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In this paper, we discuss the photonic spin Hall effect (PSHE) in the combined structure of periodic dielectric waveguides (PDWs) and photonic crystal waveguide (PCW). By controlling the polarization of the dipole source, we have successfully achieved the arbitrary manipulation of PSHE for the PDW guided mode in our structure. The advantage of PDW is that it can be designed to different shapes without the geometric constraints, and still maintains a relatively high transmission rate. Therefore, by further changing the structure, including the adjustment of the intersection angle between PDW and PCW, and the alteration of the straight PDW to the bending or bifurcated shapes, we find that the output of highly unidirectional PSHE is mainly determined by the intersection angle between PDW and PCW and almost independent of the detailed shape of PDW. [ABSTRACT FROM AUTHOR]

The spin-polarized conversion and splitting of beam are highly important for photonic researches and applications. Although the photonic spin Hall effect (PSHE) realized by the Pancharatnamâ€"Berry (PB) metasurface has shown unprecedented capabilities to control spin-polarized light, spin-polarized beam splitting metadevices suffer from the limitations of low-efficiency. Here, we present a highly efficient spin-polarized beam splitter (SPBS) based on PB metasurface comprising silicon nano elliptical cylinder (Si-NEC) arrays. Because of the electromagnetic multipole resonance inside the designed Si-NECs, the PB metasurface can achieve high transmittance and enhanced PSHE. Therefore, the SPBS based on the PB metasurface can achieve a high spin conversion efficiency of nearly 100%, while ensuring a transmittance of 87% at 622 nm wavelength. It can also maintain a good working effect within the bandwidth of 600â€"660 nm. Furthermore, by introducing spatial shift between the two reverse Si-NEC arrays, the SPBS can also be used to realize 45° polarization rotation of an incident linearly polarized light, avoiding the input polarization angle dependence. Our design may have potential applications in high-performance and broadband spin-photonic devices. [ABSTRACT FROM AUTHOR]

Light–matter interaction at dielectric interfaces usually manifests as spin-dependent correction to light propagation, known as classical Imbert–Fedorov (IF) shift or photonic spin Hall effect, ruled by the general spin–orbit interaction (SOI) of light. Even though vector wave equations and strong SOI-based perturbation theory in a wave picture can offer good solutions to describe the modal dispersion in optical fibers, it is difficult for all these to provide an intuitive insight into the walking off for twisted (or vortex) light beams carrying orbital angular momentum (OAM). Here we present a new perspective to the topologically spin-dependent modal splitting for the twisted light highly confined in optical fibers based on the classical IF shift on geometric optics. We verify this topologically IF-shift-based walking off by comparing the analytical results of modal splitting degrees with the solutions of eigen equation, and associate the longitudinal projection of IF shift with an interesting resonance of fiber Bragg gratings locked by the signs of SAM or OAM. This interpretation provides an insight supplement to describe light ray propagating in optical fibers together with both longitudinal Goos–Hänchen and transverse IF shift under the total internal reflection, and may benefit the development of nanoscale fiber-based light on optically classical or quantum communication and metrology. [ABSTRACT FROM AUTHOR]

Spinâ€"orbit interaction deals with the interaction and coupling of spin and orbital angular momentum degrees of freedom of spinning particles, which manifests in diverse fields of physics, ranging from atomic, condensed matter to optical systems. In classical light beams, this has led to a number of non-trivial optical phenomena like spin and orbital Hall effect of light, optical Rashba effect, photonic Aharonovâ€"Bohm effect, rotational Doppler effect, transverse spin, Belinfante’s spin-momentum and spin-momentum locking etc. These have been observed in diverse micro- and nano-scale optical systems. These have generated a new area in photonics, namely, spin-orbit photonics that not only deals with fundamental lightâ€"matter interaction effects but also opened up the feasibility of a new generation of miniaturized and on-chip integrable multifunctional photonic devices based on the angular momentum and geometrical phase of light. This paper will introduce the emerging field of spin-orbit photonics and will cover the representative spin-orbit photonic effects in a variety of light-matter interactions with examples. In this regard, we also present proof-of-concept demonstrations of two interesting techniques based on the geometrical phase of light, namely, geometrical phase polarimeter and weak value polarimeter. [ABSTRACT FROM AUTHOR]

We theoretically and experimentally study the spin separations of femtosecond laser pulses reflected from an air–glass interface. Due to the broad spectral bandwidths, the displacements induced by the spin Hall effect of light and the in-plane spin separation of light are dependent on the pulse durations. Their lower frequency components acquire larger separations. To accurately measure the separations, the intensity distribution of the pulses and the spectral responsivity of the detector should be taken into account. [ABSTRACT FROM AUTHOR]

