Recent advances in optical metasurfaces for polarization detection and engineered polarization profiles

Metasurfaces, planar metamaterials consisting of a single layer or several layers of artificial structures, not only form the basis for fundamental physics research but also have considerable technological significance. Metasurfaces can locally modify the optical property within a subwavelength range, which can facilitate device miniaturization and system integration. Metasurfaces have shown extraordinary capabilities in the local manipulation of the light's amplitude, phase, and polarization, leading to a plethora of novel applications such as generalized Snell's law of refraction and photonic spin Hall effect. This progress report is focused on the recent advancements in the fundamental research of geometric metasurfaces and their applications in ultrathin optical devices, including planar metalenses, helicity multiplexed holograms, functionality switchable devices, polarization beam splitters, vector beam generation, arbitrary polarization control, and so on. The compactness, ease of fabrication, and unusual functionalities of these devices render geometric optical metasurface devices very attractive for new applications such as encryption, imaging, anti‐counterfeiting, optical communications, quantum science, and fundamental physics. This paper aims to bring readers some new insights, and to broaden the applications of geometric metasurfaces in more research fields of science and technology.

Guest Editorial

Nowadays, the coherent optical communication system plays an important role in communication field because of large capability and bandwidth. A coherent optical communication, based on high-order modulation and digital signal processing technologies, consists of optical transmitters, optical fiber lines, optical amplifiers and optical receivers. In the high-speed coherent optical communication system, the phase noise from the transmitter laser and the local oscillator laser can significantly degrade the performance of the signal transmission and detection, especially for the systems using high-order modulation format, such as m-ary phase shift keying (mPSK) and m-ary quadrature amplitude modulation (m-QAM). Therefore, investigations on laser phase noise compensation algorithm based on digital signal processing technologies has become more and more significant. In this work, a multi-ring carrier phase recovery algorithm is developed for compensating the laser phase noise in optical fiber communication systems using high-order modulation formats. Degradations on the performance of communication systems due to the laser phase noise have been investigated. The system performance using the proposed algorithm and the conventional Viterbi-Viterbi algorithm were also evaluated in 9-channel and 15- channel, 32-Gbaud, Nyquist-spaced QPSK, 16-QAM, 64-QAM and 256-QAM coherent transmission systems with considering the impact of the laser phase noise. It is found that the phase noise leads to stricter constraints on the linewidths of transmitter-side and receiver-side lasers, and it can greatly degrade the achievable information rates in communication systems. Besides, compared to the conventional Viterbi-Viterbi algorithm, which is usually applied in the QPSK system, our proposed algorithm can also well mitigate the laser phase noise in 16-QAM, 64-QAM and 256-QAM optical communication systems.
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Each photon in a vortex beam carries an orbital angular momentum (OAM). OAM detection plays a very important role in unlocking the new capability of vortex beams in both fundamental research and technological advancements. As the number of information channels continues to grow, there is an urgent need to improve the detection capability of the OAM sorting system. The unprecedented capability of optical metasurfaces has provided an opportunity to realize OAM detection in a compact platform. Different OAM modes are typically separated into distinct light spots in OAM sorting. However, each spot in an OAM sorting system is tailored to recognize only one predesigned OAM mode. A metasurface approach to detecting more OAM modes with the same number of light spots in the observation plane is demonstrated. The conservation of momentum in the design is realized with a customized multifoci metalens. The efficacy of this method is exemplified through the detection of two OAM modes in an individual light spot, dramatically increasing the detection capability of the OAM sorter. It is anticipated that this design will reduce the demand for spatial resources, creating new opportunities to explore OAM applications across many research fields such as optical communications and quantum science.

Deep neural networks (DNNs) have been applied to recover signals in optical communication systems and have shown competence of mitigating linear and nonlinear distortions. However, as the data throughput increases, the heavy computational cost of DNNs impedes them from rapid and power-efficient processing. In this paper, we propose an optical communication signal recovery technology based on a photonic convolutional processor, which is realized by dispersion delay unit and wavelength division multiplexing. Based on the photonic convolutional processor, we implement an optoelectronic convolutional neural network (OECNN) for signal post-equalization and experimentally demonstrate on 16QAM and 32QAM of an optical wireless communication system. With system parameters optimization, we verify that the OECNN can achieve accurate signal recovery where the bit error ratio (BER) is below the 7% forward error correction threshold of 3.8×10-3 at 2Gbps. With adding the OECNN-based nonlinear compensation, compared with only linear compensation, we improve the quality (Q) factor by 3.35 dB at 16QAM and 3.30 dB at 32QAM, which is comparable to that of an electronic neural network. This work proves that the photonic implementation of DNN is promising to provide a fast and power-efficient solution for optical communication signal processing.

