Cylindrical Polarization Research Articles

Gain improvement of conformal circularly polarized Fabry-Perot resonator antenna based on metasurface

Conformal antenna can keep consistent with the shape of the carrier perfectly, which greatly saves the space of the carrier, so it is widely used in military, aerospace and other fields. In this paper, a cylindrical conformal circular polarization Fabry-Perot resonator antenna is designed, and the adverse effect of conformal on antenna gain is overcome by transmission phase compensation. Firstly, a linear to circular polarization conversion metasurface with adjustable transmission phase is designed. The main reason for choosing the metasurface here is to take advantage of its excellent electromagnetic control ability. The metal structures of the metasurface are printed on the ultra-thin dielectric plates, and then attached to the support structure machined by 3D printing, so as to achieve the conformal design of the metasurface. Secondly, the proposed metasurface is used as the superstructure and is combined with a conformal feed to form a cylindrical conformal circularly polarized Fabry Perot resonator antenna. The conformal feed is a patch antenna which is conformal designed in the same way as the metasurface. The conformal feeder is placed under the conformal metasurface to ensure that the electromagnetic wave radiated by the feeder can enter the resonant cavity composed of the feeder and the metasurface. The curvature of the cylindrical conformal of feed and metasurface is the same, which can ensure the high unity of the different positions in the resonator. Next, by adjusting the transmission phase of the unit on the metaurface, the phase compensation of the transmitted electromagnetic wave is realized, so that the electromagnetic wave radiated by the conformal antenna still presents the characteristic of plane wave. Finally, the conformal Fabry-Perot resonator antenna obtains the same high-gain radiation characteristics as the planar resonator antenna. The designed antenna achieves a maximum gain of 13.1dBic at 10 GHz. The size of the antenna is 2.9λ × 2.9λ, so the aperture efficiency is about 19.9 %. The antenna proposed in this paper overcomes the influence of conformal design on the gain of the antenna, so that it can be conformal on the surface of a cylindrical object and obtain better gain characteristics.

Energy rule for enhancing the spin Hall effect in superposition of axisymmetric beams with cylindrical and linear polarization

We study the spin angular momentum of a superposition of two vector light beams radial symmetry, one has cylindrical polarization and another – linear. Both beams can have an arbitrary radial shape. An analytical expression is obtained for the spin angular momentum and two its properties are proven. The first one is that changing weight coefficients of the superposition does not changes the shape of the spin angular momentum density distribution, whereas the intensity shape can change. The second property is that the maximal spin angular momentum density is achieved when both constituent beams in the superposition have equal energy.

Enhancing the Spin Hall Effect of Cylindrically Polarized Beams.

Two linked gear wheels in a micromachine can be simultaneously rotated in opposite directions by using a laser beam that has in its section areas the spin angular momentum (SAM) of the opposite sign. However, for instance, a cylindrical vector beam has zero SAM in the focus. We alter a cylindrical vector beam so as to generate areas in its focus where the SAM is of opposite signs. The first alteration is adding to the cylindrical vector beam a linearly polarized beam. Thus, we study superposition of two rotationally symmetric beams: those with cylindrical and linear polarization. We obtain an expression for the SAM and prove two of its properties. The first property is that changing superposition coefficients does not change the shape of the SAM density distribution, whereas the intensity changes. The second property is that maximal SAM density is achieved when both beams in the superposition have the same energy. The second perturbation is adding a spatial carrier frequency. We study the SAM density of a cylindrical vector beam with a spatial carrier frequency. Due to periodic modulation, upon propagation in space, such a beam is split into two beams, having left and right elliptic polarization. Thus, in the beam transverse section, areas with the spin of different signs are separated in space, which is a manifestation of the spin Hall effect. We demonstrate that such light beams can be generated by metasurfaces, with the transmittance depending periodically on one coordinate.

Spin-orbit coupling induced polarization transform in the autofocusing of ring Airy beams with hybrid polarizations.

