Inhomogeneous Polarization Distribution Research Articles

Polarization detection for cylindrical vector beams empowered by pixelated metasurfaces

Polarimetry plays an indispensable role in the light–matter interactions. Nevertheless, conventional components developed for polarization measurements suffer from bulky volume and spatial alignment schemes, causing them to reveal limited performance in determining inhomogeneous polarization distributions. Here, we propose a polarization detection scheme based on pixelated all-dielectric metasurfaces using spin-multiplexing coding techniques. The polarization resolving capability of the pixelated metasurface under homogeneous linearly polarized illumination was first evaluated, and the extracted peak coordinates were used to establish an exact functional relationship with the azimuthal angle. Subsequently, the measurement of spatial inhomogeneous polarization was further explored with a focusing matrix assembled from pixelated metasurfaces. The proposed polarization detection strategy can be extended to other spectral bands without discrimination, stimulating potential applications in high-resolution imaging, sensing and data communication.

Polarization singularity index determination using wedge plate lateral shear interferometry

Polarization singularity is a point in an inhomogeneous polarization distribution at which the azimuth of the polarization ellipse is indeterminate. The azimuth gradient is solenoidal around the polarization singular point. In metrology, shear interferometry enables the visualization of phase gradients in optical beams. Since the polarization azimuth gradient is related to the Stokes phase gradient, we adopt the shear interferometry technique to visualize these phase gradients for the detection of polarization singularity. We use a wedge plate lateral shear interferometer (LSI) in our experiments for the detection of the ellipse and vector field singularities. Being a self-referencing and intensity-based technique, this method is convenient for practical use. Unlike the traditional Stokes polarimetry method, where the polarization distribution at every spatial point is evaluated, this technique can distinguish degenerate states of polarization singularities simply by visual observation of intensity fringes. Index and helicity measurement of these polarization singularities without using polarization measurement methods are novel aspects of this scheme. Both theoretical and experimental results are presented to support our claim.

Influence of Primary Coma on the Tightly Focusing Characteristics of Circular Basis Hybrid Order Poincaré Sphere Beams

Analogous to the Poincaré sphere, a hybrid order Poincaré sphere is used to represent the ellipse field singularities (C-points). We study the tight focusing properties of generic bright and dark hybrid order Poincaré sphere beams in the presence of primary coma. The role of the polarization singularity index and handedness of the polarization of the hybrid order Poincaré sphere beams on the focused structure has been discussed. Results have been presented for the total intensity, component intensities, and component phase distributions for left- and right-handed bright and dark star and lemon types singularities. The presence of primary coma distorted the focal plane intensity distributions for both positive and negative index generic C-points. Coma is known to disturb the circular symmetry of the focal plane intensity distribution. Similarly in tight focusing polarization is known to disturb the symmetry. Therefore, a beam with structured and inhomogeneous polarization distribution tightly focused under the influence of coma is a fit case to study. It is found that the presence of primary coma aberration in the focusing system produces a positional shift of the high-intensity peaks and a reduction of the intensity on one side of the center. As the strength of the primary coma increases, the focal plane intensity distributions shift more and more toward the right from the initial position. Unlike the scalar vortex case, in the case of hybrid order Poincaré sphere beams, the focal plane intensity distribution undergoes rotation, as the helicity of the hybrid order Poincaré sphere beams is inverted, in addition to shifting. All the component phase distributions are found to be embedded with phase vortices of charge ±1.

Controlling nonlinear collapse of ellipticity and orientation of a co-variant vector optical field.

A vector optical field with inhomogeneous spatial polarization distribution offers what we believe to be a new paradigm to form controllable filaments. However, it is challenging to steer multiple performances (e.g. number, orientation, and interval) of filaments in transparent nonlinear media at one time. Herein, we theoretically self-design and generate a kind of believed to be novel ellipticity and orientation co-variant vector optical field to interact with Kerr medium to solve this issue. The collapsing behaviors of such a new hybrid vector optical field reveal that, by judiciously adjusting the inherent topological charge and initial phase of incident optical field, we are able to give access to stable collapsing filamentation with tunable numbers, orientations and interval. Additionally, the collapsing patterns presented are immune nearly to the extra random noise. The relevant mechanism behind the collapse of the vector optical field is elucidated as well. The findings in this work may have huge potential in optical signal processing, laser machining, and other related applications.