A new class of planar optical components and devices has emerged using subwavelength metastructures with a strong contrast in refractive indices. High-contrast metastructures have shown promises to manipulate optical fields in an extraordinary way and to replace conventional bulky optical elements by their low-profile analogs, typically with subwavelength-scale features. We elucidate the underlying principle, how these seemingly low-profile geometries render unique optical responses, using the coupled-mode analysis in a multimode waveguide. Moreover, strong field localization in high-index structures allows us to interpret each single element in the metastructures as a low-quality-factor resonator (or a localized scatterer), permitting us to realize designer surface that shapes phase, amplitude, and polarization of light in free space, also known as an optical metasurface. The remainder of the thesis is devoted to explore novel applications in optics using high-contrast metastructures. One of the particularly interesting applications is to use them in an optical resonator. Specifically, we demonstrate to incorporate high-contrast subwavelength grating reflectors and dielectric metasufaces in a vertical Fabry–Perot cavity, and show that we can flexibly tune the resonance frequency by the subwavelength patterning. With this technique, we envision the realization of compact, on-chip spectrometers when integrating them on a photodetector array. Secondly, we investigate the use of high-contrast subwavelength gratings in visible wavelengths. We perform the optimization of their geometries and demonstrate a set of RGB color filters, down to near a micrometer in the pixel size. This platform exhibits unique performances such as high efficiency, angular insensitivity, and color tunability by the design. A novel device concept is also explored, where a high-contrast subwavelength grating reflector is integrated on a silicon platform to constitute an active resonant antenna, enabling high-speed, phase-dominant modulation by means of thermo-optic effect of silicon. We demonstrate an array of such active antennas, yielding a beam deflection capability. This justifies the robustness of our device design, enabling a large-scale integration of high-speed, phase-dominant spatial light modulators. Finally, we introduce a disorder-engineered metasurface in the context of wavefront shaping. Recently, wavefront shaping with disordered media has demonstrated optical manipulation capabilities beyond those of conventional optics, but translating this class of technology into a practical use has remained challenging due to enormous amounts of information needed to be characterized as the input-output responses. As a paradigm shift, we propose the use of disorder-engineered metasurface in wavefront shaping, where the disorder is programmatically designed and makes the system characterization-free prior to use. With this approach, we demonstrate high numerical aperture focusing in an extended volume as well as wide-field fluorescence imaging with unprecedented performances.

光纤通信已经成为目前通信网络发展的主流方向。但是随着光纤通信系统向着超远距离、超大容量、超高速率的方向发展，光纤色散已经成为其巨大的阻碍。如何有效地控制光通信系统中产生的色散，使光信号在大容量、高速率、长距离传输中无误码地到达接收端，已经成为人们迫切需要解决的问题。对光通信系统中的色散进行监测并进行动态补偿的研究，使得超大容量、超长距离、超高速率的光纤通信系统成为可能，有助于最终实现全光智能自动交换网络。在众多测量光纤色散的方法中，调制相移法具有结构简单，可靠性强等优点。本文中，我们对使用调制相移法测量光纤色散进行了研究和仿真，讨论了在不同光纤长度和不同调制频率下，影响调制相移法测量结果精度的因素。通过适当地选取调制频率，我们可以使用调制相移法测得不同光纤长度下色散的精确值。另外在本文中，我们通过仿真验证了调制相移法可以测量具有任意色散–波长曲线的光纤。因此在实际应用中，调制相移法是可以应用于高精度光纤色散的测量。 Optical fiber communication is the mainstream of communication development at present. However, with the development of optical fiber communication system which trending towards ultra-long length, ultra-large capacity and ultra-high speed, fiber dispersion turned to be a great obstacle. How to control the dispersion created in the optical communication system effectively while force optical signal to reach the receiving end in high-capacity, high-rate and error-free long-distance transmission has increasingly become a research focus of optical fiber communication systems. It is a great significance to research about monitor and dynamic compensate dispersion in the optical communication systems, so that the ultra-large capacity, ultra-long length and ultra-high speed optical fiber communication systems would not be impossible, all-optical intelligent network with automatic switching functions would come true. Among the varieties of methods to measure fiber dispersion, the phase-modulation shift method is the unique one, which has the advantages of simple structure, high reliability. In this paper, we make a research and simulate to use the phase-modulation shift method, meanwhile we also discuss about the factors which affect the accuracy of the results in the different fiber lengths and modulation frequencies. By the means that selecting appropriate modulation frequency, we can use modulation phase shift method to measure the exact value of dispersion comes out from different lengths of fiber. Moreover, this paper also proves that the phase-modulation shift method is able to measure the optical fiber with arbitrary dispersion-wavelength curve, which is already verified by our simulation. Therefore, the phase-modulation shift method is well to be applied to take practice in the high-precision fiber dispersion measurement.

