Table of contents

This letter presents an efficient algorithm for estimating the three-dimensional (3D) location of a photodiode (PD) receiver via visible light positioning. It solely works on measured powers from different light-emitting diode (LED) sources and does not require any prior knowledge of the PD receiver height. It is found that four LEDs are required that are not on the same circle, in order to unambiguously determine the 3D location. The algorithm is optimized towards a minimized calculation time in view of real-time operation on energy-constrained lightweight and mobile devices such as drones.

The successful exfoliation of graphite has initiated new science in many research fields and is employing a huge number of scientists in the world investigating the chemical, structural, mechanical and optoelectrical properties of the atomic-thick sheets of graphene and graphene oxide (GO). Similarly to other carbon-based materials, the graphene family have shown exceptional optical responses and nowadays it is engineered to produce efficient photonic components. In this review, we aim to summarize the main results in nonlinear optical response and fluorescence of GO. Moreover, its laser printing is reviewed as a novel promising lithographic technique.

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.

Bragg-reflection waveguides (BRWs) fabricated from AlGaAs provide an interesting nonlinear optical platform for photon-pair generation via parametric down-conversion (PDC). In contrast to many conventional PDC sources, BRWs are made of high refractive index materials and their characteristics are very sensitive to the underlying layer structure. First, we show that the design parameters like the phase matching wavelength and the group refractive indices of the interacting modes can be reliably controlled even in the presence of fabrication tolerances. We then investigate how these characteristics can be taken advantage of when designing quantum photonic applications with BRWs. We especially concentrate on achieving a small differential group delay between the generated photons of a pair and then explore the performance of our design when realizing a Hong–Ou–Mandel interference experiment or generating spectrally multi-band polarization entangled states. Our results show that the versatility provided by engineering the dispersion in BRWs is important for employing them in different quantum optics tasks.

Recent studies have highlighted that the rotational Doppler effect arises from rotational motion and optical orbital angular momentum (OAM), which has potential to detect rotating objects. Here, we investigate the frequency shift when an OAM beam is scattered from a surface with a radially periodic structure moving in the radial direction. This kind of frequency shift is related to the radial velocity and the harmonic components of the radially periodic structure of the surface. We support our conclusion by means of calculating time-evolution phase of light beam theoretically and measuring the rotating interference patterns through coherent detection. Furthermore, a more general theoretical model for the complex frequency shift resulting from the concurrence of both the angular and radial Doppler effects, is proposed. The complex frequency shift characteristics are associated with harmonic components and the velocity of the structural surface in both radial and angular directions. Interestingly, the frequency shift resulting from the moving surface may be spatially variant, because of the combination of the angular and radial Doppler effects. This scheme is useful for analyzing the light-moving matter interaction. Meanwhile, it might be applicable to transverse velocity measurement and frequency modulation.

Fano resonance is a kind of resonant phenomenon which manifests an asymmetric line-shape. Here, the Fano-type spectra of scattering are found in a coaxial cylindrical metallic meta-structure, both in the microwave and optical regime. It is found that the peak of Fano resonance is mainly contributed from the toroidal dipole, whereas the dip is due to the destructive interference of the electric and toroidal dipole modes. At the peak of the Fano resonance, the magnetic field shows a 'vortex-like' configuration at the gap of metallic structure and the corresponding magnitude can be enhanced remarkably. This toroidal-based Fano resonance in the proposed meta-structures may find potential applications in nanophotonics, such as optical sensing, spectral engineering, etc.

We numerically investigated the effect of chemical potential on Dyakonov–Tamm waves (DTWs) guided by a graphene-coated structurally chiral medium in the terahertz and the visible spectral regimes. Only one DTW can propagate in a specific direction in wide angular sectors, but multiple DTWs can propagate in a specific direction in narrow angular sectors, in both spectral regimes. Although the phase speed of a DTW depends weakly on the chemical potential in both spectral regimes, the propagation distance can be strongly dependent on the chemical potential in the terahertz regime but not in the visible regime. This difference can be attributed to the real part of the surface conductivity of graphene, which varies significantly with the chemical potential in the terahertz regime but not in the visible regime.