The orbital angular momentum (OAM) has been widely studied and applied to many scientific fields, especially in optical communication and quantum information. In order to give full play to the high-dimensional characteristics of OAM, separating different OAM states is a fundamental requirement. Here we find a way to introduce the mode dependent phase shift by using Porro prism, design and manufacture a new kind of two-output OAM sorter. Our OAM sorter can separate two OAM states with different topological charges without destroying them. Because it is composed of several linear commonly optical elements, which can be closely pasted together, our OAM sorter is very compact and sturdy. Furthermore, by means of cascading, our design can be used for constructing an OAM sorter with multiple output ports. Therefore, our research should have great potential application prospects, especially in the communication protocol based on OAM. [ABSTRACT FROM AUTHOR]

Manifestations of orbital angular momentum induced effects in the diffraction of a radially polarized vector beam by an off-axis tilted aperture are studied both experimentally and theoretically. Experiments were carried out to extract the degree of circular polarization, which was shown to be proportional to the on-axis component of the spin angular momentum density. We report a clear separation of the regions of dominance of the right and left circular polarizations associated with positive and negative topological charges respectively, which is reminiscent of the standard vortex-induced transverse shift, albeit in the diffraction scenario. The experimental results are supported by model simulations and the agreement is quite satisfactory. The results are useful to appreciate the orbit-orbit related effects due to unavoidable misalignment problems (especially for vortex beams). [ABSTRACT FROM AUTHOR]

Nonlinear topological photonics has been attracting increasing research interest, as it provides an exciting photonic platform that combines the advantages of active all-optical control offered by nonlinear optics with the unique features of topological photonic systems, such as topologically-protected defect-immune light propagation. In this paper, we demonstrate that topological interface modes and trivial edge modes of a specially designed graphene metasurface can be coupled in a tunable and optically controllable manner, thus providing an efficient approach to transfer optical power to topologically protected states. This is achieved in a pump-signal configuration, in which an optical pump propagating in a bulk mode of the metasurface is employed to tune the band structure of the photonic system and, consequently, the coupling coefficient and wave-vector mismatch between edge and topological interface modes. This tunable coupling mechanism is particularly efficient due to the large Kerr coefficient of graphene. Importantly, we demonstrate that the required pump power can be significantly reduced if the optical device is operated in the slow-light regime. We perform our analysis using both ab initio full-wave simulations and a coupled-mode theory that captures the main physics of this active coupler and observe a good agreement between the two approaches. This work may lead to the design of active topological photonic devices with new or improved functionality. [ABSTRACT FROM AUTHOR]

We study the energy and momentum of the surface plasmon-polariton (SPP) excited in a symmetric three-layer ‘insulatorâ€"metalâ€"insulator’ structure, which is known to support the symmetric (S) mode (which, under certain conditions, possesses the negative group velocity) as well as the antisymmetric (AS) mode with always positive energy flow. The electric and magnetic field vectors are calculated via both the phenomenological and the microscopic approach; the latter involves the hydrodynamic model accounting for the quantum statistical effects for the electron gas in metal. Explicit representation for the energy and momentum constituents in the dielectric and in the metal film are obtained, and the wavenumber dependences of the energy and momentum contributions for the whole SPP are analyzed numerically. The various energy and momentum constituents are classified with respect to their origin (‘field’ or ‘material’), and the physical nature (orbital (canonical) and spin (Belinfante) momentum contributions). The pictures characteristic for the S and AS modes are systematically compared. The results can be useful for the studies and applications of the SPP-induced thin-film effects, in particular, for the charge and spin dynamics in thin-film plasmonic systems. [ABSTRACT FROM AUTHOR]

Surface plasmon polaritons (SPPs), surface electromagnetic waves propagating along metal-dielectric interfaces, have found numerous applications in integrated photonic devices, optical storage, and optical sensing, etc. In recent years, there has been a surge of interest in the fundamental and applications of SPPs carrying orbital angular momentum, namely SPP vortices or plasmonic vortices. In this review, we summarize the fundamental concepts of plasmonic vortices, and highlight recent advances in the generation and applications of plasmonic vortices, from SPPs at lightwave frequencies to spoof SPPs at microwave and Terahertz frequencies. [ABSTRACT FROM AUTHOR]

Polarization is a fundamental physical dimension of electromagnetic waves. However, experimental determination of polarization states of light is inherently challenging and inconvenient due to the loss of phase information in conventional optical characterization. In particular, the Stokes parameters are a critical indicator to fully describe arbitrary polarization state, which, however, require multiple intensity measurements in three orthogonal polarization bases. Here, we design and experimentally demonstrate a multi-tasking geometric phase element array to unveil all Stokes parameters in one shot. Such a method is convenient, broad-band, real-time with compact size to accurately characterize the complex polarization states of coherent light, which may find visionary applications in structured light microscopy, polarizable quantum computation, and communication, etc. [ABSTRACT FROM AUTHOR]