Manipulating polarization is of significance for the application of light. Spin-orbit coupling provides a prominent pathway for manipulating the polarization of light field but generally requires tight focusing conditions or anisotropic media. In this paper, we construct ring Airy beams with hybrid polarizations and reveal the controllable polarization transforms in their autofocusing dynamics by manipulating concomitant spin-orbit coupling in free space. The numerical and experimental results show that the polarization transform is dependent on the azimuthal orders of amplitude and vortex phases of two spin constituents of ring Airy beams, that the focal spots present pure linear polarization whose orientation is determined by the initial phase when the vortex phase topological charge is equal to the amplitude angular factor, otherwise, the focal fields present cylindrical vector polarizations whose orders depend on the difference of amplitude angular orders and topological charges. Our work provides new insights for studying spin-orbit interactions and the depolarization of complex polarization.

Vertical Emitting Nanowire Vector Beam Lasers.

Due to the peculiar structured light field with spatially variant polarizations on the same wavefront, vector beams (VBs) have sparked research enthusiasm in developing advanced super-resolution imaging and optical communications techniques. A compact VB nanolaser is intriguing for VB applications in miniaturized photonic integrated circuits. However, determined by the diffraction limit of light, it is a challenge to realize a VB nanolaser in the subwavelength scale because the VB lasing modes should have laterally structured distributions. Here, we demonstrate a VB nanolaser made from a 300 nm thick InGaAs/GaAs nanowire (NW). To select the high-order VB lasing mode, a standing NW as-grown from the selective-area-epitaxial (SAE) growth process is utilized, which has a bottom donut-shaped interface with the silicon oxide growth substrate. With this donut-shaped interface as one of the reflective mirrors of the nanolaser cavity, the VB lasing mode has the lowest threshold. Experimentally, a single-mode VB lasing mode with a donut-shaped amplitude and azimuthally cylindrical polarization distribution is obtained. Together with the high yield and uniformity of the SAE-grown NWs, our work provides a straightforward and scalable path toward cost-effective co-integration of VB nanolasers on potential photonic integrated circuits.

Optical lattices engineered by vector polarization and multisector amplitude modulation

Optical lattices have several applications including super-resolution imaging, lithography, and light-tweezers. Compared with the conventional multibeam interference method, optical lattices in a tightly focused light fields are presented by cylindrical vector polarization illumination and multisector amplitude modulation. The polarization and phase difference of the illumination beam, including the size and relative position of the sectors, were analyzed using the vector diffraction theory. The obtained results indicate that the primitive cell shape of optical lattices can be controlled by the polarization direction of the illumination beam when the relative positions of sectors in the amplitude modulation mask are set. In addition, the period and primitive cell shape of optical lattices with hyper or complex constructions can be controlled by the relative positions of sectors, while the optical lattice zone primarily depends on the sector size. By combining polarization and amplitude modulation in a high numerical aperture optical system, these engineered optical lattices are potentially beneficial in expanding their applications.

Improving working volume and total scattering cross section using polarization convertor stirrer in a reverberation chamber

AbstractIn our paper, a stirrer made of a polarization converter for a reverberation chamber (RC) is proposed. A cylindrical polarization converter (PC) stirrer with two unit structures arranged alternatively is placed in RC to take the place of a traditional metallic stirrer. Because of the scattering property of the PC stirrer, it is possible to enlarge the working volume and improve the scattering performance of the stirrer in the RC while maintaining the field uniformity and field anisotropy coefficient to meet the testing standard of the International Electrotechnical Commission 61000‐4‐21. Metallic stirrer and PC stirrer are both placed in the RC simultaneously to make a measurement for guaranteeing the same testing environment. Some parameters of the RC stirred by the PC stirrer or metallic stirrer have been measured and analyzed, such as field uniformity, field anisotropy, coefficient independent sample number and total scattering cross‐section (TSCS), and so on. Testing results verify that the RC stirred by a PC stirrer outperforms the classic RC stirred by a conventional metallic stirrer in terms of some performance, indicating that the proposal has a significant impact on RC testing technology that requires a larger working volume and better TSCS.

Formation of high-order cylindrical vector beams with sector sandwich structures

Complex polarization-phase transformations are considered, which are implemented using easy-to-manufacture optical elements. The manufacturing technology of such elements is based on the axially symmetric discretization of the required polarization and phase distributions. This representation leads to optical elements in the form of sector sandwich structures consisting of polarizing and phase plates stacked together. The paper numerically and experimentally investigates the main types of such sector sandwich structures for the formation of second-order cylindrical polarizations.

