Far-field Polarization Research Articles

Nonreciprocal polarized transmission via bound states in the continuum.

We realize the observation of near-unity nonreciprocal polarized transmission via the bound states in the continuum (BICs) in a double-layer grating structure. By introducing out-of-plane perturbations and topological defects that break the mirror symmetry between the upper and lower layers, the far-field polarization states in momentum space are inverted vertically and horizontally, showing mirrored polarization characteristics for incident channels from different upper and lower ports. During the process of introducing mirror perturbations in the upper and lower layers, a π/2 phase inversion occurs in the Г-M direction, making chirality possible. Utilizing this bidirectionally tunable nonreciprocal spatiotemporal phase transition enables multiple modulations of polarization states and opens up more possibilities for asymmetric light manipulation in chiral optical effects.

Mode coupling with Fabry–Perot modes in photonic crystal slabs

Abstract Fabry–Perot (FP) modes are a class of fundamental resonances in photonic crystal (PhC) slabs. Owing to their low quality factors, FP modes are frequently considered as background fields with their resonance nature being neglected. Nevertheless, FP modes can play important roles in some phenomena, as exemplified by their coupling with guided resonance (GR) modes to achieve bound states in the continuum (BIC). Here, we further demonstrate the genuine resonance mode capability of FP modes PhC slabs. Firstly, we utilize temporal coupled-mode theory to obtain the transmittance of a PhC slab based on the FP modes. Secondly, we construct exceptional points (EPs) in both momentum and parameter spaces through the coupling of FP and GR modes. Furthermore, we identify a Fermi arc connecting two EPs and discuss the far-field polarization topology. This work elucidates that the widespread FPs in PhC slabs can serve as genuine resonant modes, facilitating the realization of desired functionalities through mode coupling.

Probing phase transition of band topology via radiation topology

Topological photonics has received extensive attention from researchers because it provides brand new physical principles to manipulate light. Band topology is characterized using the Berry phase defined by Bloch states. Until now, the scheme for experimentally probing the topological phase transition of band topology has always been relatively lacking in topological physics. Moreover, radiation topology can be aroused by the far-field polarization singularities of Bloch states, which is described by the Stokes phase. Although such two types of topologies are both related to Bloch states on the band structures, it is rather surprising that their development is almost independent. Here, in optical analogs of the quantum spin Hall effects (QSHEs) and Su-Schrieffer-Heeger model, we reveal the correlation between the phase transition of band topology and radiation topology and then demonstrate that the radiation topology can be employed to study the band topological transition. We experimentally demonstrate such an intriguing phenomenon in optical analogs of QSHEs. Our findings not only provide an insightful understanding of band topology and radiation topology, but also can serve as a route to manipulate light.

Merging diverse bound states in the continuum: from intrinsic to extrinsic scenarios

Bound states in the continuum (BICs) in photonic crystal slabs are characterized as vortex centers in far-field polarization and infinite quality (Q) factors, which can be dynamically manipulated in momentum space to construct the singularity configurations with functionalities such as merging BICs for further suppress scattering loss of nearby resonance. However, the vast majority of research focuses on two types of intrinsic BICs for simplicity, because these polarization singularities affect each other, and are even prone to annihilation. Here, we introduce the extrinsic (Fabry-Pérot) BICs and combine them with the intrinsic BICs to merge diverse BICs in momentum space. The extrinsic BICs can move independently of the intrinsic BICs, providing an unprecedented degree of freedom to reduce the complexity of constructing merging BIC configurations. Interestingly, an interaction of oppositely charged BICs that is collision beyond annihilation is revealed, which only exchanges the topological charge of BICs but not affect their existence. Following the proposed strategy, four-types-BICs merging and steerable three-types merging are achieved at the Γ and off-Γ points, further boosting the Q factor scaling rule up to Q∝k x −14 and Q∝k x −6 respectively. Our findings suggest a systematic route to arrange abundant BICs, may facilitate some applications including beam steering, optical trapping and enhancing the light-matter interactions.