We demonstrate the application of a two stage machine learning algorithm that enables one to correlate the electrical signals from a GaAs x N 1 âˆ' x circular polarimeter with the intensity, degree of circular polarization (DCP) and handedness of an incident light beam. Specifically, we employ a multimodal logistic regression to discriminate the handedness of light and a six-layer neural network (NN) to establish the relationship between the input voltages, the intensity and DCP. We have developed a particular NN training strategy that substantially improves the accuracy of the device. The algorithm was trained and tested on theoretically generated photoconductivity and on photoluminescence experimental results. Even for a small training experimental dataset (70 instances), it is shown that the proposed algorithm correctly predicts linear, right and left circularly polarized light, misclassifying less than 1.5 % of the cases and attaining an accuracy larger than 97 % in the vast majority of the predictions ( 92 %) for intensity and DCP. These numbers are significantly improved with larger theoretically generated datasets (4851 instances). The algorithm is versatile enough that it can be easily adjusted to other device configurations where a map needs to be established between the input parameters and the device response. Training and testing data files as well as the algorithm are provided as supplementary material. [ABSTRACT FROM AUTHOR]

A weakly focused light beam is studied numerically in the focal plane, to observe the linear polarization separation and circular polarization accumulation in the beam cross section. This is an example of spin-orbit interaction of light, where the extrinsic orbital angular momentum of light encountered due to focusing is converted into spin angular momentum and causes circular polarization generation and linear polarization separation in different parts of the beam cross section. This effect is studied by evaluating the polarization vector and Stokes vector and angle parameters in the beam cross section in detail. Because of the lens’s spherical geometry and origination of geometric phase, the accumulation of the spin component in the beam cross section is observed to be circularly symmetric, while the linear polarization shows splitting into orthogonal polarization components in the beam cross section. The underlying results may have an important role for understanding the spin-orbit interaction in a weakly focused system at a fundamental level and can be useful in many applications of nanophotonics. [ABSTRACT FROM AUTHOR]

Topological photonics is an incipient research area where the well-developed theory and applications of so-called topological insulators is applied to photonic systems. In that vein, specially-designed ring waveguides, arranged in a periodic structure and evanescently coupled, have shown the ability to propagate edge states that are robust against defects in the lattice. Here, we propose the application of photonic topologically-protected edge states (TPES) in the anomalous Floquet photonic topological insulator structures to develop a device that is able to behave as OR, AND, and XOR logic gates, depending on the characteristics of the excitation field. Materials and dimensions of the device are amenable to conventional fabrication methods, opening the possibility for implementation in on-chip photonic communication technology. We conclude by applying our results to the implementation of an all-optical two bit calculator based on TPES, a potential building block for future computational technology. [ABSTRACT FROM AUTHOR]

The angular momentum (AM) of light, comprising spin and orbital AMs, is conserved and produces a spin-Hall shift in this process for paraxial beams. For nonparaxial beams, the spin and orbital AMs are non-separable and produce many changes in the beams' spatial profile contrary to paraxial beams. These changes can be manifested as polarization modulation in the transverse plane, and conversion to orbital angular momentum (OAM) structured beams in the transverse and longitudinal planes, which can be estimated by studying the electric field vector in detail. We have calculated theoretically and simulated numerically the electric field vector components in the focal plane, to study the polarization modulation and AM conservation for OAM and Gaussian light beams of circular and linear polarizations and compared the results. Further, we have calculated and simulated the Poynting vector components for the corresponding fields to study the energy flow. We have considered the focusing of light beams using a high Numerical Aperture objective lens to obtain the nonparaxial beam, and presented a detailed theoretical analysis therein. The interpretation studies presented here are new, which may have many applications in nanophotonics and help in understanding the spin–orbit interaction at the fundamental level. [ABSTRACT FROM AUTHOR]

In this paper co-polarization spatial Goos–Hänchen (GH) shifts (Δss/λ and Δpp/λ) of periodic chiral metamaterials are investigated. The numerical results show that the enhancement of GH shifts is affected by the incident frequency, the incident angle, the period and the volume fraction of the dispersive chiral layer. For s polarized waves, GH shifts are always enhanced negatively with one Brewster angle, but two Brewster angles arise when the period is large and specific. For p polarized waves there are two Brewster angles in general, at which the GH shifts can be greatly enhanced with negative and positive peaks. More Brewster angles appear when the period is sufficiently large and specific. The transition of negatively and positively enhanced GH shifts is sensitive to large frequencies, and can be tuned by changing the frequency slightly. The transition can also be adjusted by varying the volume fraction of the dispersive chiral layer and the period of the structure. [ABSTRACT FROM AUTHOR]

As an approximate theory that is highly regarded for its computational efficiency, geometrical optics (GO) is widely used for modeling waves in various areas of physics. However, GO fails at caustics, which significantly limits its applicability. A new framework, called metaplectic geometrical optics (MGO), has recently been developed that allows caustics of certain types to be modeled accurately within the GO framework. Here, we extend MGO to the most general case. To illustrate our new theory, we also apply it to several sample problems, including calculations of two-dimensional wavefields near fold and cusp caustics. In contrast with traditional GO solutions, the corresponding MGO solutions are finite everywhere and approximate the true wavefield well near these caustics. [ABSTRACT FROM AUTHOR]