A canonical problem configured in three different arrangements of periodic multilayered isotropic dielectric material layers with a dissipative dielectric defect, to guide multiple compound Tamm waves with Uller–Zenneck wave characteristics, was formulated and solved. The numerical solutions showed multiple Tamm waves guided at a fixed wavelength along the dissipative defect with same polarization state. These waves propagate with different phase speeds, different propagation distances, different field localizations, and different field profiles. The results identified excitation of symmetric and anti-symmetric solutions of p-polarized Tamm waves and p- and s-polarized waveguide modes. The high phase speed solutions ceased to exist beyond a certain limit of dissipative dielectric defect thickness, but the low phase speed solutions were computed for a wide range of dissipative dielectric defect thickness. The Tamm waves were compounded when the thickness of defect is very small and transmuted into waveguide modes for increasing thickness of dissipative dielectric defect because most of the energy is then strongly confined to the dissipative dielectric defect. The excitation of multiple Tamm waves and waveguide modes at a given dissipative dielectric defect thickness and fixed wavelength can be considered for multi-channel optical communication and sensing applications.

Using Jones matrices and vectors, we show that a metasurface-based optical element composed of a set of subwavelength diffraction gratings, whose anisotropic transmittance is described by a matrix of polarization rotation by angle mφ, where φ is the polar angle, generate an mth order azimuthally or radially polarized beam, when illuminated by linearly polarized light, or an optical vortex with topological charge m, when illuminated by circularly polarized light. Such a converter performs a spin–orbit transformation, acting similarly to a liquid-crystal half-wave plate. Using the FDTD-aided numerical simulation, we show that uniform linearly or circularly polarized light passing through the above-described optical metasurface with m = 2 and then tightly focused with a binary zone plate generates an on-axis near-focus energy backflow comparable in magnitude with the incident energy. Notably, the magnitude of the reverse energy flow is shown to be the same when focusing a circularly polarized optical vortex with topological charge m = 2 and a light beam with the second-order polarization singularity.

In the field of metasurface-based light focusing, both convex metalenses with parabolic phase profile and autofocused Airy (AFA) beams play essential roles. AFA beam generation as a combination of two convergent mirrored Airy beams leaves the space between the two launched Airy profiles inefficient with zero amplitude transmission and constant phase distribution. In this paper, we propose using this inutile space as an independent metalens. We show that coincidence of the focal spot of the metalens to that of the AFA beam will increase the focusing intensity more than 24 percent at its focal point. It is shown that using the nonoperative space between the two launched Airy beams as an independent metalens not only does not disrupt the AFA focusing efficiency, but also enables us to use the full potential of the space to increase the focusing efficiency or flexibly assigning the focusing spots.

We present an electrically tunable metasurface which modulates the amplitude of the transmission spectra and works independently of the light polarization. The tunable metasurface comprises circular metal–insulator–metal (MIM) plasmonic resonators, using indium tin oxide (ITO) as a gate-tunable material. The property of the ITO film is tuned by applying a DC bias voltage through a 2D array of DC connections. The transmission amplitude at the resonance wavelength of 1650 nm can be modulated up to 29 dB by applying an electric voltage. The symmetric DC connections on the four sides of the circular MIM resonators enable polarization independence. This active metasurface could work as a signal modulator and an optical switch.

The exceptional points (EPs) in photonic crystals (PCs) with PT-symmetry involve the band coalescence at which the real bands are transformed into conjugate complex bands. The PC EPs are usually in the extended bands and cannot be excited in a guided form. To achieve some guided EPs, we design a two-dimensional photonic crystal waveguide with localized PT-symmetry. The waveguide edges are made of balanced gain and loss materials. With the increasing of material gain or loss coefficient, the coupled mode bands clearly undergo dynamical evolution. An interesting band attraction and coalescence behavior occurs, which leads to the formation of the first-order EP and the second-order EP. The second-order EP can be locally excited with multiple field distributions which is dependent on the line source position. In contrast to the other modes whose frequencies clearly change with material parameter, the EPs in this structure have unique frequency stability against the change of material parameters.

An asymmetric transmission device composed of all dielectric phase gradient metasurfaces on the dielectric substrate in the visible wavelengths is proposed and designed. In order to verify its operation principle and investigate its asymmetric transmission performance, ray and wave analyses are employed together. Specifically, ray tracing and finite-difference time-domain techniques are carried out in the study. The analytical calculations of the designed structure are confirmed with the results of the ray and wave analysis. It is also demonstrated that broadband and high contrast asymmetric transmission occur across nearly the entire visible spectrum from wavelengths of 500–715 nm. Especially, the difference of transmission between forward and backward illuminations in the design wavelength of 532 nm is found to be nearly 90% under TM polarization. In addition, it is indicated that a slight degradation in the asymmetric transmission performance of the structure occurs due to the change of the light polarization. It is shown that the asymmetric transmission has been directly related to the total reflection of the light for only one direction excitation case. The proposed structure can be fabricated with emerging nano-fabrication techniques in conformal metasurfaces and can be realized in different wavelength ranges.

