Complete Polarization Conversion Research Articles

Exploiting Topological Darkness in Photonic Crystal Slabs for Spatiotemporal Vortex Generation.

Spatiotemporal optical vortices (STOVs) with swirling phase singularities in space and time hold great promise for a wide range of applications across diverse fields. However, current approaches to generate STOVs lack integrability and rely on bulky free-space optical components. Here, we demonstrate routine STOV generation by harnessing the topological darkness phenomenon of a photonic crystal slab. Complete polarization conversion enforced by symmetry enables topological darkness to arise from photonic bands of guided resonances, imprinting vortex singularities onto an ultrashort reflected pulse. Utilizing time-resolved spatial mapping, we provide the first observation of STOV generation using a photonic crystal slab, revealing the imprinted STOV structure manifested as a curved vortex line in the pulse profile in space and time. Our work establishes photonic crystal slabs as a versatile and accessible platform for engineering STOVs and harnessing the topological darkness in nanophotonics.

Observation of Topologically Enabled Complete Polarization Conversion

AbstractExploiting topological ideas has been a major theme in modern photonics, which provides unprecedented opportunities to design photonic devices with robustness against defects. While most previous works in topological photonics have focused on band theory, recent theories extend the topological concepts to the analysis of scattering matrices and suggest a topological route to complete polarization conversion (CPC). Here, the experimental observation of the topological CPC is reported. Using angle‐resolved reflection measurements, it is unveiled experimentally that the CPC between arbitrary two polarizations occurs at vortex singularities of reflection coefficients in momentum space, reflecting its topological nature. Besides, it is visualized directly that for a given input polarization, the output one can cover the entire Poincaré sphere over a wide frequency range by varying the incident angle, guaranteed by the topological nature of CPC, which is in sharp contrast to the conventional polarization–conversion approaches that usually suffer from the bulky volume, limited choice of eigen‐polarization states, or narrow operation bandwidths. Remarkably, that BICs lie on the critical coupling curves that define the condition for CPC is experimentally demonstrated. This work paves the way to exploring the topological properties in scattering matrices for controlling light polarization and creating robust photonics devices.

Ultrabroadband RCS Reduction Design by Exploiting Characteristic Complementary Polarization Conversion Metasurfaces

A new design method for ultrabroadband radar cross section (RCS) reduction by exploiting characteristic complementary polarization conversion metasurfaces (PCMs) is proposed and validated. The proposed method lifts the conventional bandwidth limitation enforced by the performance of a single PCM and expands the RCS reduction bandwidth. A systematic strategy for ultrabroadband RCS reduction design is developed and an effective reflection coefficient amplitude of the composite surface ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Gamma _{ {eff}}$ </tex-math></inline-formula> ) is derived as a new RCS reduction indicator. Based on this indicator, complete polarization conversion (CPC) frequency points of PCM are identified as important characteristics, and PCM pairs with interleaved CPC points, termed as initial PCM (I-PCM) and complementary PCM (C-PCM), are designed to compensate each other for ultrabroadband RCS reduction. A scaling-and-tuning two-step method for designing the C-PCM of a specific I-PCM is developed and validated. To further improve RCS reduction performance, taking the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Gamma _{ {eff}}}$ </tex-math></inline-formula> as an indicator, the particle swarm optimization (PSO) algorithm is used to select the optimal I-PCMs, C-PCMs, and their area ratios. Finally, an ultrabroadband RCS reduction surface is designed, fabricated, measured, and validated. It realizes 10-dB monostatic RCS reduction ranging from 7.6 to 26.2 GHz (110.7% relative bandwidth) which exceeds the polarization conversion bandwidth of each individual PCM. Besides, good polarization insensitivity and bistatic RCS reduction performance are also obtained. The proposed method provides a new route for designing broadband RCS reduction surface based on PCMs with unsatisfactory characteristics and greatly alleviates the performance requirement for PCMs.