In this article we show that in a three dimensional (3D) optical vector field there exist two types of hidden singularities, one is spin density (SD) phase singularity and the other is SD vector singularity, which are both unique to 3D fields. The nature of these SD singularities is discussed and their connection with traditional optical singularities is also examined. Especially it is shown that in a 3D field with purely transverse SD ('photonic wheels'), these two types of singularities exhibit very interesting behaviors: they are exactly mapped to each other regardless of their different physical meanings and different topological structures. Our work supplies a fundamental theory for the SD singularities and will provide a new way for further exploration of 3D vector fields. [ABSTRACT FROM AUTHOR]

The spatial and angular Goos-Hänchen (GH) and Imbert-Fedorov (IF) shifts are examined for fundamental Gaussian beam (633 nm) reflected from a monolayer of MoS2 using the 'Stokes polarimetric' technique. This technique enables us to promptly determine the magnitudes of the reflection coefficients and their relative phase differences which gives the optical beam shifts. The GH and IF shifts are measured with different polarization basis along with varying angle of incidence. This is a precision technique depending only upon the polarization components of the light beam and is quite independent of the fluctuations of the light intensity. The response of GH and IF shifts for MoS2 substrate is significant and thus may open up new possibilities in the optical applications of 2D materials. [ABSTRACT FROM AUTHOR]

Metasurfaces have demonstrated revolutionary capabilities in manipulating the polarization, phase, and amplitude of light. Pancharatnam–Berry (P–B) phase metasurfaces can operate over a broad spectrum and robust phase distributions against fabrication tolerance. However, the conjugated characteristic of the modulated phases for different spin photons introduces an obstacle for multifunctional photonics applications. This problem seriously hinders the development of P–B phase metasurfaces. We proposed a method through symmetry breaking of photonic spin–orbit interactions and achieving spin-independent manipulation of spin–orbit interactions. As a proof-of-principle demonstration, we realized polarization-multiplexed multifunctional metalenses at visible and infrared wavelengths with high numerical aperture, and the focal lengths of different foci can be adjusted independently. The proposed metalens has potential applications in imaging, optical communications and optical trapping, and also the approach of independent manipulation of spin–orbit interactions can facilitate the development of P–B phase metasurfaces toward spin-controlled multifunctional photonics. [ABSTRACT FROM AUTHOR]

Spin-dependent splitting of the reflected beam for the chiral metamaterial slab is investigated by analyzing the spatial transverse shifts (TSs) of the two spin-dependent components of the reflected beam. Two components refer to the right-circularly polarized and left-circularly polarized reflected wave components. The influences of the physical parameters and the polarization of the incident beam on the spin-dependent splitting are discussed. The results show that two components are asymmetric due to the cross-polarization of the electromagnetic fields in chiral slab. By adjusting the angle of incidence and the polarization state of the incident beam, the spatial splitting can be enhanced. Moreover, the effect of the thickness on the spatial TSs of the two components becomes weak with increasing the angle of incidence. To oneʼs interest, the spin-dependent splitting also can be realized for two special linear (TE and TM) polarization. Our study may provide an opportunity to realize and control the transverse splitting of the light, and open up the possibility for developing new nanophotonic devices. [ABSTRACT FROM AUTHOR]

We consider reflection and transmission of polarized paraxial light beams at a plane dielectric interface. The field transformations taking into account a finite beam width are described based on the plane-wave representation and geometric rotations. Using geometrical-optics coordinate frames accompanying the beams, we construct an effective Jones matrix characterizing spatial-dispersion properties of the interface. This results in a unified self-consistent description of the Goos–Hänchen and Imbert–Fedorov shifts (the latter being also known as the spin Hall effect of light). Our description reveals the intimate relation of the transverse Imbert–Fedorov shift to the geometric phases between constituent waves in the beam spectrum and to the angular momentum conservation for the whole beam. Both spatial and angular shifts are considered as well as their analogues for higher-order vortex beams carrying intrinsic orbital angular momentum. We also give a brief overview of various extensions and generalizations of the basic beam-shift phenomena and related effects. [ABSTRACT FROM AUTHOR]

The analogies between optical and electronic Goos-äanchen effects are established based on electron wave optics in semiconductor or graphene-based nanostructures. In this paper, we give a brief overview of the progress achieved so far in the field of electronic Goos-Hänchen shifts, and show the relevant optical analogies. In particular, we present several theoretical results on the giant positive and negative Goos-Hänchen shifts in various semiconductor or graphene-based nanostructures, their controllability, and potential applications in electronic devices, e.g. spin (or valley) beam splitters. [ABSTRACT FROM AUTHOR]

