Propagation model for vector beams generated by metasurfaces

Modern Types of Axicons: New Functions and Applications

Axicon is a versatile optical element for forming a zero-order Bessel beam, including high-power laser radiation schemes. Nevertheless, it has drawbacks such as the produced beam's parameters being dependent on a particular element, the output beam's intensity distribution being dependent on the quality of element manufacturing, and uneven axial intensity distribution. To address these issues, extensive research has been undertaken to develop nondiffracting beams using a variety of advanced techniques. We looked at four different and special approaches for creating nondiffracting beams in this article. Diffractive axicons, meta-axicons-flat optics, spatial light modulators, and photonic integrated circuit-based axicons are among these approaches. Lately, there has been noteworthy curiosity in reducing the thickness and weight of axicons by exploiting diffraction. Meta-axicons, which are ultrathin flat optical elements made up of metasurfaces built up of arrays of subwavelength optical antennas, are one way to address such needs. In addition, when compared to their traditional refractive and diffractive equivalents, meta-axicons have a number of distinguishing advantages, including aberration correction, active tunability, and semi-transparency. This paper is not intended to be a critique of any method. We have outlined the most recent advancements in this field and let readers determine which approach best meets their needs based on the ease of fabrication and utilization. Moreover, one section is devoted to applications of axicons utilized as sensors of optical properties of devices and elements as well as singular beams states and wavefront features.

Generation of cylindrical vector vortex beams using a biconical glass rod

This Letter proposes a biconical glass rod for generating a cylindrical vector vortex (CVV) beam. Based on the principle of total internal reflection and the cylindrical symmetry structure of the glass rod, a circularly polarized incident beam with a constant phase distribution can be converted into a CVV beam, which possesses both a spatially inhomogeneous polarization and a helical phase distribution. The polarization azimuth of the CVV beam can be tuned with the aid of a polarization rotator composed of two cascade half-wave plates. The design theory is presented, and the feasibility of the design is demonstrated experimentally.

Cylindrical vector beam multiplexing for radio-over-fiber communication with dielectric metasurfaces

Radio-over-fiber (ROF) technology, loading microwave signal on light beams, has attracted considerable attention in wireless access network for its superiority in processing high-frequency microwave signals. Multiplexing for achieving high-capacity density, however, remains elusive in ROF communication because the optical microwave occupies large bandwidth. Here, we introduce a cylindrical vector beam (CVB) multiplexing for ROF communication with dielectric Pancharatnam-Berry phase-based metasurfaces (PBMs). CVBs, a structured light beam possessing spatially nonuniform polarization distribution and carrying vector mode, provide an additional multiplexing dimension for optical communication with the advantages of weak scintillation in free-space and low mode injure in few-mode-fiber. Exploiting the spin-orbit interaction of the PB phase, we construct PBMs to manipulate CVBs, which show broadband working wavelengths ranging from C- to L-band. After 3 m free-space propagation, two multiplexed CVBs carrying 100 GHz microwave are successfully demultiplexed, and the 100 GHz ROF communication with 12 Gbit/s QPSK-OFDM signals is realized. The crosstalk of the multiplexed CVBs is less than -15.13 dB, and the bit-error-rates (BERs) are below 3.26 × 10^-5. With 5 km few-mode-fiber transmission, the CVBs are also demultiplexed with the BERs of 6.51 × 10^-5. These results indicate that CVB is not only capable of free-space transmission but also available for few-mode-fiber transmission, which might pave new avenues for the multiplexing of ROF communications.

Compact optical module to generate arbitrary vector vortex beams

We demonstrated a compact optical module that is capable of efficiently generating vector vortex beams (VVB). With this device, a linearly polarized input beam can be converted into a vector beam with arbitrary spatial polarization and phase distributions, accompanied by an energy utilization up to 61%. Equally important, the area utilization of the spatial light modulator, a key component in the device, is as high as 65.5%. With the designed vector-vortex-beam-generation module, several types of VVBs with different vortex topological charges and spatial polarization distributions were created experimentally. This device may find applications in optical tweezers, laser machining, and so on.

Machine Learning-Based Classification of Vector Vortex Beams

Structured light is attracting significant attention for its diverse applications in both classical and quantum optics. The so-called vector vortex beams display peculiar properties in both contexts due to the nontrivial correlations between optical polarization and orbital angular momentum. Here we demonstrate a new, flexible experimental approach to the classification of vortex vector beams. We first describe a platform for generating arbitrary complex vector vortex beams inspired to photonic quantum walks. We then exploit recent machine learning methods-namely, convolutional neural networks and principal component analysis-to recognize and classify specific polarization patterns. Our study demonstrates the significant advantages resulting from the use of machine learning-based protocols for the construction and characterization of high-dimensional resources for quantum protocols.