Complete polarization conversion using anisotropic temporal slabs

It is well known that control over the polarization of electromagnetic waves can be achieved by utilizing artificial anisotropic media such as metamaterials. However, most of the related research has been focused on time-invariant systems. Inspired by the concept of temporal boundaries, we propose a method to realize polarization conversion in real time by employing time-variant materials, whose permittivity or permeability switches between isotropic and anisotropic values. The criteria for complete polarization conversion are studied for several polarization angles, both analytically and numerically.

Polarization regulation characteristics of reflected waves at the interface of double topological insulators

The property of reflective polarization on the surface of two 3D strong topological insulators was studied, obtaining the generalized necessary conditions required for the complete polarization conversion of linearly polarized light. By analyzing the reflectivity, cross reflectivity, and polarization conversion ratios of the interface of two topological insulators, we found that the model can realize the complete polarization conversion by using the existing topological insulator material, breaking through the limitation that the complete conversion requires a new small dielectric constant topological insulator material. The process can be verified by the Kerr rotation angle. Finally, we show the design method of polarization conversion devices to realize super strong angular stability. The polarization control capability of topological insulators can also be verified by Kerr effect. This work provides a theoretical basis for the application of topological insulators in polarized devices.

Linear polarization rotation in multilayer topological insulator structures

The linear polarization conversion effect of reflected electromagnetic wave from topological insulator (TI) multilayer structure is investigated. We calculate the reflection coefficients of the anisotropic TI multilayer structure based on the transfer matrix method, and the polarization conversion effect is characterized by the polarization conversion rate and Kerr rotation angle. Polarization conversion at each interface in the TI multilayer with parallel magnetization on TI surfaces can be accumulated, and the complete polarization conversion may occur in the structure consisting of sufficiently many TI layers. The polarization conversion rate is lowered with increasing frequency and exhibits significant oscillation behavior with frequency as the layer thickness increases. Fewer TI layers are required to obtain complete polarization conversion for the structure with larger topological admittance parameter, and the rotation effect of polarization is suppressed by increasing the incidence angle. The out-of-plane uniaxial anisotropy of the TI may also affect the polarization conversion in the TI multilayer.

Optical reflection properties for the interface associated with Chern insulator and chiral metamaterial

Based on the topological nature of Chern insulator and magnetoelectric coupling of chiral metamaterial, we investigate the electrodynamics for the interface associated with the two media. The Fresnel coefficients of the interface between Chern insulator and chiral metamaterial, as well as the corresponding polarization rotation angles, are derived. The reflection characteristics of the linearly polarized incident wave at the interface, such as complete polarization conversion and change of polarization state, are discussed. Under the combined influence of Chern insulator and chiral metamaterial, the partial polarization conversion may be enhanced to the complete polarization conversion, and the chiral metamaterial may compensate for the suppression effect of longitudinal conductivity of Chern insulator on the polarization conversion.

3D-printed chiral metasurface as a dichroic dual-band polarization converter.

We propose a novel design of a 3D chiral metasurface behaving as a spatial polarization converter with asymmetric transmission. The metasurface is made of a lattice of metallic one-and-a-half-pitch helical particles. The proposed metasurface exhibits a dual-band asymmetric transmission accompanied by the effect of complete polarization conversion. Regarding circularly polarized waves, the metasurface demonstrates a strong circular dichroism. A prototype of the metasurface is manufactured for the quasi-optic experiment by using a 3D printing technique utilizing a cobalt-chromium alloy, which exhibits good performances against thermal fatigue and corrosion at high temperatures.

Topologically Protected Complete Polarization Conversion.

We consider the process of conversion between linear polarizations as light is reflected from a photonic crystal slab. We observe that, over a wide range of frequencies, complete polarization conversion can be found at isolated wave vectors. Moreover, such an effect is topological: the complex reflection coefficients have a nonzero winding number in the wave vector space. We also show that bound states in continuum in this system have their wave vectors lying on the critical coupling curve that defines the condition for complete polarization conversion. Our work points to the use of topological photonics concepts for the control of polarization, and suggests the exploration of topological properties of scattering matrices as a route towards creating robust optical devices.