Anomalous far-field polarization around bound states in the continuum in non-Bravais lattices.

It is generally believed that at-Γ bound states in the continuum (BICs) are enclosed by a linearly polarized vortex in momentum space when the structures have mirror (σz) symmetry, in-plane inversion (I) symmetry, and time reversal symmetry (T). Here, we reveal an anomalous situation in which at-Γ BICs can be enclosed by linearly and elliptically polarized far-field even when the σz, I, and T symmetries are all maintained in non-Bravais lattices, which is radically different from previous cognition. Asymmetric, diatomic structures are designed to elaborate this intriguing phenomenon. By controlling the geometric parameters or refractive indexes of the two meta-atoms, the far-field polarization around the at-Γ BICs gradually deviates from linear polarization and approaches circular polarization. Our findings reveal that non-Bravais lattices can provide a novel platform to manipulate the far-field polarization, showing important applications in quantum entanglement, structured light, and radiation modulation.

Governance of Friedrich-Wintgen bound states in the continuum by tuning the internal coupling of meta-atoms.

Bound state in the continuum (BIC) is a phenomenon that describes the perfect confinement of electromagnetic waves despite their resonant frequencies lying in the continuous radiative spectrum. BICs can be realized by introducing a destructive interference between distinct modes, referred to as Friedrich-Wintgen BICs (FW-BICs). Herein, we demonstrate that FW-BICs can be derived from coupled modes of individual split-ring resonators (SRR) in the terahertz band. The eigenmode results manifest that FW-BICs are in the center of the far-field polarization vortices. Quasi-BIC-I keeps an ultrahigh quality factor (Q factor) in a broad momentum range along the Γ-X direction, while the Q factor of the quasi-BIC-II drops rapidly. Our results can facilitate the design of devices with high-Q factors with extreme robustness against the incident angle.

Non-orthogonal cavity modes near exceptional points in the far field

Non-orthogonal eigenstates are a fundamental feature of non-Hermitian systems and are accompanied by the emergence of nontrivial features. However, the platforms to explore non-Hermitian mode couplings mainly measure near-field effects, and the far-field behaviours remain mostly unexplored. Here, we study how a microcavity with non-Hermitian mode coupling exhibits eigenstate non-orthogonality by investigating the spatial field and the far-field polarization of cavity modes. The non-Hermiticity arises from asymmetric backscattering, which is controlled by integrating two scatterers of different size and location into a microdisk. We observe that the spatial field overlap of two modes increases abruptly to its maximum value, whilst different far-field elliptical polarizations of two modes coalesce when approaching an exceptional point. We demonstrate such features experimentally by measuring the far-field polarization from the fabricated microdisks. Our work reveals the non-orthogonality in the far-field degree of freedom, and the integrability of the microdisks paves a way to integrate more non-Hermitian optical properties into nanophotonic systems.

Strongly polarized color conversion of isotropic colloidal quantum dots coupled to fano resonances.

Colloidal quantum dots (QDs) offer high color purity essential to high-quality liquid crystal displays (LCDs), which enables unprecedented levels of color enrichment in LCD-TVs today. However, for LCDs requiring polarized backplane illumination in operation, highly polarized light generation using inherently isotropic QDs remains a fundamental challenge. Here, we show strongly polarized color conversion of isotropic QDs coupled to Fano resonances of v-grooved surfaces compatible with surface-normal LED illumination for next-generation QD-TVs. This architecture overcomes the critically oblique excitation of surface plasmon coupled emission by using v-shapes imprinted on the backlight unit (BLU). With isotropic QDs coated on the proposed v-BLU surface, we experimentally measured a far-field polarization contrast ratio of ∼10. Full electromagnetic solution shows Fano line-shape transmission in transverse magnetic polarization allowing for high transmission as an indication for forward-scattering configuration. Of these QDs coupled to the surface plasmon-polariton modes, we observed strong modifications in their emission kinetics revealed by time-resolved photoluminescence spectroscopy and via dipole orientations identified by back focal plane imaging. This collection of findings indicates conclusively that these isotropic QDs are forced to radiate in a linearly polarized state from the patterned planar surface under surface-normal excitation. For next-generation QD-TVs, the proposed polarized color-converting isotropic QDs on such v-BLUs can be deployed in bendable electronic displays.