Multicolor concentric annular ultrafast vector beams

Novel multicolor concentric annular ultrafast vector beams (MUCAU-VB) are firstly generated simply by using cascaded four-wave mixing (CFWM) in a glass plate pumped by two intense vector femtosecond pulses. A proof-of-principle experiment shows that up to 10 frequency up-conversion concentric annular radially polarized sidebands are obtained simultaneously based on CFWM process, where the spectra range of the first 7 order sidebands extending from 545 nm to 725 nm. The results prove the polarization transfer property from the pump beam to the signal beams even in the CFWM, a third-order optical parametric process. The pulse duration of the first order sideband is measured to be 74 fs which is according with those of two input beams. These novel MUCAU-VB, which are manipulated in temporal, spectral, spatial domain and polarization state simultaneously, are expected to apply in wide fields, such as manipulating particles and multicolor pump-probe experiments.

Wavelength-adaptable effective q-plates with passively tunable retardance

Wave retarders having spatially varying optical axes orientations, called q-plates are extremely efficient devices for converting spin to orbital angular momentum of light and for the generation of optical vortices. Most often, these plates are designed for a specific wavelength and have a homogeneous constant retardance. The present work provides a polarimetric approach for overcoming both these limitations. We theoretically propose and experimentally demonstrate q-plates with tunable retardance, employing a combination of only standard q-plates and waveplates. A clear prescription is provided for realizing wavelength indepedent q-plates for a desired retardance, with a potential for ultrafast switching. Apart from the potential commercial value of the proposed devices, our results may find applications in quantum communication protocols, astronomical coronography, angular momentum sorting and in schemes that leverage optical vortices and spin to orbital angular momentum conversion.

Generation of Switchable Singular Beams with Dynamic Metasurfaces

Singular beams have attracted great attention due to their optical properties and broad applications from light manipulation to optical communications. However, there has been a lack of practical schemes with which to achieve switchable singular beams with sub-wavelength resolution using ultrathin and flat optical devices. In this work, we demonstrate the generation of switchable vector and vortex beams utilizing dynamic metasurfaces at visible frequencies. The dynamic functionality of the metasurface pixels is enabled by the utilization of magnesium nanorods, which possess plasmonic reconfigurability upon hydrogenation and dehydrogenation. We show that switchable vector beams of different polarization states and switchable vortex beams of different topological charges can be implemented through simple hydrogenation and dehydrogenation of the same metasurfaces. Furthermore, we demonstrate a two-cascade metasurface scheme for holographic pattern switching, taking inspiration from orbital angular momentum-shift keying. Our work provides an additional degree of freedom to develop high-security optical elements for anti-counterfeiting applications.

Metasurfaces with continuous ridges for inverse energy flux generation

In this study, we propose a new approach to construct metasurfaces for the generation of inverse energy flux near the optical axis. We derive new equations intended to create continuous subwavelength relief for transformation of a linearly polarized input field into a radially polarized beam with an arbitrary order. Proposed metasurfaces combine the polarization converter as subwavelength gratings with a varying period and the focusing element as additional structure. Such a combination increases polarization conversion efficiency and decreases the number of optical elements in an arrangement. Numerical simulations of the proposed metasurfaces, based on the finite element method, show that the higher-order polarization conversion provides the greater integrated inverse energy flux. Moreover, the shape of the inverse flux area achieved with the higher-order metasurface is annular and has bigger force area.

Generation of arbitrary vector vortex beams based on the dual-modulation method

A dual-modulation method that combines a liquid crystal on silicon spatial light modulator with an S-waveplate and a biplate consisting of double quarter-wave plates (DQWPs) or double half-wave plates (DHWPs) is proposed. The method is used to realize the phase and polarization dual modulation of an incident laser beam. This study focuses on the generation of an arbitrary vector vortex beam (VVB) based on the proposed dual-modulation method. The phase and polarization transformation effects of the proposed method are theoretically derived using the Stokes-Mueller matrix algorithm. Correspondingly, an experimental configuration is constructed to generate arbitrary VVBs, and correlation analyses are carried out to quantitatively evaluate the quality of the generated VVBs. The results indicate that the correlation coefficients of the generated VVBs can reach more than 0.94 whether the biplate in the experimental configuration is DQWP or DHWP.

