Space-variant polarization manipulation for far-field polarimetry by use of subwavelength dielectric gratings

Recent Advances in Photoalignment Liquid Crystal Polarization Gratings and Their Applications

Liquid crystal (LC) circular polarization gratings (PGs), also known as Pancharatnam–Berry (PB) phase deflectors, are diffractive waveplates with linearly changed optical anisotropy axes. Due to the high diffraction efficiency, polarization selectivity character, and simple fabrication process, photoalignment LC PGs have been widely studied and developed especially in polarization management and beam split. In this review paper, we analyze the physical principles, show the exposure methods and fabrication process, and present relevant promising applications in photonics and imaging optics.

Synthesis and characterization of non-uniformly totally polarized light beams: tutorial

Polarization of a light beam is traditionally studied under the hypothesis that the state of polarization is uniform across the transverse section of the beam. In such a case, if the paraxial approximation is also assumed, the propagation of the beam reduces to a scalar problem. Over the last few decades, light beams with spatially variant states of polarization have attracted great attention, due mainly to their potential use in applications such as optical trapping, laser machining, nanoscale imaging, polarimetry, etc. In this tutorial, an introductory treatment of non-uniformly totally polarized beams is given. Besides a brief review of some useful parameters for characterizing the polarization distribution of such beams across transverse planes, from both local and global points of view, several methods for generating them are described. It is expected that this tutorial will serve newcomers as a starting point for further studies on the subject.

Polarization-independent metasurface lens employing the Pancharatnam-Berry phase

Metasurface optical elements, optical phased arrays constructed from a dense arrangement of nanoscale antennas, are promising candidates for the next generation of flat optical components. Metasurfaces that rely on the Pancharatnam-Berry phase facilitate complete and efficient wavefront control. However, their operation typically requires control over the polarization state of the incident light to achieve a desired optical function. Here, we circumvent this inherent sensitivity to the incident polarization by multiplexing two metasurfaces that were designed to achieve the same optical function with incident light of opposite helicity. We analyze the optical performance of different multiplexing approaches, and demonstrate a subwavelength random interleaved polarization-independent metasurface lens operating in the visible spectrum, providing a diffraction-limited spot size for the shared-aperture.

Analysis of interference fringes based on three circularly polarized beams targeted for birefringence distribution measurements

As a potential means of measuring birefringence distribution, we analyzed the interference fringes based on three circularly polarized beams: a right-handed signal beam, a left-handed reference beam, and a right-handed reference beam. All beams were crossed at the same angle on the interfering plane, creating a two-dimensional interference fringe with three grating vectors. We proposed that by analyzing the interference fringes, we can measure the anisotropic phase shift in the signal beam. The obtained features of the anisotropic phase shift can be extended to the measurement of two-dimensional birefringence distributions without rotational manipulations of the objectives or polarizers. The fringes were generated by monolithic gratings, which can generate three-beam interfering fields precisely and easily. Finally, we confirmed the feasibility of a birefringence measurement system without any rotational manipulations of optics.

Multifunctional interleaved geometric-phase dielectric metasurfaces

Shared-aperture technology for multifunctional planar systems, performing several simultaneous tasks, was first introduced in the field of radar antennas. In photonics, effective control of the electromagnetic response can be achieved by a geometric-phase mechanism implemented within a metasurface, enabling spin-controlled phase modulation. The synthesis of the shared-aperture and geometric-phase concepts facilitates the generation of multifunctional metasurfaces. Here shared-aperture geometric-phase metasurfaces were realized via the interleaving of sparse antenna sub-arrays, forming Si-based devices consisting of multiplexed geometric-phase profiles. We study the performance limitations of interleaved nanoantenna arrays by means of a Wigner phase-space distribution to establish the ultimate information capacity of a metasurface-based photonic system. Within these limitations, we present multifunctional spin-dependent dielectric metasurfaces, and demonstrate multiple-beam technology for optical rotation sensing. We also demonstrate the possibility of achieving complete real-time control and measurement of the fundamental, intrinsic properties of light, including frequency, polarization and orbital angular momentum.

Integrated plasmonic metasurfaces for spectropolarimetry

Plasmonic metasurfaces enable simultaneous control of the phase, momentum, amplitude and polarization of light and hence promise great utility in realization of compact photonic devices. In this paper, we demonstrate a novel chip-scale device suitable for simultaneous polarization and spectral measurements through use of six integrated plasmonic metasurfaces (IPMs), which diffract light with a given polarization state and spectral component into well-defined spatial domains. Full calibration and characterization of our device is presented, whereby good spectral resolution and polarization accuracy over a wavelength range of 500-700 nm is shown. Functionality of our device in a Müller matrix modality is demonstrated through determination of the polarization properties of a commercially available variable waveplate. Our proposed IPM is robust, compact and can be fabricated with a single photolithography step, promising many applications in polarization imaging, quantum communication and quantitative sensing.

Design and laboratory demonstration of an achromatic vector vortex coronagraph

A vector vortex coronagraph (VVC) is one of promising means for imaging extremely faint objects around bright stars such as exoplanets. We present a design of an achromatic VVC, in which an axially-symmetric half-wave plate (AHP) is placed between crossed polarization filters (circular polarizer and analyzer). The circular polarizer and the analyzer are both composed of a polarizer and a quarter-wave plate (QWP). We demonstrate, via Jones calculus and Fourier analysis, that the achromatic stellar elimination can theoretically be realized by optimal polarization filters, even when chromatic AHP and QWPs are used. We carried out laboratory demonstrations of the designed VVC using a photonic-crystal AHP. As a result, we observed achromatic coronagraphic performance, a light suppression level of 7 × 10(-5), over a wavelength from 543 nm to 633 nm.