Poincaré Beams for Magnetic Field Sensing

Poincaré beam (PB), carrying spin angular momentum (SAM) and orbital angular momentum (OAM), plays a significant role in a wide range of optical applications. The inhomogeneous polarization distribution of PB can be represented as a point on the surface of hybrid-order Poincaré sphere (HyOPS). Here, we have investigated the properties of PB propagation in a magnetic fluid under an applied magnetic field. The PB consists of a Gaussian and a vortex beam, which are in orthogonally linear polarization. The Stokes parameters of PB are measured in the experiment to build the Stokes phase profile. Besides, one of the phase contours is marked to analyze the changes of the phase profile. The results show that the phase contours rotate, and the rotation angle is well linearly to magnetic field strength with sensitivity of 0.934°/mT in the range of 2.42 to 27.79 mT, which is consistent with the expectation of the theoretical formula. Our results may contribute to the effort of exploring potential optical sensing at the onset of PB.

Generation of broadband terahertz focused vector beam using multifunctional metasurface

Focused vector beams are special beams with spatially inhomogeneous polarization distribution, which have broad application prospects in photonics applications such as high-resolution imaging, communications, and optical sensing. Recently, multifunctional metasurfaces for generating focused vector beams have attracted great interest due to their highly integratable, miniaturized, and compact properties. However, in terahertz (THz) range, related research remains largely unexplored. In this paper, a multifunctional broadband metasurface with an all-dielectric structure is proposed for generating focused vector beams in the terahertz region, enabling efficient linear polarization conversion and complete phase control of the transmitted field. Without the help of a polarization diversity scheme, this planar focused vector beam metasurface (FVBM) can simultaneously convert x- and y-polarized incident beams into focused radially polarized (RP) and azimuthally polarized (AP) beams, respectively. The simulation results show that the proposed metasurface exhibits excellent average conversion efficiencies (CEs) higher than 85.47% and 80.94% in the frequency range of 1.7 THz - 2.0 THz under linear x-polarized and y-polarized excitation, respectively. At 1.85 THz, the proposed metasurface can focus RP and AP beams with focusing efficiencies of 70.25% and 69.88%, respectively. The proposed metasurface has potential applications in integrating multiple functionalities into single, compact and ultrathin terahertz devices.

Vector beam generation based on spin-decoupling metasurface zone plate

As singular beams, vector beams with inhomogeneous polarization distribution have found extensive application prospects in many fields, including optical information processing, high-resolution imaging, and quantum optics. Metasurfaces with strong ability to manipulate the polarization and phase of electromagnetic waves have been harnessed to generate vector beams. Among them, the proposal of metasurfaces with different responses to orthogonally polarized waves can undoubtedly enhance the flexibility of optical device design. Here, we introduce the diffractive optics (Fresnel zone plates) into the spin-decoupling metasurfaces, called the metasurface zone plates, utilized to engender desired vector beams. As a demonstration, we design two metasurface zone plates: one can generate a focused vector beam under linearly polarized incidence, and the other is exploited to engender a vector vortex beam. The simulated and measured results simultaneously demonstrate the feasibility and practicability of such a concept. Our work unlocks the potential of optical devices and may further promote the development of metasurface polarizers toward miniaturization and versatility.

Multifunctional all-dielectric metasurface quarter-wave plates for polarization conversion and wavefront shaping.