Polarization conversion of reflected electromagnetic wave from topological insulator

Based on the special electromagnetic properties of a 3D strong topological insulator (TI), we discuss, theoretically, the reflection of electromagnetic wave at the interface between a dielectric and a TI, and focus on the polarization conversion between the incident field and reflected field. Two cases, linear polarization and elliptical polarization at oblique incidence are considered. We derive the conditions required for the complete polarization conversion from incident s polarization into reflected p polarization, and vice versa. Furthermore, elliptical polarization incidence also can be modulated to linear or circular polarization after reflection, under special conditions, and the corresponding reflectivity can approach 1. All these special polarization behaviors originate from the intrinsic topological magnetoelectric coupling response in TI. This work provides promising applications of TIs on polarized devices and the polarization splitters.

Novel Passive Polarization Rotator Based on Spiral Photonic Crystal Fiber

In this letter, a novel design of a passive ultra-compact polarization rotator based on soft glass equiangular spiral photonic crystal fiber (ES-PCF) is proposed and analyzed using full vectorial finite difference approaches. The suggested ES-PCF has eight arms with a central elliptical air hole, which can be shifted in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$x$</tex> </formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$y$</tex> </formula> directions to achieve complete polarization conversion. In addition, the major axis of the elliptical hole is rotated by 45 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$^{\circ}$</tex></formula> , which offers 99% polarization conversion ratio with ultra-compact device length of 96 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\mu{\rm m}$</tex></formula> . It is also evident from the numerical results that the polarization rotation would be more than 98% over the 1.5–1.6 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> wavelength range.

Cavity-involved plasmonic metamaterial for optical polarization conversion

We experimentally demonstrate a plasmonic assisted Fabry–Perot cavity in a metal/insulator/metal trilayer structure with L-shaped hole arrays inside, which significantly contribute to the mechanism to realize a nearly complete polarization conversion (=0.93) in optical transmissions at near-infrared wavelength. This interesting property is found arising from an overlap of the cavity and plasmonic modes in two orthogonal polarization states. This discovered physics remarkably endows this plasmonic metamaterial with good optical performance and looser fabrication requirement, not only indicating practical applications but also providing fruitful inspirations in future nanophotonic designs.

Magnetic response induced by a moving dielectric slab

Magnetic response or magneto-optical effect is generally caused by the interaction of light with magnetized materials or some metamaterials. In this work, an exact solution for the scatterings of electromagnetic wave by the homogeneous dielectric slab moving in an arbitrary direction is presented. We find that the magnetic response can also be achieved by the dielectric slab moving along some certain directions. Such a kind of magnetic response depends on many factors such as the moving velocity of the object, the thickness, and dielectric property of the slab. At some certain velocities and incident directions of the wave, a complete polarization conversion of the reflected wave can be obtained. This means that the polarizations of the electromagnetic wave can be manipulated through reflections by the moving dielectric materials.

Complete polarization conversion using one crystal with dual transverse Pockels effect

Based on dual transverse Pockels effect, complete polarization conversion can be achieved by using only one electro-optic crystal and its two externally applied voltages. The electro-optic phase retardation and the azimuth angle of the field-induced principal dielectric axes of the crystal can be independently and linearly controlled by the amplitude and direction of the applied electric field, and the formulas for this correlation are deduced for arbitrary input and output polarization states. The candidate crystals mainly include the uniaxial crystals of 3 m, 62 m, and 32 symmetry groups, and the cubic crystals of 43 m and 23 symmetry groups. Theoretical analysis demonstrates that one crystal exhibiting both dual transverse Pockels effect and optical activity can also be used for complete polarization converter. The continuous polarization rotation of a linearly polarized light from 0 degrees to 180 degrees has been performed experimentally by use of single lithium niobate crystal with four lateral electrodes. In addition the light beam position-dependent polarization conversion by using a bulk electro-optic crystal is also measured in the experiment. This new type of polarization converter will have potential applications in many fields due to its simple configuration, explicit control logic of polarization conversion, and lower power consumption.