Optical metasurfaces is a rapidly developing research field driven by its exceptional applications for creating easy-to-integrate ultrathin planar optical devices. The tight confinement of the local electromagnetic fields in resonant photonic nanostructures can boost many optical effects and offer novel opportunities for the nanoscale control of light-matter interactions. However, once the structure-only metasurfaces are fabricated, their functions will be fixed, which limits it to make breakthroughs in practical applications. Recently, persistent efforts have led to functional multiplexing. Besides, dynamic light manipulation based on metasurfaces has been demonstrated, providing a footing ground for arbitrary light control in full space-time dimensions. Here, we review the latest research progress in multifunctional and tunable metasurfaces. Firstly, we introduce the evolution of metasurfaces and then present the concepts, the basic principles, and the design methods of multifunctional metasurface. Then with more details, we discuss how to realize metasurfaces with both multifunctionality and tunability. Finally, we also foresee various future research directions and applications of metasurfaces including innovative design methods, new material platforms, and tunable metasurfaces based metadevices.

Here, we demonstrate low-cost, scalable manufacturing of optical metasurfaces with three approaches: 1) increasing a refractive index of resin with dielectric particle embedding for single-step nanoimprinting, 2) suppressing optical losses of hydrogenated amorphous silicon (a-Si:H) to apply complementary-metal-oxide-semiconductor technologies, and 3) high-index atomic layer deposited (ALD) structural resin. We demonstrate the effectiveness of these materials in creating optical metasurfaces operating at different wavelengths in the infrared, visible, and ultraviolet spectra. Firstly, we achieve high efficiencies of up to 90.6%, 47%, and 60% with a-Si, TiO2 , and ZrO2 PER at wavelengths of 940, 532, and 325 nm, respectively. Furthermore, we obtain a measured efficiency of 30% at a wavelength of 248 nm using ZrO2 PER metasurfaces. Secondly, by adjusting the deposition conditions of plasma-enhanced chemical vapor deposition, we engineer the bandgap of a-Si:H to enable low-loss operation, with minimum extinction coefficients as low as 0.082 at 450 nm. Using low-loss a-Si:H, we demonstrate efficient beam-steering metasurfaces with measured efficiencies of 42%, 65%, and 75% at 450, 532, and 635 nm, respectively, marking the first Si-typed metasurfaces working at the full visible, as well as demonstrating high-efficiency near-infrared metalenses. Finally, we manufacture highly efficient metalenses using hybrid ALD structural resin with deep-ultraviolet lithography in the visible and ultraviolet. Their measured efficiencies approach 60.9%, 77.8%, and 64.8% at 450, 532 and 635 nm, making them suitable for ultrathin virtual reality devices. Our approaches using PER, low-loss a-Si:H, and hybrid ALD structural resin enables the low-cost, large-area manufacturing of efficient optical metasurfaces across different wavelengths, facilitating the commercialization of metasurface-based photonic devices .