Polarization states and far-field optical properties in dielectric photonic crystal slabs.

We study the role of topological singularities like Bound States in a Continuum (BICs) or Circularly Polarized States (CPSs) in determining ellipticity of the far-field polarization in dielectric metasurfaces. Using finite-difference time-domain as well as rigorous coupled-wave analysis simulations, we determine the behavior of the Stokes parameter S3 in the whole k space above the light cone, with special regard to the region close to the singularities. Moreover, we clarify the relation between the topological singularities and the circular dichroism in reflectivity.

Topological enhancement of evanescent field localization in all-dielectric metasurfaces

Bound states in a continuum in periodic photonic structures can be described as topological defects in the k-space. These vortices can migrate within the Brillouin zone when the geometrical parameters of the system vary. In this study, we focus on the migration of the polarization vortex across the light line, from the radiation continuum to the waveguide mode domain. We found that during this transition, the far-field polarization vortex transforms into a near-field vortex. This near-field vortex is characterized by an enhanced localization of the evanescent field due to the vanishing fundamental Fourier harmonic of the Bloch amplitude. This mechanism can be used to improve polaritonic devices and optical sensors based on all-dielectric metasurfaces by increasing the overlap between optical modes and 2D excitons or thin layers of bio-analytes.

Far-Field Polarization Optics Control the Nanometer-Scale Pattern of High-Fluorescence Dissymmetry Emission from Achiral Molecules near Plasmonic Nanodimers

Chiroptical spectroscopies like circular dichroism report on the handedness of molecules but are often limited to the high-concentration regime because of the mismatch between the wavelength of propagating visible light and the size of molecules. Recent work has demonstrated that the electromagnetic fields generated near chiral plasmonic nanostructures can have high optical chirality that bridges these length scales and can induce high-fluorescence dissymmetry signals from achiral dyes by redirecting their emission. As a step toward single-molecule chiral sources, the induced fluorescence dissymmetry and apparent emission pattern near chiral plasmonic nanoparticles must be quantified. Here, we measure the induced fluorescence dissymmetry from the achiral dye Cy5.5 near a chiral gold nanodimer structure, and we map out the apparent emission pattern of the dyes using super-resolution single-molecule microscopy. Our high-sensitivity approach quantifies the fluorescence dissymmetry from sampling only ∼100 zeptomoles of Cy5.5 and measures from Cy5.5 near gold nanodimers median fluorescence dissymmetry factors that are 2 orders of magnitude greater than those of synthesized chiral fluorescent molecules. We observe a spatial correlation between the simulated optical chirality about a plasmonic gold nanodimer and experimentally obtained super-resolution apparent emission patterns, and we use the incident polarization to control our detection bias for the brightest molecules to sample nanometer-scale regions of high electric flux.

Multimode Lasing in All-Solution-Processed UV-Nanoimprinted Distributed Feedback MAPbI3 Perovskite Waveguides

In this work, we present an all-solution fabrication approach for external second-order 1D distributed feedback (DFB) gratings using soft UV-nanoimprint lithography (UV-NIL) above archetypical methylammonium lead iodide (MAPbI3) perovskite films. This high-throughput method can be carried out in an ambient environment and requires only slightly elevated temperatures as low as 70 °C, gentle imprint pressure, and the use of compatible UV-NIL resin. Under stripe-shaped optical excitation, we observe simultaneously occurring optical phenomena in our high-gain strong-scattering perovskite films, namely amplified spontaneous emission, random lasing, and 1D DFB lasing. In pursuit of distinguishing these mechanisms, we explore far-field emission patterns and output polarization. Additionally, the DFB lasing is hardly attenuated when a thin absorbing indium tin oxide (ITO) film, commonly used as an electrode in fully contacted electrical devices, is inserted between the perovskite film and the DFB grating. As a result, we reproducibly achieve single and multimode, low-threshold (below 100 μJ·cm–2), narrow linewidth (below 0.2 nm), and strongly polarized (extinction ratio above 50) optically pumped DFB lasing for MAPbI3 waveguides with and without an adjacent ITO layer. We believe that the proposed resonator integration approach can be extended toward complete electrically active devices, enabling an alternative integration scheme to achieve current-injection lasing.