The fundamental goals of display technology are achieving more vivid colors and higher resolutions. In this study, we propose a unique nanoarray silicon structure called a U-shaped structure array. This structure exhibits several Mie resonances for incident light, making it capable of reflecting specific colors by manipulating the geometric parameters of its structure. The bottom thickness of an individual U-shaped structure can be decreased to make one dominant resonance peak, thereby achieving a clearer reflected color. Based on this structure, we investigate all color responses to different geometric properties. Our results demonstrate that full vivid color can be reflected by this U-shaped structure array by manipulating its Mie resonances. Our results of all-dielectric U-shaped nanostructures can be applied to reflected displays and color filters.

We report on controlling the bi-photon orbital angular momentum (OAM) eigenmodes in the spontaneous parametric down conversion process by simply adjusting the asymmetry of the pump vortex beam. Adjusting the optic axis of the spiral phase plate of phase winding corresponding to OAM mode, l, with respect to the beam propagation axis, we have transformed a Gaussian beam into an asymmetric vortex beam with OAM modes, l, l–1, l–2 ...0 with different weightages. Pumping the nonlinear crystal with such asymmetric vortices and controlling their asymmetry we have tailored the spiral spectrum of the bi-photon OAM eigenmodes. Calculation of azimuthal Schmidt number of the bi-photons showed an increase in the spiral bandwidth of the OAM eigenmodes and hence the dimensionality of the system. Although we have restricted our study to show the increase in spiral bandwidth of the bi-photons by simply controlling the asymmetry of the pump vortices, the dimensionality of the bi-photon states can be enhanced further by manipulating the pump beam size and crystal length.

We have studied a femtosecond (fs) ultrafast all-optical photonic crystal (PhC) switch, and a new dynamic mechanism of the interplay between nonlinearity and Bragg scattering is revealed. Almost all phenomena observed in the experiments are reproduced in our numerical simulations. Completely different from the traditional band-gap shift mechanism, we propose a new dynamic mechanism that can explain the considerable increase of transmission and almost all other complex transmission behaviors of the switch. First, the fs-level ultrashort-lifetime nonlinearity generated by the fs pumping pulse, which cannot generate a band-gap shift, only causes a sudden phase change on the signal field that is still inside the PhC. Second, such a phase change can partially destroy the destructive interference of Bragg scattering, which is the physical reason for the low transmission in the photonic gap of the PhC. The study by Fourier transform of the temporally transmitted field can clearly support our new explanation. New phenomena, such as the larger than unity sum of transmission and reflection, are predicted. The new dynamic mechanism could be widely used in other systems as a new method to amplify nonlinear effects, or as a detector for ultrafast (femto- or attosecond) electronic processes.

Sum-frequency generation in the case of coaxial interaction of elliptically polarized Gaussian beams of fundamental radiation in isotropic chiral medium was studied. The system of three equations of nonlinear diffraction was solved numerically without the assumption of undepleting fundamental ('pump') waves. It has been shown that only for small (less than 0.1) values of nonlinear coupling coefficient of the interacting waves can their propagation be described within the framework of such approximation with precision of a few tenths of a percent. However, the results of the undepleting pump approximation are remarkably different from those obtained in the numerical solution of a system of nonlinear equations for greater values of nonlinear coupling coefficient. In the first turn, this difference appears in the amplitude and polarization of the sum frequency beam. When the nonlinear coupling coefficient is of the order of unity, its further increase does not lead to the growth of the conversion efficiency, which remains of the order of 10⁻². At the same time, there appear to be notable changes of intensity and polarization of light in the transverse sections of the beams at fundamental frequencies.