We study the Goos-Hanchen and Imbert-Fedorov shifts for quasi-monochromatic Gaussian beams near the Brewster angle, when the reflection is from a denser to a rarer medium. This is the case of interest in the usual experiments on reflectometry, etc. We have incorporated the effects of the finite linewidth of the quasi-monochromatic light and treated the cases of a Lorentzian and a Gaussian lineshape of the input light spectrum. This study of light with a finite spectral width was carried out for the more frequently studied case, namely reflection from a dense to a rare medium. We found that the shift is increased as compared to the monochromatic Gaussian beam, and is zero at the Brewster angle for appolarized beam. The shift variation with angle of incidence near the Brewster and critical angles at different values of refractive index ratios is found. We also studied the shift variation for Hermite-Gauss beams around the Brewster angle when the reflection is from a rarer to a denser medium and compare this with our earlier results for the case when the reflection was from a denser to a rarer medium. [ABSTRACT FROM AUTHOR]

We study monochromatic, scalar solutions of the Helmholtz and paraxial wave equations (PWEs) from a field-theoretic point of view. We introduce appropriate time-independent Lagrangian densities for which the Euler–Lagrange equations reproduces either Helmholtz and PWEs with the z-coordinate, associated with the main direction of propagation of the fields, playing the same role of time in standard Lagrangian theory. For both Helmholtz and paraxial scalar fields, we calculate the canonical energy-momentum tensor and determine the continuity equations relating ‘energy’ and ‘momentum’ of the fields. Eventually, the reduction of the Helmholtz wave equation to a useful first-order Dirac form, is presented. This work sheds some light on the intriguing and not so acknowledged connections between angular spectrum representation of optical wavefields, cosmological models and physics of black holes. [ABSTRACT FROM AUTHOR]

Flat metalenses with tunable focal length have wide applications in modern optical systems with high integration requirement. Here, by combining propagation phase and geometric phase, we numerically realize a metalens with a tunable focal length by controlling the polarization state of incident light. The tuning range can be as large as 15λ at the designer wavelength. Furthermore, the tunable performance can operate at the entire visible band with an efficiency over 35%. We believe that the proposed polarization-enabled tunable focusing metalens will find significant applications in imaging and sensing applications. [ABSTRACT FROM AUTHOR]

We report the observation of splitting, rotation and spiraling behavior in the polarization components of the Laguerre–Gaussian beam, reflected around the Brewster angle for a TM-polarized input beam. The measurements carried out using Stokes parameters in addition show that the strength and behavior of the above-mentioned effects depend on the order of the Laguerre–Gaussian beam. These polarization effects appear to be a manifestation of the collaborative behavior of the diffraction-corrective longitudinal Goos–Hänchen and spin-orbit interaction induced transverse spin-Hall shifts, wherein the coupling between these shifts play a critical role. Except for the polarization splitting, other observed effects are absent for a TE polarized input beam. These studies are useful to understand the topological aspect of complex light field in various applications, including optical manipulation, wherein the source optical angular momentum has been point of discussion. Additionally, they can be used as a promising tool in the identification of the thickness of thin layer and optical sensing. [ABSTRACT FROM AUTHOR]

The capability of curved hyperbolic metamaterials to guide the emission of a dipole-emitter into two separate channels depending on the emission helicity with high confinement provides new possibilities for polarization detection. In this work, we numerically demonstrate a novel design of miniature Stokes polarimeter using hyperbolic metamaterial structure coupled with two dipole-emitters. Through determining the four intensity signals arising from the two directional radiation channels of each dipole-emitter, the state of polarization of the incident photons can be analyzed. Numerical simulations also demonstrated that such a design is capable of working with a bandwidth of at least 40 nm. Such a compact full Stokes parameter polarimeter could be a potential candidate for high spatial resolution polarization sensor and imager for a wide variety of applications. [ABSTRACT FROM AUTHOR]

Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters. In addition, metasurfaces have recently been proposed to confine electromagnetic waves, thereby avoiding undesired leakage of energy and increasing the overall efficiency of electromagnetic instruments and devices. The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Due to these advantages, they are promising candidates for real-world solutions to overcome the challenges posed by the next generation of transmitters and receivers of future high-rate communication systems that require highly precise and efficient antennas, sensors, active components, filters, and integrated technologies. This Roadmap is aimed at binding together the experiences of prominent researchers in the field of metasurfaces, from which explanations for the physics behind the extraordinary properties of these structures shall be provided from viewpoints of diverse theoretical backgrounds. Other goals of this endeavour are to underline the advantages and limitations of metasurfaces, as well as to lay out guidelines for their use in present and future electromagnetic devices. This Roadmap is divided into five sections: 1. Metasurface based antennas. In the last few years, metasurfaces have shown possibilities for advanced manipulations of electromagnetic waves, opening new frontiers in the design of antennas. In this section, the authors explain how metasurfaces can be employed to tailor the radiation properties of antennas, their remarkable advantages in comparison with conventional antennas, and the future challenges to be solved. 2. Optical metasurfaces. Although many of the present demonstrators operate in the microwave regime, due either to the reduced cost of manufacturing and testing or to satisfy the interest of the communications or aerospace industries, part of the potential use of metasurfaces is found in the optical regime. In this section, the authors summarize the classical applications and explain new possibilities for optical metasurfaces, such as the generation of superoscillatory fields and energy harvesters. 3. Reconfigurable and active metasurfaces. Dynamic metasurfaces are promising new platforms for 5G communications, remote sensing and radar applications. By the insertion of active elements, metasurfaces can break the fundamental limitations of passive and static systems. In this section, we have contributions that describe the challenges and potential uses of active components in metasurfaces, including new studies on non-Foster, parity-time symmetric, and non-reciprocal metasurfaces. 4. Metasurfaces with higher symmetries. Recent studies have demonstrated that the properties of metasurfaces are influenced by the symmetries of their constituent elements. Therefore, by controlling the properties of these constitutive elements and their arrangement, one can control the way in which the waves interact with the metasurface. In this section, the authors analyze the possibilities of combining more than one layer of metasurface, creating a higher symmetry, increasing the operational bandwidth of flat lenses, or producing cost-effective electromagnetic bandgaps. 5. Numerical and analytical modelling of metasurfaces. In most occasions, metasurfaces are electrically large objects, which cannot be simulated with conventional software. Modelling tools that allow the engineering of the metasurface properties to get the desired response are essential in the design of practical electromagnetic devices. This section includes the recent advances and future challenges in three groups of techniques that are broadly used to analyze and synthesize metasurfaces: circuit models, analytical solutions and computational methods. [ABSTRACT FROM AUTHOR]

In this paper, the reflection and transmission properties of the plane electromagnetic waves are investigated at the interface of an asymmetric graphene-based hyperbolic metamaterial composed of alternating stacks of graphene monolayers and dielectric layers. It is shown that the Brewster angle can be changed by tailoring the orientation of the optical axis of the considered structure as well as the chemical potential of the graphene. Furthermore, the propagation of the wave vectors and Poynting vectors are studied in the given structure. Such structures have singlet dispersion behavior for the transverse electric wave and triplet dispersion behavior for the transverse magnetic wave. These types of dispersion can be switched by modifying the orientation of the optical axis, the chemical potential of the graphene, and the frequency of the impinging wave. Such tunability characteristics of the graphene-based metamaterials may be practical to design optical devices such as hyperlenses and tunable optical filters. [ABSTRACT FROM AUTHOR]

Metasurfaces provide the freedom to manipulate electromagnetic (EM) waves by controlling the peculiarity of subwavelength artificial structures. Many exciting devices have been developed using metasurfaces that can even perform multiple functionalities. In this paper, we propose to transform linearly polarized EM waves into high-order orbital angular momentum (OAM) beams and specifically reflected split beams for different polarizations based on anisotropic coding metasurfaces. By means of precisely adjusting the geometry of the element structure, the reflection phases of two perpendicular polarizations can be encoded without interference, which is conductive to encode the metasurfaces independently in two different directions. Under this benefit, two kinds of 2-bit phase-coded metasurfaces are constructed and verified by experiments to demonstrate their versatility of generating effective high-order OAM beams and specified reflected split beams, respectively. Beyond that, by adjusting the coding scheme, more features can be implemented under this merit. [ABSTRACT FROM AUTHOR]

Superoscillations are band-limited functions with the counterintuitive property that they can vary arbitrarily faster than their fastest Fourier component, over arbitrarily long intervals. Modern studies originated in quantum theory, but there were anticipations in radar and optics. The mathematical understanding—still being explored—recognises that functions are extremely small where they superoscillate; this has implications for information theory. Applications to optical vortices, sub-wavelength microscopy and related areas of nanoscience are now moving from the theoretical and the demonstrative to the practical. This Roadmap surveys all these areas, providing background, current research, and anticipating future developments. [ABSTRACT FROM AUTHOR]

The physics of optical vortices, also known as twisted light, is now a well-established and a growing branch of optical physics with a number of important applications and significant inter-disciplinary connections. Optical vortex fields of widely varying forms and degrees of complexity can be realised in the laboratory by a host of different means. The interference between such beams with designated orbital angular momenta and optical spins (the latter is associated with wave polarisations) can be structured to conform to various geometrical arrangements. The focus of this review is on how such tailored forms of light can exert a controllable influence on atoms with which they interact. The main physical effects involve atoms in motion due to application of optical forces. The now mature area of atom optics has had notable successes both of fundamental nature and in applications such as atom lasers, atom guides and Bose–Einstein condensates. The concepts in atom optics encompass not only atomic beams interacting with light, but atomic motion in general as influenced by optical and other fields. Our primary concern in this review is on atoms in structured light where, in particular, the twisted nature of the light is made highly complex with additional features due to wave polarisation. These features bring to the fore a variety of physical phenomena not realisable in the context of atomic motion in more conventional forms of laser light. Atoms near resonance with such structured light fields become subject to electromagnetic fields with complex polarisation and phase distributions, as well as intricately structured intensity gradients and radiative forces. From the combined effect of optical spin and orbital angular momenta, atoms may also experience forces and torques involving an interplay between the internal and centre of mass degrees of freedom. Such interactions lead to new forms of processes including scattering, trapping and rotation and, as a result, they exhibit characteristic new features at the micro-scale and below. A number of distinctive properties involving angular momentum exchange between the light and the atoms are highlighted, and prospective applications are discussed. Comparison is made between the theoretical predictions in this area and the corresponding experiments that have been reported to date. [ABSTRACT FROM AUTHOR]