Vectorial far-field method for subwavelength grating based VCSELs

We proposed in detail a procedure for the vectorial far-field analysis of high-contrast subwavelength gratings (HCGs) for vertical cavity surface emitting lasers (VCSELs). The vectorial far-field analysis method (VFM) is based on the vectorial angular spectrum method. The transmission and reflection coefficients of the HCG are used to modulate the polarized far-field components, and the modulated far-field components are vectorially composited to obtain the final transmitted and reflected far-field distribution. The reflectance and transmittance of the HCG to a two-dimensional finite-size beam can be solved by the integral of the output beam. The near field of the transmitted and reflected beam can be solved by an inverse Fourier transform of the far-field distribution. The VFM can be effectively applied in the transmit and reflect far-field beam shaping design and polarization analysis of HCG-VCSELs. The near field reconstructed by VFM can be applied to the calculation of coupling efficiency and lens design. The results of VFM are verified by a three-dimensional finite-difference time domain based on the licensed RSoft FullWAVE tool. VFM only needs to calculate a single-period structure, and the response of HCG to various near-field distributions can be obtained by one calculation, which provides an efficient and accurate method for the design of polarized HCG-VCSELs. The VFM reveals the underlying physics of the shaping effect of subwavelength grating on polarized beams and can be easily extended to the far-field analysis of arbitrary periodical meta-surfaces.

Phase and Polarization Digitally Modulated Array Using Reconfigurable DR Element: Proposal, Design, and Verification

The proposal, design, and verification of a phase and polarization digitally reconfigurable / modulated array using the reconfigurable dielectric resonator (DR) elements are developed in this paper. Different from the conventional phased arrays and reconfigurable antennas, the radiated electromagnetic (EM) wave can be flexibly controlled by simultaneously modulating its phase and polarization. To realize phase switching, a clever 1-bit phase shifter (0° / 180°) is introduced in the element design. The polarization states are controlled by integrating the switch-loaded feeding structure. And a compact 2-bit polarization tuner (0°-LP, 90°-LP, RHCP, and LHCP) is finally realized. Since every element can realize 1-bit phase and 2-bit polarization switching, the far-field beams and polarization types of the array format can be arbitrarily modulated by designing the coding sequence. To verify the proposed working principle, a compact reconfigurable metasurface-loaded DR element, and a four-element digitally reconfigurable linear array are implemented, respectively. The overlapped -10 dB impedance and 3 dB axial ratio bandwidths are 2.35-2.55 GHz (12.5 %) and 2.42-2.48 GHz (2.4 %), respectively (Covering 2.40 GHz WIFI / BT band). More importantly, it can achieve 16 different beam & polarization modes. The proposed antennas exhibit a low-cost, compact size, good bandwidth, and flexible radiation modulation capacity. They are promising candidates for future smart antennas to manipulate and control electromagnetic (EM) waves.

Microscopic analysis of spin-momentum locking on a geometric phase metasurface

We revisit spin-orbit coupling in a plasmonic Berry metasurface composed of rotated nanoapertures, which is known to imprint a robust far-field polarization response. We present a scattering formalism that shows how that spin-momentum locking emerges from the geometry of the unit cell without requiring global rotation symmetries. We find and confirm with Mueller polarimetry measurements that spin-momentum locking is an approximate symmetry. The symmetry breakdown is ascribed to the elliptical projection of circularly polarized light into the planar surface. This breakdown is maximal when surface waves are excited, and a new set of spin-momentum locking rules is presented for this case.