Formation of hybrid higher-order cylindrical vector beams using binary multi-sector phase plates

Nowadays, the well-known cylindrical vector beams (CVBs) – the axially symmetric beam solution to the full-vector electromagnetic wave equation – are widely used for advanced laser material processing, optical manipulation and communication and have a great interest for data storage. Higher-order CVBs with polarisation order greater than one and superpositions of CVBs of various orders (hybrid CVBs) are especially of interest because of their great potential in contemporary optics. We performed a theoretical analysis of the transformation of first-order CVBs (radially and azimuthally polarised beams) into hybrid higher-order ones using phase elements with complex transmission functions in the form of the cosine or sine functions of the azimuthal angle. Binary multi-sector phase plates approximating such transmission functions were fabricated and experimentally investigated. The influence of the number of sectors and a height difference between neighbouring sectors, as well as the energy contribution of the different components in the generated hybrid higher-order CVBs were discussed in the context of polarisation transformation and vector optical field transformation in the focal region. The possibility of polarisation transformation, even in the case of weak focusing, is also demonstrated. The simple structure of the profile of such plates, their high diffraction efficiency and high damage threshold, as well as the easy-to-implement polarisation transformation principle provide advanced opportunities for high-efficient, quickly-switchable dynamic control of the generation of structured laser beams.

Integrated radiation and scattering performance of a multifunctional artificial electromagnetic surface

In this paper, a multifunctional artificial electromagnetic surface (AEMS) with integrated radiation and scattering performance is proposed and realized. Different from previous AEMS designs that mainly focus on scattering performance, this AEMS design takes both radiation and scattering properties into consideration in the designing process. Inspired by the design concept of antenna, a feeding structure is added to each AEMS element to achieve radiation performance. Meanwhile, the concerned characteristics of AEMS elements are almost maintained. For achieving wideband low-scattering performance, two different kinds of AEMS elements are designed and arranged in a chessboard configuration. Simulated and measured results prove that our method offers an effective strategy to design multifunctional AEMS that achieve radiation and scattering performance simultaneously.

Switchable phase and polarization singular beams generation using dielectric metasurfaces

Singular beams which possess helical phase wavefront or spatially inhomogeneous polarization provide new freedom for optical field manipulation. However, conventional schemes to produce the singular beams have difficulty in realizing the flexible switch between different singular beams. In this work, we have experimentally demonstrated the capability of dielectric metasurfaces to generate three types of singular beams and switch between them at working wavelength of 1550  nm. We have shown vortex beam and cylindrical vector beam generation with single metasurface and cylindrical vector vortex beam generation with two cascaded metasurfaces. Moreover, experimental demonstration on switching cylindrical vector beam into vortex beam has also been done by combining one quarter-wave plate and a Glan laser polarizer. The experimental results match well with the analysis from the Jones matrix calculations. The average conversion efficiency of cylindrical vector beam to vortex beam was estimated to be 47.7%, which was about 2.3% lower than the theoretical prediction.

Efficient generation of vector beams by calibrating the phase response of a spatial light modulator

The spatial light modulator (SLM) is considered as an effective device to create beams with inhomogeneous phases and polarizations, such as vortex beams and vector beams. However, the nonlinear responses of SLM severely reduce the generation efficiency of these beams. In this paper, by calibrating the SLM to present a linear phase response in the scope of 0-2π, we propose a convenient and efficient method of creating vector beams with arbitrary polarizations based on phase encoding. Compared with the common methods of generating vector beams, our approach can distinctly enhance the generation efficiency.

Geometric phase Doppler effect: when structured light meets rotating structured materials

We examine the geometric phase Doppler effect that appears when a structured light interacts with a rotating structured material. In our scheme the structured light possesses a vortex phase and the structured material works as an inhomogeneous anisotropic plate. We show that the Doppler effect manifests itself as a frequency shift which can be interpreted in terms of a dynamic evolution of Pancharatnam-Berry phase on the hybrid-order Poincaré sphere. The frequency shift induced by the change rate of Pancharatnam-Berry phase with time is derived from both the Jones matrix calculations and the theory of the hybrid-order Poincaré sphere. Unlike the conventional rotational Doppler effect, the frequency shift is proportional to the variation of total angular momentum of light beam, irrespective of the orbital angular momentum of input beams.