Different from conventional optical waveplates, which suffer from limited functionalities and bulky configurations, metasurfaces provide full-range birefringence control along with unprecedented capabilities of wavefront shaping at any wavelength range of interest with properly designed anisotropic meta-atoms, thereby resulting in miniaturized planar meta-waveplates with excellent and fancy functionalities beyond the conventional counterparts. In this Letter, we design a set of dielectric metasurface quarter-wave plates (QWPs) that enable efficient circular-to-linear polarization conversion along with complete phase control over the converted linearly polarized beam under circularly polarized (CP) excitation. Capitalizing on this meta-QWP platform, we numerically demonstrate two advanced multifunctional meta-QWPs (i.e., a beam-steerer and a focusing metalens) to generate different wavefronts with homogeneous and inhomogeneous linear polarization distributions under CP excitation, mimicking the functionalities of cascaded multi-stage optical components. Owing to the compactness, flexibility, and versatility, such meta-QWPs are capable of integrating more advanced applications in polarization optics.

Polarization and Holography Recording in Real‐ and k‐Space Based on Dielectric Metasurface

AbstractThe diverse design freedom and mechanisms of metasurfaces motivate the manipulation of polarization in an ultrashort distance with subwavelength resolution and make metasurfaces outperform conventional polarization optical elements. However, in order to enhance the information capability and encryption security of metasurface holograms, polarization manipulation together with multiplexing technologies are still highly desired. Here, a birefringent dielectric metasurface with the capability of encoding a grayscale image in real‐space based on Malus's law by utilizing the inhomogeneous polarization distribution and realizing the reconstruction of a vectorial holographic image in k‐space with the help of the phase profiles of different polarization components of output light is demonstrated. This novel functionality is realized by exploiting the manipulation of polarization and phase of output light simultaneously offered by the dielectric metasurface. The proposed method may enhance the information capability and security level of applications such as the anticounterfeiting and encryption.

Efficient generation of complex vectorial optical fields with metasurfaces

Vectorial optical fields (VOFs) exhibiting arbitrarily designed wavefronts and polarization distributions are highly desired in photonics. However, current methods to generate them either require complicated setups or exhibit limited functionalities, which is unfavorable for integration-optics applications. Here, we propose a generic approach to efficiently generate arbitrary VOFs based on metasurfaces exhibiting full-matrix yet inhomogeneous Jones-matrix distributions. We illustrate our strategy with analytical calculations on a model system and an experimental demonstration of a meta-device that can simultaneously deflect light and manipulate its polarization. Based on these benchmark results, we next experimentally demonstrate the generation of a far-field VOF exhibiting both a vortex wavefront and an inhomogeneous polarization distribution. Finally, we design/fabricate a meta-device and experimentally demonstrate that it can generate a complex near-field VOF—a cylindrically polarized surface plasmon wave possessing orbital angular momentum—with an efficiency of ~34%. Our results establish an efficient and ultracompact platform for generating arbitrary predesigned VOFs in both the near- and far-fields, which may find many applications in optical manipulation and communications.

Generation of super strong longitudinal electric field via vector superposition of linearly polarized laser beams

Radially polarized laser beams, which could generate longitudinal electric fields (LEF) in the far field after being focused have drawn increasing interest in wide fields. However, it is difficult to boost a radially polarized laser pulse to high peak power due to its spatially inhomogeneous polarization distribution. Here, we propose a vector superposition approach, firstly partition a linearly polarized beam into two parts in the near field, secondly modulate their polarization direction separately using the electro-optical effect and finally focus them using a high numerical aperture lens, thereby generating the LEF in the far field owing to vector superposition. This approach can amplify a laser pulse to the required peak power by using traditional schemes of the linearly polarized seed-pulse generation and multi-pass amplification, thereby efficiently scaling the LEF to the required magnitude. The simulation results support the theory in concluding that the approach would be a competent way to achieve super strong LEF in the far field of a laser beam.

Kelvin's chirality of optical beams

Geometrical chirality is a property of objects that describes three-dimensional mirror-symmetry violation and therefore it requires a non-vanishing spatial extent. In contrary, optical chirality describes only the local handedness of electromagnetic fields and neglects the spatial geometrical structure of optical beams. In this manuscript, we put forward the physical significance of geometrical chirality of spatial structure of optical beams, which we term "Kelvin's chirality". Further, we report on an experiment revealing the coupling of Kelvin's chirality to optical chirality upon transmission of a focused beam through a planar medium. Our work emphasizes the importance of Kelvin's chirality in all light-matter interaction experiments involving structured light beams with spatially inhomogeneous phase and polarization distributions.

