept constitutes an approach for designing two-dimensional chiral media tailored at the nanoscale, allowing for high tunability of their optical response.

Circular Dichroism Metamirrors with Near-Perfect Extinction

The efficient analysis and engineering of the polarization state is imperative in diverse disciplines, including physics, materials science, biology and quantum optics. For instance, scientists apply circularly polarized light to manipulate the spin state of electron for quantum information processing. Chrysina gloriosa (jeweled beetles) under left-handed circularly polarized light illumination appear more brilliant than those under right-handed circularly polarized light illumination, and circular dichroism spectroscopy is of critical importance to identify the structure of chiral molecules. Metallic mirrors are basic elements and widely used in optical setup to control the path of light. However, the state of circular polarization is reversed, or even degrades to elliptical polarization when it is reflected off a surface. Therefore, the original handedness of the optical signals is lost after multiple reflections in a complex optical system. Here, we propose and demonstrate a new concept of circular dichroism metamirrors, which enables selective, near-perfect reflection of designated circularly polarized light without reversing its handedness, yet complete absorption of the other polarization state. Such a metamirror can be considered as the optical analogy of Chrysina gloriosa in nature, while exhibits nearly maximal efficiency. A general method to design the circular dichroism metasmirror is presented under the framework of Jones calculus. It is analytically shown that the building block of such a metamirror needs to simultaneously break the n-fold rotational (n > 2) symmetry and mirror symmetry. By combining two layers of anisotropic metamaterial structures, we design a circular dichroism metamirror in the mid-infrared region, which shows perfect reflectance (94.7%) for left-handed circularly polarized light without reversing its handedness, while almost completely absorbs (99.3%) right-handed circularly polarized light. These findings offer new methodology to implement novel photonic devices for a variety of applications, including polarimetric imaging, molecular spectroscopy and quantum information processing.