Free-Space Transmission and Detection of Variously Polarized Near-IR Beams Using Standard Communication Systems with Embedded Singular Phase Structures.

We propose to achieve multichannel information transmission in free space by means of variously polarized beams. The interaction of vortex beams of various orders with the main polarization states is theoretically analyzed. The passage of beams with different polarization states through multi-order diffractive optical elements (DOEs) is simulated numerically. Using the simulation results, tables of code correspondence of diffraction order numbers to the presence of phase vortices in the analyzed beams are constructed, which allow one to determine diffraction orders that carry information about various polarization states. The performed experiment made it possible to study the recognition of the first order cylindrical polarization state formed by a Q-plate converter using a phase DOE. In the experiment, these elements were built into a commercial fiber-optic communication system operating at the near-IR frequencies. After detecting the beam polarization state, beams of the required diffraction orders are efficiently coupled into optical fiber using an additional phase element. The developed optical detection system also provides channel suppression of homogeneously polarized components, which are supposed to be used for transmission of other channels.

Direct emission of broadband terahertz cylindrical vector Bessel beam

As one kind of special beams, a terahertz (THz) cylindrical vector Bessel beam processes centrosymmetric polarization distribution and nondiffractive propagation properties. Such a vector Bessel beam is potential in various practical applications ranging from THz communication and electron acceleration to sensing and imaging. Here, we propose and experimentally demonstrate a method to directly emit a broadband THz cylindrical vector Bessel (CVB) beam using nonlinear effects. By photo-exciting an indium tin oxide film coated axicon with a circularly polarized femtosecond laser pulse, a high-quality CVB beam is obtained in a broadband THz frequency range. The proposed method is universal and low-cost, opening a simple avenue for the emission of broadband THz specialty beams.

Spin-Orbital Conversion of a Strongly Focused Light Wave with High-Order Cylindrical-Circular Polarization.

We discuss interesting effects that occur when strongly focusing light with mth-order cylindrical–circular polarization. This type of hybrid polarization combines properties of the mth-order cylindrical polarization and circular polarization. Reluing on the Richards-Wolf formalism, we deduce analytical expressions that describe E- and H-vector components, intensity patterns, and projections of the Poynting vector and spin angular momentum (SAM) vector at the strong focus. The intensity of light in the strong focus is theoretically and numerically shown to have an even number of local maxima located along a closed contour centered at an on-axis point of zero intensity. We show that light generates 4m vortices of a transverse energy flow, with their centers located between the local intensity maxima. The transverse energy flow is also shown to change its handedness an even number of times proportional to the order of the optical vortex via a full circle around the optical axis. It is interesting that the longitudinal SAM projection changes its sign at the focus 4m times. The longitudinal SAM component is found to be positive, and the polarization vector is shown to rotate anticlockwise in the focal spot regions where the transverse energy flow rotates anticlockwise, and vice versa—the longitudinal SAM component is negative and the polarization vector rotates clockwise in the focal spot regions where the transverse energy flow rotates clockwise. This spatial separation at the focus of left and right circularly polarized light is a manifestation of the optical spin Hall effect. The results obtained in terms of controlling the intensity maxima allow the transverse mode analysis of laser beams in sensorial applications. For a demonstration of the proposed application, the metalens is calculated, which can be a prototype for an optical microsensor based on sharp focusing for measuring roughness.

Efficient generation of annular cylindrical vector beams by refractive axicons with high-transmission thin-film retarders

A new type of energy-efficient thin-film-coated (TF) conical optics is presented that combines annular beam shapers and cylindrical polarization converters to transform a linearly polarized laser beam into annular cylindrical vector beams. The main operational steps of the polarization converters include the generation of a circularly polarized vortex beam, the compensation of a quarter-wave shift between its p- and s-polarized components, and the summation of these components, with the last two steps being carried out simultaneously by means of such key elements as conically illuminated high-transmission TF retarders. Three optical configurations are considered and analyzed, of which the TF retarders are located on the conical surfaces of two refractive beam-shaping axicons. The total light throughput of such configurations approach 100% and they are suitable for operation at deep UV wavelengths and high power density. As potential elements of these configurations, five TF retarders are designed and optimized at a wavelength of 266 nm. Numerical simulations and comparison of the angular and spectral characteristics of these TF retarders confirm that they comply with the tolerances on the acceptable deviations of axicon wedge angles and wavelengths of typical high-power CW lasers with the above wavelength. Also presented are the results of a tolerance analysis of the TF retarders with respect to their TF thicknesses.