Optical polarization is an important characteristic of electromagnetic waves that has a significant impact on number of applications, such as information delivery, 3D imaging, and quantum computation. Metasurfaces, sort of artificially designed planar structure, have attracted immense attention due to their ability to control the amplitude and phase of electromagnetic waves at a subwavelength scale. Metasurfaces hold promise for the fields of nonlinear dynamics, light beam shaping, quantum computation, etc. Beside these promising applications, metasurfaces can also be used for versatile polarization generation in a compact device dimension. Therefore, metasurfaces can be used for creation of flat optical devices with novel functionalities. In this talk, I will discuss how we can generate versatile polarization states by using metasurfaces. Firstly, I will describe geometric phase metasurfaces, which can be used for passive polarization control. We demonstrate six metasurface chips integrated on a single sample, in which each chip is responsible for generating one specific polarization state thus generating versatile polarization sates. Subsequently, I will discuss a scheme of active polarization modulation by using indium tin oxide (ITO)-based tunable metasurfaces. By suitably biasing the metasurface, the linearly-polarized incident light can be actively converted to a cross-polarized, circularly-polarized or elliptically-polarized light.

Optical metasurfaces have attracted much attention in recent years, due to their versatile capabilities to manipulate electromagnetic waves within subwavelength scale. Dielectric metasurface holograms can significantly improve resolution, efficiency, view angle and suppression of the high order diffraction compared to traditional devices. The ability to completely control the optical properties of light using metasurface is sensitive to the geometric parameters of the nanostructures. However, current metasurface fabrication methods often result in unavoidable structural deformations. Here we introduce the rounded angles (fillets) to the sharp corners of the meta-atoms, such as rectangular pillars and isosceles triangular pillars, in dielectric metasurface and numerically study the effect of these fillets under different symmetric conditions. We find that with specific radii of the fillets, bound states in continuum (BICs) appear, making the structure act as an absorber. Apart from these BICs, fillets that do not break the original symmetry of the meta-atoms have almost no effect on the manipulation of incident light by the metasurface. However, for fillets that break the original symmetry of the meta-atoms, the amplitude and phase manipulation by metasurface may be distorted.

The efficient and accurate evaluation of the transmission performance of high-capacity optical communication systems has always attracted significant research attentions. The enhanced Gaussian noise (EGN) model is considered as an excellent solution to predict the system performance taking into account linear and nonlinear transmission impairments. Since the conventional form of the EGN model is complicated and intractable for a fast computation, the closed-form simplification has been regarded as a direction to significantly reduce the computational complexity. However, the accuracy of such a closed-form EGN model becomes a main concern in the application of ultra-wideband optical communication systems. In this work, we have investigated the accuracy of the closed-form EGN model for ultra-wideband optical fiber communication systems, where the performance of the system using electronic dispersion compensation, multi-channel nonlinearity compensation and full-field nonlinearity compensation has been evaluated in terms of symbol rate, number of channels and signal power. Our work will provide an insight on the application of the EGN model in next-generation ultra-wideband long-haul optical fiber communication networks.