We present a simple method of spatial-division multiplexing/demultiplexing with orbital angular momentum (OAM) based on a multi-ring optical fiber. Multi-ring fibers are ideal spatial division multiplexing carriers, providing more communication channels including orthogonal OAM multiplexing channels and spatial channels. We propose an OAM encoding scheme using multiplexing perfect vortex array (MPVA) to make full use of these channels and achieve high-capacity data communication links. For improving the coupling efficiency, perfect vortices whose radii independent of topological charge are employed to generate the intensity rings in each ring core region. Furthermore, a practical encoding scheme of multiplexing OAM is proposed to optimize the use of available OAM states. The encoded information in one core could reach L bits as there are L available OAM states in the fiber. To decode the multiplexing perfect vortex, an additional correction phase is employed to transform the perfect vortex into conventional optical vortex. By detecting the states of multiplexing OAM of each spot in this array with a perfect vortex detection phase plate, information encoded by perfect vortices is decoded. The efficient data encoding and decoding method with MPVA could be expanded to almost all kinds of multi-ring fibers to achieve high coupling efficiency and high-capacity data transmission.

In this work, we propose a very simple, and efficient interferometric setup for transforming a homogeneous linearly polarized beam into a radially polarized beam. The proposal is based in a double-aperture common-path interferometer adapted for phase-shifting of π radians per quadrant. It is carried out by placing two composed grating in each aperture, which are built by joining two gratings displaced by a half-period, on horizontal and vertical direction, respectively. The input optical beams are orthogonal and linearly polarized, and their amplitudes are modulated in quadrature. We show that the combination of amplitude-only filters and the π phase-shifting per quadrant can achieve the implementation of complex filters such as the sinusoidal ones. The combination of the spatial modulation of both amplitude and phase generates an optical field with radial polarization. Specialized optical elements of high cost and SLM devices are avoided. We show the theoretical model and experimental results.

Imaging of periodic objects in free space brings us to the Talbot effect in optics, for x-rays and for matter waves. X-ray dynamical diffraction imaging of periodic objects inside a crystal has scientific and applied interest. Thus, we arrive at the concept of the dynamical diffraction Talbot effect in a crystal. This work is the first attempt to investigate the Talbot effect inside a medium. Using the Green function formalism, an exact formula for the diffracted wave amplitude is found. By means of 'paraxial' approximation, which is an analogue of the paraxial approximation in optics, it is shown that the dynamical diffraction Talbot effect takes place. Expressions for polarization sensitive Talbot and corrected Talbot distances are obtained. We analyse the influences of absorption, Bragg filtration of harmonics and polarization on the dynamical diffraction Talbot effect. For the first time, simulated Talbot carpets inside the crystal are obtained, which show that the predictions, obtained by 'paraxial' approximation, are true. We present the dynamical diffraction Talbot carpets observation method by means of a wedge-shaped crystal. The dynamical diffraction Talbot effect can be used for investigation of objects and crystal defects and deformations. Dynamical diffraction Talbot effect investigations in optics, for electrons and neutrons, are possible.

The stability and security of communication is very important, and a ghost imaging mechanism is useful to transmit the object signal. In this paper, the image transmission based on temporal ghost imaging (TGI) is demonstrated. The image is encoded into a sequence of binary numbers which are considered as the temporal object. The influences of the realization times and the reference information are presented. The larger realization times lead to the higher resolution of the image, and the bit error rate drops to 0.63% with 300 measurements. The stability of the image transmission is analyzed, and the scheme presents a natural ability against the noise disturbance and the network congestion, which demonstrates that about 75% of the packet loss probability is permitted in the transmission process. The security has been discussed from four kinds of attack modes and the reference information, and the results indicate the secure technique. These properties confirm the potential applications in encryption and telecommunication with the advantages of high security and quality of reconstructed information.

Most studies on ghost imaging focus on high-quality and high-resolution imaging with a few measurements. However, as far as we know, continuous multi-resolution imaging is rarely mentioned. In this work, we both theoretically and experimentally demonstrate a method that uses the Hadamard derived pattern to realize continuous multi-resolution imaging simply and quickly, which we call multi-resolution progressive computational ghost imaging, whereby both the reconstruction time and measurements required for multi-resolution images can be significantly reduced. This approach improves the flexibility of ghost imaging, and can be extended to multi-resolution image-dependent practical applications, such as target tracking and recognition.

This paper presents a robust and non-iterative algorithm for phase retrieval from two interferograms changed by an arbitrary unknown phase-step. First, the object phase is algebraically eliminated; second, the background and modulation light are approximated to 2D polynomials of degree K; third, an error function is algebraically derived; fourth, the least squares method is applied for estimating the phase-step, background, and modulation light, and finally the object phase is retrieved. With this approach, it is not necessary to do additional measurements. The important advantages of this method are its simple computer implementation and its capacity to support high spatial variations in the illumination. This approach is described theoretically, and verified numerically and experimentally.