A transverse energy flow at the tight focus of light with higher-order circular-azimuthal polarization

Tight focusing of light with mth-order circular-azimuthal polarization was investigated. This is a new type of inhomogeneous hybrid polarization that combines the properties of mth order cylindrical polarization and circular polarization. Using the Richards-Wolf formalism, we obtained analytical expressions in the focal spot for the projections of the electric and magnetic field, the intensity distribution, the projections of the Poynting vector, and the spin angular momentum. It was shown theoretically and numerically that at the focus, the intensity has 2(m+1) local maxima located on a circle centered on an on-axis intensity null. It was shown that 4m vortices of a transverse energy flow were produced at the focus, with their centers located between the local intensity maxima. It was also shown that in the focal plane, the transverse energy flow changes the direction of rotation 2(2m+1) times around the optical axis. It is interesting that the longitudinal projection of the spin angular momentum at the focus changes sign 4m times. In those areas of the focal plane where the transverse energy flow rotates counterclockwise, the longitudinal projection of the spin angular momentum is positive, and the polarization vector rotates counterclockwise in the focal plane. Conversely, if the energy flow rotates clockwise, the polarization vector rotates clockwise, and the longitudinal projection of the spin angular momentum is negative. Numerical simulations are in agreement with the theoretical investigation.

Experimental investigation of the energy backflow in the tight focal spot

Using two identical microobjectives with a numerical aperture NA = 0.95, we experimentally demonstrate that the on-axis intensity near the tight focal spot of an optical vortex with a topological charge 2 is zero for right-handed circular polarization and nonzero for left-handed circular polarization. This serves to confirm that in the latter case there is a reverse energy flow on the optical axis, as testified by a very weak local maximum (the Arago spot) detected at the center of the measured energy flow distribution, caused by diffraction of the direct energy flow by a 300 nm circle (the diameter of a reverse energy flow tube). The comparison of numerical and experimental intensity distributions shows that it is possible to determine the diameter of the reverse energy flow "tube", which is equal to the distance between the adjacent intensity nulls. For NA = 0.95 and a 532 nm incident wavelength, the diameter of the on-axis reverse energy flow "tube" is measured to be 300 nm. It is also experimentally shown that when an optical beam with second-order cylindrical polarization is focused with a lens with NA = 0.95, there is a circularly symmetric energy flow in the focus with a very weak maximum in the center (the Arago spot), whose distribution is determined by diffraction of the direct energy flow by a 300 nm circular region, where the energy flow is reverse. This also confirms that in this case, there is a reverse energy flow on the optical axis.

Sector sandwich structure: an easy-to-manufacture way towards complex vector beam generation.

Complex polarization-phase transformations that are realized using easy-to-manufacture optical elements are considered. The manufacturing technology of such elements is based on the angular discretization of the required polarization and phase distributions, which allows one to make optical elements in the form of sector sandwich structures consisting of polarized and phase plates stacked together. We analyze analytically and study numerically the main types of such sector sandwich structures for the formation of cylindrical polarizations of various orders. New effects are observed, which result in the appearance of complex polarized beams with vortices of various orders, arising after the passage through polarizing plates and their combinations with differently rotated phase plates. The results of the experimental study of the formed beams using a multichannel diffraction filter are consistent with theory.