Complete Polarization Conversion Characteristics of Dielectric Grating Composed of Left-Handed Materials

The TE/TM polarization conversion characteristics of dielectric grating composed of left-handed materials for the case of plane wave oblique incidence are carefully investigated by a method which combines the multimode network theory with the rigorous mode matching method. It is indicated that complete polarization conversion between TE and TM modes can be realized using left-handed gratings. This can hardly achieve in the conventional right-handed gratings. Comparisons of polarization conversion properties between right-handed gratings and left-handed gratings are given with physical explanations. This unique merit of left-handed periodic structure is of important significance for accurate design of new millimeter wave TE/TM polarization converters.

Realization of complete polarization conversion using periodic structure composed of left‐handed materials

AbstractThe polarization conversion characteristics of dielectric periodic structure composed of left‐handed materials for the case of plane wave oblique incidence are carefully investigated by a method which combines the multimode network theory with the rigorous mode matching method. It is indicated that complete polarization conversion between TE and TM modes can be realized using left‐handed gratings. This can hardly achieve in the conventional right‐handed gratings. This unique merit of left‐handed periodic structure is an important significance for accurate design of new millimeter wave polarization converters. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2862–2864, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22826

Effects of polarization rotation in optical ring-resonator-based devices

In this paper, polarization rotation due to optical waveguide bending is analytically and numerically investigated. It is shown that the process of polarization conversion in the bend strictly depends on the characteristics of the straight waveguide. The strength of polarization coupling linearly increases with the waveguide refractive index contrast, whereas birefringence is responsible for the phase mismatch that limits the conversion efficiency. Waveguides with refractive index profile symmetrical in the direction orthogonal to the plane of the bend do not exhibit polarization rotation. Inside optical ring resonators, polarization rotation induced by waveguide bending is found to be highly enhanced by the cavity effect, which gives rise to strongly frequency dependent polarization rotation and dramatically alters the ring's spectral response. In the case of single-ring phase shifters, polarization rotation is effectively described by means of an approach based on the theory of poles-zeros loci, which straightforwardly provides conditions for complete polarization conversion, also in the presence of birefringence and losses. The detrimental effects caused by undesired polarization coupling on the spectral response of widespread optical devices based on ring resonators are also discussed. From this point of view, birefringence is demonstrated to be beneficial to make optical components less sensitive to the effects of polarization rotation.

Compact polarization converter in InP-based material

We present a polarization converter using one-dimensional grating principles. The device is based on slanted slots etched deeply into an InP/InGaAsP heterostructure. Almost complete polarization conversion, with a 14 dB extinction ratio, is observed for a device less than 2 microm long.

Application of a fully vectorial beam propagation method

A new vectorial beam propagation method has successfully been applied to a passive polarization splitter and a passive polarization converter on InGaAsP/InP. The propagating fields are calculated and the power attenuation is evaluated. The calculated crosstalk values of the splitter are in good agreement with the realized best performance. A complete polarization conversion in the converter has also been simulated.

Quasi-optics of a thin-film polarization converter

A planar thin isotropic film sandwiched between an isotropic cover and an electro-optic substrate crystal can support transverse electric and transverse magnetic guided modes. When an external electric field is applied to the substrate and is harmonically modulated suitably in the propagation direction, the two modes have a phase-matched interaction and exchange energy in the propagation direction. The governing equations of this interaction are deduced, using a quasi-optical technique for two orientations of the substrate crystal, and are analyzed to yield the requirement for the complete polarization conversion. Representative numerical results are obtained to show that for practically realizable film thicknesses, complete polarization conversion can be achieved in acceptable values of the interaction length and with acceptable frequency selectivity.