Gaussian beams reflected from a multi-layered dielectric experience a shift in their centroid that is different than that from a single interface. This has been previously investigated for linearly polarized beams and, to a much lesser extent, beams with spin angular momentum. Here a combination of theoretical and computational analysis is used to provide a unified quantification of these shifts in layered dielectrics, for light endowed with an intrinsic orbital angular momentum (OAM)—i.e. vortex beams. Two geometries are considered: air/glass/air and glass/air/glass multi-layers. Destructive interference causes singular lateral shifts in the centroid of the reflected vortex beam for which spin alone produces only a mild modulation. In the case of total internal reflection, both spin and intrinsic OAM contribute to an enhancement of these lateral shifts as the interlayer thickness is decreased. This is just the opposite of the trend associated with longitudinal shifts. We find that vortex beams undergo centroid shifts up to tens of microns, more than an order of magnitude larger than for Gaussian beams. [ABSTRACT FROM AUTHOR]

In this paper, several novel spin-selected metalenses composed of rotary silicon nanobrick array have been designed and numerically investigated at the telecommunication wavelength of 1500 nm, which can be focused at different positions along transverse or longitudinal direction for different spin light. They can not only directly distinguish the spin states but also manipulate photons with different spin states. Our optimized silicon nanobricks with varying orientation angles can achieve 98% cross-polarized transmitted efficiency, and possess Pancharatnam–Berry phase. Furthermore, a spin-independent metalens has also been designed, which can be well focused at the same spot for the light with different spin states. Our work opens up new avenues toward establishing novel spin-related metadevices, and it is significant for the development of functionality switchable optical devices and spin photonics. [ABSTRACT FROM AUTHOR]

We introduced here a reconfigurable topological phase using optically pumped graphene nanoribbon arrays with homogeneous spacing at terahertz frequencies. By adjusting the profile of imaginary potential, various topologically non-trivial dimer chains composed of graphene nanoribbons can be selectively produced. These dimer chains support different kinds of midgap states. Moreover, we numerically demonstrated that these midgap states are robust against some perturbations, such as non-Hermitian and lattice disorders, and can be further adjusted by varying the pumping strength on the specific graphene nanoribbon. Consequently, the reconfigurable topological phase can be obtained by only tuning the profile of imaginary potential without altering the geometry of the system. These tunable topological phases may provide a convenient method to tune the spatial profile of graphene-based topological lasers and have potential applications in topologically protected waveguiding in terahertz frequency range. [ABSTRACT FROM AUTHOR]

Although a relatively new area of nanoscience, nonlinear plasmonics has become a fertile ground for the development and testing of new ideas pertaining to light–matter interaction under intense field conditions, ideas that have found a multitude of applications in surface science, active photonic nanodevices, near-field optical microscopy, and nonlinear integrated photonics. In this review, we survey the latest developments in nonlinear plasmonics in three-dimensional (metallic) and two-dimensional (graphene) nanostructures and offer an outlook on future developments in this field of research. In particular, we discuss the main theoretical concepts, experimental methods, and computational tools that are used together in modern nonlinear plasmonics to explore in an integrated manner nonlinear optical properties of metallic and graphene based nanostructures. [ABSTRACT FROM AUTHOR]

We investigate the asymmetric transmission for forward and backward propagation of tilted circular polarized optical waves in subwavelength epsilon-near-zero (ENZ) hyperbolic slabs. This chiral-optical effect is solely triggered by anisotropy without resorting to any breaking of reciprocity and chiral symmetries or spatial nonlocal effects. Remarkably, we show that the asymmetric transmission undergoes a dramatic enhancement near the ENZ condition. This happens since, close to the zero-crossing point, the extraordinary waves can accumulate the desired propagation phase even though the slab is ultrathin and, by varying excitation angles and slab thickness, we engineer this phase thus achieving a huge asymmetric transmission. The proposed strategy holds promise for realizing ultra-compact and efficient polarization devices in different frequency range even at very high frequencies (ultraviolet) since the effect is merely due to anisotropy and it is available without resorting to nanofabrication processes. [ABSTRACT FROM AUTHOR]