Mechanism of formation of an inverse energy flow in a sharp focus

A reverse energy flow is theoretically demonstrated to occur in the interference pattern generated by four plane waves with linear polarization. In some regions of the interference pattern, the right-handed triplets of plane-wave vectors k $({k}_{z}g0)$, E, and H (comprising a wave vector and $\mathbf{E}$- and H-field vectors) sum up to form an electromagnetic field described by a right-handed triplet of vectors k $({k}_{z}l0)$, E, and H. It is in these regions that the negative propagation of light occurs. On the optical axis the orbital energy flow, proportional to the light intensity, is shown to be positive, while the spin flow is negative and exceeds the orbital flow in magnitude. That is why the on-axis summary energy flow is negative. The magnitude of the reverse flow on the optical axis is two times lower than that of the intensity. A similar mechanism may apply to the case of sharply focusing a laser beam with second-order polarization or phase singularity. Using two identical micro-objectives with a numerical aperture of 0.95, it has been demonstrated experimentally that the intensity on the optical axis near the focus of an optical vortex with a topological charge of 2 is zero for right circular polarization and nonzero for left circular polarization. This confirms that in the latter case there is the reverse flow of light energy on the optical axis, since in the center of the measured energy flow distribution there is a very weak local maximum (the Arago spot) aroused due to diffraction of the forward flow by a circle with a diameter of 300 nm (the diameter of the tube with the reverse flow). Comparing the numerical and experimental intensity distributions, it is possible to determine the diameter of a ``tube'' with the reverse flow. For a numerical aperture of 0.95 and a wavelength of 532 nm, the diameter of the tube of the reverse flow along the optical axis is 300 nm. It is also shown experimentally that when an optical beam with second-order cylindrical polarization is focused with a numerical aperture of 0.95, there is a circularly symmetric energy flow in the focus with a very weak flow in the center (the Arago spot), whose distribution is determined by diffraction of the forward flux by an $\ensuremath{\sim}300$-nm-diameter circular area, where the energy flow is reverse. This also confirms that in the latter case, there is a reverse energy flow on the optical axis.

Astigmatic transformation of optical vortex beams with high-order cylindrical polarization

We investigate the astigmatic transformation of vortex and nonvortex cylindrical vector beams (CVBs) of different orders performed by a cylindrical lens. The total intensity does not change when the cylindrical lens rotates; however, the individual components can be significantly transformed. We show that the modulus of the polarization order |p| of the investigated CVBs can be defined by the astigmatic intensity distribution. In addition, in the case of vortex CVBs, it is only possible to determine a change in the sign of the order of the optical vortex. The experimentally obtained results are in agreement with the modeling results. The results obtained can be useful in optical communications for the detection of vortex beams with different polarization states and in polarization-dependent laser nanopatterning of various materials.

Sub-terahertz wideband vector beam generator based on superwavelength lattice dielectric grating

Segmented orientation of a half wave plate will convert a linear polarization into a cylindrical vector polarization. In this work, the half wave plate is achieved by 3D printed low-index dielectric grating, whose segmented rotation forms a vector beam generator. The radially and azimuthally polarized beam is experimentally measured at 140 GHz when passing through the printed sample. Further study shows that the grating lattice affects the bandwidth of the vector beam generator. The frequently used subwavelength lattice design shows narrow bandwidth due to the undesired eigenmode dispersion and interaction. In contrast, the superwavelength lattice design offers wideband and efficient vector beam generation thanks to the weak diffraction of the low-index material and the proper eigenmode dispersion. The mechanism can be applied similarly to other type of polarization control to enrich sub-terahertz elements.

Recognition of polarization and phase states of light based on the interaction of non-uniformly polarized laser beams with singular phase structures.

We propose to analyze the polarization and phase states of laser beams using a fixed set of non-polarizing phase elements. The experimental implementation of the proposed method is based on the use of multi-order phase-diffractive optical elements (DOEs). The presence or absence of intensity maxima (information bits) corresponding to the numbers of diffraction orders allows an identification code (a codeword) to be obtained. The resulting codeword makes it possible to uniquely determine the order of the vortex singularity and the order of the cylindrical polarization of the laser beam in various combinations based on simple relations.

Diffractive optical elements for the generating cylindrical beams of different orders

The paper proposes a method for calculating the phase function of subwavelength diffractive optical elements. The method is based on diffraction gratings with a varying period for generating vector beams with arbitrary-order cylindrical polarization. Formulas for the phase function of the grating are obtained with due regard for the period variation for increasing the efficiency of the polarization conversion of the incident beam. The obtained phase functions are supposed to be used for creating polarization-conversion diffractive optical elements for noise-resistant optical communication systems.