Polarization changes at Lyot depolarizer output for different types of input beams

Lyot depolarizers are optical devices made of birefringent materials used for producing unpolarized beams from totally polarized incident light. The depolarization is produced for polychromatic input beams due to the different phase introduced by the Lyot depolarizer for each wavelength. The effect of this device on other types of incident fields is investigated. In particular two cases are analyzed: (i) monochromatic and nonuniformly polarized incident beams and (ii) incident light synthesized by superposition of two monochromatic orthogonally polarized beams with different wavelengths. In the last case, it is theoretically and experimentally shown that the Lyot depolarizer increases the degree of polarization instead of depolarizes.

Polarization-gratings approach to deformed-helix ferroelectric liquid crystals with subwavelength pitch

Electro-optical properties of deformed helix ferroelectric liquid crystal (DHFLC) cells are studied by using a general theoretical approach to polarization gratings in which the transmission and reflection matrices of diffraction orders are explicitly related to the evolution operator of equations for the Floquet harmonics. In the short-pitch approximation, a DHFLC cell is shown to be optically equivalent to a uniformly anisotropic biaxial layer where one of the optical axes is normal to the bounding surfaces. For in-plane anisotropy, orientation of the optical axes and birefringence are both determined by the voltage applied across the cell and represent the parameters that govern the transmittance of normally incident light passing through crossed polarizers. We calculate the transmittance as a function of the electric field and compare the computed curves with the experimental data. The theoretical and experimental results are found to be in good agreement.

Surface-relief polarization gratings for visible light

Polarization gratings are space-variant subwavelength-structured photonic devices that control electromagnetic wave propagation by local modulation of the state of polarization of light. Using electron beam lithography, we have fabricated such devices in the form of dielectric and metallic surface-relief profiles for operation in the visible wavelength region, where structural features with dimensions on the order of 100 nm are required. We provide experimental demonstrations of various laser-beam splitting elements with diffraction efficiencies exceeding values that could be achieved by diffractive elements operating in the framework of scalar optics.

Structure of the dielectric tensor in nematic liquid crystals with topological charge

A q-plate is a stratified medium composed of a uniaxial nematic liquid crystal with an inhomogeneous orientation of the optical axis possessing a topological charge q, which looks promising as a switching device. This work reports an approach to diagonalizing the dielectric tensor in q-plates, giving a detailed study of nontrivial tensor coordinate transformations. The relationship between the diagonal dielectric tensor and the topological charge of the plate is established. From that, the dielectric tensor for a given q-plate can be explicitly calculated in a convenient frame, and hence the field propagation equations can be derived. These studies establish a theoretical basis that could further deepen the research on these particular structures for nonconventional manipulation of the light.

Real-time measurement of unique space-variant polarizations

A configuration for real-time measurement of unique, space-variant, polarizations is presented. The experimental results reveal that the full state of polarization at each location within the beam can be accurately obtained every 10 msec, limited only by the camera frame rate. We also present a more compact configuration which can be modified to determine the real-time wavelength variant polarization measurements.

Optical Vectorial Vortex Coronagraphs using Liquid Crystal Polymers: theory, manufacturing and laboratory demonstration

In this paper, after briefly reviewing the theory of vectorial vortices, we describe our technological approach to generating the necessary phase helix, and report results obtained with the first optical vectorial vortex coronagraph (OVVC) in the laboratory. To implement the geometrical phase ramp, we make use of Liquid Crystal Polymers (LCP), which we believe to be the most efficient technological path to quickly synthesize optical vectorial vortices of virtually any topological charge. With the first prototype device of topological charge 2, a maximum peak-to-peak attenuation of 1.4x10(-2) and a residual light level of 3x10(-5) at an angular separation of 3.5 lambda/d (at which point our current noise floor is reached) have been obtained at a wavelength of 1.55 microm. These results demonstrate the validity of using space-variant birefringence distributions to generate a new family of coronagraphs usable in natural unpolarized light, opening a path to high performance coronagraphs that are achromatic and have low-sensitivity to low-order wavefront aberrations.

Miniature plasmonic wave plates

Linear birefringence, as implemented in wave plates, is a natural way to control the state of polarization of light. We report on a general method for designing miniature planar wave plates using surface plasmons. The resonant optical device considered here is a single circular aperture surrounded by an elliptical antenna grating. The difference between the short and long axis of each ellipsis introduces a phase shift on the surface plasmons which enables the realization of a quarter wave plate. Furthermore, the experimental results and the theoretical analysis show that the general procedure used does not influence the optical coherence of the polarization state and allows us to explore completely the surface of the unit Poincaré sphere by changing only the shape of the elliptical grating.

Manipulation of polarization-dependent multivortices with quasi-periodic subwavelength structures

Multiple vortices with different topological charges are formed by the use of two sequential geometric phase elements. These elements are realized by quasi-periodic subwavelength gratings. The first element is a spiral phase element and the second element is a spherical phase element. We provide a theoretical analysis and an experimental demonstration of the formation of the multiple vortices that comprise scalar vortices and a vectorial vortex.