Metalens for Generating a Customized Vectorial Focal Curve.

Three-dimensional (3D) light fields with spatially inhomogeneous polarization and intensity distributions play an increasingly important role in photonics due to their peculiar optical features and extra degrees of freedom for carrying information. However, it is very challenging to simultaneously control the intensity profile and polarization profile in an arbitrary manner. Here we experimentally demonstrate a metalens that can focus light into an arbitrarily shaped focal curve with a predefined polarization distribution. The efficacy of this approach is exemplified through the demonstration of focused curves in 3D space ranging from simple shapes such as a circle to topologically nontrivial objects such as a 3D knot with controlled local polarization states. This powerful control of the light field would be technically challenging with their conventional counterparts. Our demonstration may find applications in beam engineering and integration optics.

Research progress of hybrid vector beams (Invited)

Polarization is an important property of light field. According to the spatial polarization distribution, optical field can be divided into the scalar field and the vector field. Since the vector field with inhomogeneous polarization distribution performs some interesting behaviors in light propagation, studying the vector beam is of either scientific significance or engineering applied importance. The hybrid vector beams, which contains all kinds of polarization state in the same wave front, has attracted a lot of interests since 2010. Due to the richer polarization states than the normal vector beam, the hybrid vector beam shows attractive prospect on the optical communication, optical manipulation and quantum communication fields. Here, the generation, focusing, propagating and nonlinear optics of the hybrid vector beam were summarized.

Detection of degenerate Stokes index states

Stokes phase is the phase difference between orthogonal component states in the decomposition of any polarization state. Phase singularities in the Stokes phase distribution are Stokes singularities of an inhomogeneous polarization distribution. Under circular decomposition, Stokes phase distribution (phi _{12}) represents polarization azimuth (gamma ) distribution and the singularities present in it are polarization singularities. Therefore, the charge of the Stokes vortices depicted as Stokes index sigma _{12} is an important parameter associated with the polarization singularity. The Hybrid order Poincaré sphere (HyOPS)/Higher order Poincaré sphere (HOPS) beams, all having same Stokes index, contain a Stokes singularity at the center of the beam as these beams are constructed by vortex superposition. These beams, being superposition of orthogonal orbital angular momentum (OAM) states in orthogonal spin angular momentum (SAM) states can offer great multiplexing capabilities in communication. In this article, we identify these degenerate Stokes index states and discuss the ways and means of lifting this degeneracy. Otherwise, there are limitations on intensity based detection techniques, where demultiplexing or segregation of different HOPS/HyOPS beams is warranted. The method adduced here uses the diffraction of these beams through an equilateral triangular aperture in combination with polarization transformation as a probe to lift the Stokes index/Stokes phase degeneracy. Successively, the novelty of the detection scheme is discussed in the context of beams with alike polarization distributions where even the technique of Stokes polarimetry fails to predict the OAM and SAM content of the beam.

High-efficiency metadevices for bifunctional generations of vectorial optical fields

AbstractVectorial optical fields (VOFs) exhibiting tailored wave fronts and spatially inhomogeneous polarization distributions are particularly useful in photonic applications. However, devices to generate them, made by natural materials or recently proposed metasurfaces, are either bulky in size or less efficient, or exhibit restricted performances. Here, we propose a general approach to design metadevices that can efficiently generate two distinct VOFs under illuminations of circularly polarized lights with different helicity. After illustrating our scheme via both Jones matrix analyses and analytical model calculations, we experimentally demonstrate two metadevices in the near-infrared regime, which can generate vortex beams carrying different orbital angular momenta yet with distinct inhomogeneous polarization distributions. Our results provide an ultracompact platform for bifunctional generations of VOFs, which may stimulate future works on VOF-related applications in integration photonics.