Tensor impedance metasurfaces have attracted much attention recently due to their ability to flexibly manipulate both propagating and surface waves. So far, the analysis of tensor impedance has not been fully supported by commercial electromagnetic simulation software. Also, the representation of formulas of penetrable tensor impedance for both propagating and surface waves is limited in the literature. Hence, a unified analysis framework is proposed for this purpose. It allows the calculation of wave reflection and transmission with arbitrary incident angles, or inversely, the calculation of surface impedance tensors given the desired wave propagation properties. It also allows surface eigenmode analysis, where surface modes with novel isofrequency contours are found for penetrable tensor impedance surfaces. The relationship between penetrable and impenetrable metasurfaces is derived so that the resultant formulas are applicable in both scenarios. A finite difference method code is implemented to validate the derived formulas. The proposed unified formulas are of closed form, hence offering very fast operation for research and design of tensor impedance metasurfaces. [ABSTRACT FROM AUTHOR]

The dynamical characteristics of electromagnetic fields include energy, momentum, angular momentum (spin) and helicity. We analyze their spatial distributions near the planar interface between two transparent and non-dispersive media, when the incident monochromatic plane wave with arbitrary polarization is totally reflected, and an evanescent wave is formed in the medium with lower optical density. Based on the recent arguments in favor of the Minkowski definition of the electromagnetic momentum in a material medium (Philbin 2011 Phys. Rev. A 83 013823; Philbin and Allanson 2012 86 055802; Bliokh et al 2017 Phys. Rev. Lett. 119 073901), we derive the explicit expressions for the dynamical characteristics in both media, with special attention to their behavior at the interface. In particular, the ‘extraordinary’ spin and momentum components orthogonal to the plane of incidence are described, and a canonical (spin–orbital) momentum decomposition is performed that contains no singular terms. The field energy, helicity, the spin momentum and orbital momentum components are everywhere regular but experience discontinuities at the interface; the spin components parallel to the interface appear to be continuous, which testifies to the consistency of the adopted Minkowski picture. The results supply a meaningful example of the electromagnetic momentum decomposition, with separation of spatial and polarization degrees of freedom, in inhomogeneous media, and can be used in engineering the structured fields designed for optical sorting, dispatching and micromanipulation. [ABSTRACT FROM AUTHOR]

Optical spin angular momentum, an intrinsic part of optical angular momemtum, can induce a spinning motion of a trapped particle around its own axis in optical manipulation. Focusing of a type of double-vortex (DV) input field obtained by linearly superposing two optical vortex beams with equal but opposite topological charges, yields a multi-lobe focal field, each of which has non-vanishing optical spin angular momentum, and is capable of trapping particle while spinning the particle around a certain axis. Significantly, both the focusing properties and the spinning dynamics are strongly polarization dependent. For instance, the focused field of a circularly polarized double-vortex (CPDV) beam carries transverse and longitudinal spin angular momenta, inducing axial spinning of the trapped particles, whereas the focused field of a radially polarized double-vortex (RPDV) beam possesses purely transverse spin angular momentum and can drive the particles to spin transversely to the optical axis. These results may find potential applications in light beam shaping and optical manipulations. [ABSTRACT FROM AUTHOR]

Engineering local angular momentum of structured light fields in real space enables applications in many fields, in particular, the realization of unidirectional robust transport in topological photonic crystals with a non-trivial Berry vortex in momentum space. Here, we show transverse angular momentum modes in silicon topological photonic crystals when considering transverse electric polarization. Excited by a chiral external source with either transverse spin angular momentum or transverse phase vortex, robust light flow propagating along opposite directions is observed in several kinds of sharp-turn interfaces between two topologically-distinct silicon photonic crystals. A transverse orbital angular momentum mode with alternating phase vortex exists at the boundary of two such photonic crystals. In addition, unidirectional transport is robust to the working frequency even when the ring size or location of the pseudo-spin source varies in a certain range, leading to the superiority of the broadband photonic device. These findings enable one to make use of transverse angular momentum, a kind of degree of freedom, to achieve unidirectional robust transport in the telecom region and other potential applications in integrated photonic circuits, such as on-chip robust delay lines. [ABSTRACT FROM AUTHOR]

A transmission-type terahertz (THz) variable-bandwidth cross-polarization converter consisting of single-layer ultrathin metasurface is designed, fabricated and experimentally demonstrated. Both the simulated and measured results show that the bandwidth of linear polarization conversion is about 0.45 THz with a polarization conversion ratio over 90%. Different from the previous studies that mainly contributed to the geometry phase, the excellent characteristic of our designed device results from field-suppression of co-polarization components and field-enhancement of cross-polarization components, respectively. Even more intriguingly, the bandwidth of cross-polarization conversion is tunable by consecutively breaking the symmetry of the micro-structure, that is, the lower-order rotational symmetry, leading to a lower-order-symmetry-controlled THz cross-polarization conversion. Our study is an important step forward in developing compact, integrated, and bandwidth-controllable THz circuits and functional devices. [ABSTRACT FROM AUTHOR]