Spatial Processing Using High-Fidelity Models of Dual-Polarization Antenna Elements

This paper generalizes a recent improvement to a traditional spatial-processing algorithm to optimally use body-mounted arrays of dual-polarization radio-frequency antenna elements rather than single-polarization antenna elements. The paper’s generalized algorithm exploits high-fidelity far-field gain and polarization data, generated most practically by a computational electromagnetic solver (CES), to characterize the antenna array’s individual dual-polarization elements. Using this characterization and that of the desired and undesired communication nodes’ antennas, the generalized algorithm determines the array’s optimal weights. The subsequent application of a CES to a practical scenario, in which an optimally weighted array of dual-polarization antenna elements is mounted on a representative body, demonstrates the generalized algorithm’s exceptional spatial-processing performance.

Self-frequency-conversion nanowire lasers

Semiconductor nanowires (NWs) could simultaneously provide gain medium and optical cavity for performing nanoscale lasers with easy integration, ultracompact footprint, and low energy consumption. Here, we report III–V semiconductor NW lasers can also be used for self-frequency conversion to extend their output wavelengths, as a result of their non-centrosymmetric crystal structure and strongly localized optical field in the NWs. From a GaAs/In0.16Ga0.84As core/shell NW lasing at 1016 nm, an extra visible laser output at 508 nm is obtained via the process of second-harmonic generation, as confirmed by the far-field polarization dependence measurements and numerical modeling. From another NW laser with a larger diameter which supports multiple fundamental lasing wavelengths, multiple self-frequency-conversion lasing modes are observed due to second-harmonic generation and sum-frequency generation. The demonstrated self-frequency conversion of NW lasers opens an avenue for extending the working wavelengths of nanoscale lasers, even to the deep ultraviolet and THz range.

Perfect transverse spin splitting by a single particle with bianisotropy

Transverse spin splitting is a novel optical phenomenon which stems from the spin-orbit interaction of light. In this paper, we reveal the perfect transverse spin splitting in the scattered far-field of a single particle with bianisotropy. It is the result of the polarization twist effect caused by the omega-type bianisotropy of the structure in which the magnetic and electric fields interact in an unusual way and form a transverse spin dipole moment. Based on the dipole model, we conducted a detailed theoretical analysis of the relationship between the magnetoelectric coupling and the polarization state of the scattered far field. Our method reveals a physical mechanism of splitting the spin state of incident light which is regarded as a kind of giant spin Hall effect. Moreover, this perfect transverse spin splitting can also be understood by the separation of far-field polarization singularities. Our results provide a new platform for manipulating the spin state in the scattered far field and open an avenue for designing the spin structure.

Polarization modulation by surface plasmons in Young's double-slit setup

A simple phenomenological model, justified by rigorous electromagnetic analysis, is used to examine far-field polarization modulation introduced by a metallic screen with two subwavelength slits in uniform, partially polarized illumination. The accuracy of this intuitive model is assessed numerically, by employing the Fourier modal method, as regards the wavelength, the screen metal, and the slit depth, width, and separation. The Stokes parameters and the degree of polarization are evaluated paraxially in the far zone of the slits. We demonstrate that a uniform, partially linearly polarized incident beam can be rendered unpolarized by the nanoslits. Our work provides insight into the pivotal role that surface plasmons have in polarization modulation in photonics.

Polarization Singularities of Photonic Quasicrystals in Momentum Space.

We report the observation of polarization singularities in momentum space of 2D photonic quasicrystal slabs. Supercell approximation and band-unfolding approach are applied to obtain approximate photonic dispersions and the far-field polarization states defined on them. We discuss the relations between the topological charges of the polarization vortex singularities at Γ points and the symmetries of photonic quasicrystal slabs. With a perspective of multipolar expansions for the supercell, we confirm that the singularities are protected by the point-group symmetry of the photonic quasicrystal slab. We further uncover that the polarization singularities of photonic quasicrystal slab correspond to quasibound states in the continuum with exceptionally high-quality factors. Polarization singularities of different topological charges are also experimentally verified. Our Letter introduces core concepts of optical singularities into quasiperiodic systems, providing new platforms for explorations merging topological and singular optics.