Nonlinear frequency conversion of vectorial optical fields with a Mach-Zehnder interferometer

Vectorial optical fields with inhomogeneous polarization states have attracted great interest for their unique properties in both quantum and classical physics. In this paper, we propose a method to realize such nonlinear frequency conversion of the vectorial optical fields, which is by using a Mach–Zehnder interferometer. The separated arms enable two nonlinear processes to simultaneously proceed in two vertical directions. In our experiment, nonlinear frequency conversion of fundamental frequency vectorial optical fields with topological charge l=3 is studied. To illustrate the flexibility of this method, Taiji and clover patterns with inhomogeneous polarization distribution are also generated at second harmonic wavelengths. This research proposes a more flexible method to realize nonlinear frequency conversion of the vectorial optical fields and may have potential applications in the generation of ultrafast vectorial optical fields.

Polarization induced switching in PZT back gated multilayer MoS2 FETs for low power non-volatile memory

We demonstrated polarization induced switching in lead zirconium titanate (PZT) back gated multilayer MoS2 ferroelectric field effect transistors (Fe-FETs) for low power non-volatile memory devices. In this work, we investigate the influence of the interface between ferroelectric and 2D (MoS2) materials on the transistor characteristics for non-volatile memory applications. We fabricated Fe-FET devices with multilayer MoS2 as a channel material and polycrystalline and single crystal PZT thin films as the ferroelectric gate materials. The Fe-FET with polycrystalline PZT as a gate shows good memory characteristics; however, the hysteresis in transfer characteristics of the Fe-FET is in the clockwise direction (anti-hysteresis), which indicates that the transistor characteristics are not purely controlled by the ferroelectric charge. On the other hand, the Fe-FET devices with a single crystal PZT as the gate shows stable memory characteristics with a clear anti-clockwise hysteresis, which shows that the transistor characteristics are purely controlled by the ferroelectric charge. To understand the source of the anti-hysteresis and the interface between the ferroelectric and 2D materials, we have performed polarization switching at nanoscale using piezo force microscopy (PFM). The local piezoelectric hysteresis loop measured by PFM reveal that the polarization reversal in polycrystalline MoS2/PZT heterostructures is due to inhomogeneous polarization distribution, oxygen vacancies and high surface roughness of the polycrystalline PZT thin films. Our MoS2 Fe-FET devices with epitaxial PZT as a gate exhibit low switching voltages ≤2 V (much lower than the reported 2D-FET devices (8–20 V)), high ON-OFF ratios ≥104 (comparable to state of the art Fe-FET devices) and reproducible hysteresis behaviour with good sustainability over 200 cycles of switching operations. This study helps with the understanding of the fundamental phenomenon for anti-hysteresis, and the realization of low power and reliable non-volatile memory devices.

Tunable polarization singularity array enabled using superposition of vector curvilinear beams

In this paper, we present an approach for creating a polarization singularity array (PSA) along a curvilinear structure by exploring a scheme of coaxially superposing pre-designed component vector beams. Each component vector beam independently and azimuthally modulates inhomogeneous polarization distribution based on a combination of holographic beam shaping techniques and vector beam generation schemes such that the polarization singularity lattice arranged in a tunable curvilinear structure and locations appear after the superposition of component vector beams. The proposed PSA is proven optically in the vector beam generator system, which is based on a multiplex computer-generated hologram.

Polarization properties of superposed vector Laguerre-Guassian beams during propagation.

We use analytical and numerical techniques to study the superposition of two vector Laguerre-Guassian (LG) beams. Vector LG beams contain rich polarization information and have many interesting intensity and polarization features. The composited electric field shows an inhomogeneous polarization distribution. We focus on looking for the distribution laws for the singularities obtained. The number and positions of singularity points are calculated and confirmed by numerical methods. We also study the fundamental cases for nonconcentric superposed vector LG beams and discover the difference distribution laws of several types of singular points in all the possible combinations.