Preface. Unconventional Imaging and Processing. Coherence Methods for Unconventional Imaging Processes E.N. Leith. Some Diffractive - Optical Systems Based on Novel Gratings A.W. Lohmann. Technologies and Potential Applications of Adaptive Optics J.C. Dainty. Pattern Recognition with Joint Transform Correlators - Recent Developments E. Marom, et al. Storage of 3D Information on 2D Elements J. Shamir. Generalized Super Resolution Approach based on the Degrees of Freedom Hyper-Space Adaptation D. Mendlovic, et al. Diffractive Elements and Microoptics. Real-Time Holographic Interferometry in Photorefractive BTO Crystals Using Optical Phase Conjugation with a Low Power Laser Diode V.I. Vlad, et al. Planar Diffractive Elements for Compact Optics Y. Amitai, et al. Design and Fabrication Issues in Subwavelength Diffractive Optical Elements P. Chavel, P. Lalanne. Subwavelength and quasi-Subwavelength Diffractive Optical Elements L.M. Bernardo. Microsystems for Optical Imaging and Interconnects R. Dandliker, et al. Micro-Optical Components for Information Technology Fabricated via LIGA Technique H.-D. Bauer, et al. Nonlinear and Ultrafast Optics. Nonlinear Photonic Engineering: Physics and Applications J. Zyss, et al. Novel Optical Phenomena in Near-Critical Photorefractive Crystals: Diffusion Driven Diffraction and Spontaneous Self-Trapping B. Crosignani, et al. Frequency Upconversion of Lasers with Optical Parametric Oscillators O. Aytur. Characterization of Nonlinear Optical Materials for Photonic Applications S. Couris, et al. Adaptive Techniques in Ultrafast Optics Y. Silberberg, et al. Nonlinear Spatio-Temporal Information Processing with Femtosecond Laser Pulses Y. Fainman, etal. Optical Communication. All-Optical Semiconductor Switching Devices for Applications in Optical Communication Systems S. Diez, et al. Computer Modeling of Optical Fibers and Waveguides M.S. Al Salameh, S.A. Alkhawaldeh. Tbit/sec Optical ATM Switch Utilizing Dynamic WDM Technology D. Sadot. Towards Uniform Channel Performance in Dynamic WDM Systems and Networks A.E. Willner. Devices, Interconnects and Switching. Resonant Structures for Optical Processing and Communication A.A. Friesem, et al. Liquid Crystal Spatial Light Modulators as Intracavity Laser Elements and Dynamic Holographic Correctors for Distortions in Optical Systems V. Berenberg, et al. Fundamental Issues in Optical Interconnections H.M. Ozaktas. The Electroholographic Optical Switches and Interconnects A.J. Agranat, et al. VIVALDI -- Variations on a Theme of Optical Crossbars T.J. Hall. Laser-Material Interactions and Optical Storage. Laser Materials Processing for Optoeelectronics and Information Systems N.A. Vainos. Recording in Quantum Dot Glasses by Pulsed Laser Irradiation Y. Kaganovskii, et al. Photorefractive Volume Holographic Memories for Associative Storage and Processing in a Multimedia Environment P.A. Mitkas, G. Betzos. Characteristics and Applications of Volume Phase Holographic Gratings Recorded in Photo-Thermo-Refractive Glasses M.G. Moharam, S. Dunn. Optical Research in Israel. Electrooptics in Israel M. Oron. Subject Index. Author Index.

Planar photonics technology is expected to facilitate new physics and enhanced functionality for a new generation of disruptive optical devices. To analyze such planar optical metasurfaces efficiently, we propose a prismatic discontinuous Galerkin time domain (DGTD) method with a generalized dispersive material (GDM) model to conduct the full-wave electromagnetic simulation of planar photonic nanostructures. Prism-based DGTD allows for triangular prismatic space discretization, which is optimal for planar geometries. In order to achieve an accurate universal model for arbitrary dispersive materials, the GDM model is integrated within the prism-based DGTD. As an advantage of prismatic spatial discretization, the prism-based DGTD with GDM has fewer elements than conventional tetrahedral methods, which in turn brings higher computational efficiency. Finally, the accuracy, convergence behavior, and efficiency improvements of the proposed algorithm is validated by several numerical examples. A simulation toolkit with the proposed algorithm has been released online, enabling users to efficiently analyze metasurfaces with customized pixel patterns.

The design, fabrication, optical and mechanical characteristics, and reliability of several kinds of the latching-type single-mode-fiber (SMF) switch are reported. Practical applications of these switches to optical fiber communication systems and optical testing and sensing systems are also demonstrated. In the developed switches, a movable fiber is actuated by the magnetic attraction force produced between permanent magnets and a micromagnetic-alloy pipe fixed around the fiber. The ultralight micropipes (0.13 (i.d.)-0.18 (o.d.)/spl times/5/sup L/ mm, 400 /spl mu/g) were successfully massproduced by a fiber-micromachining technique based on metallizing and etching of sacrificial fibers. We used a refractive-index-matching oil in the switches to reduce insertion loss and reflected light. The several hundred fabricated 1/spl times/2 standard SMF switches exhibited excellent optical characteristics, such as typical insertion loss of 0.36 dB, return loss of 49 dB and crosstalk of -70 dB. They exhibited stable latching-type operation with loss changes of below 0.1 dB during 10/sup 5/ switching operations with minimum driving power of 9 mW and switching time of 2 ms. The switches were subjected to endurance (10/sup 8/ switching operations), heat cycles, shock and vibration, and long-term (6.5 years) intermittent operation tests, and revealed no significant optical or mechanical degradation. A 1/spl times/8 latching-type SMF switch was constructed using seven 1/spl times/2 switches cascaded in three stages and uniform insertion losses of below 0.7 dB and return losses of over 47 dB at each of eight ports were obtained. The 1/spl times/2 and 1/spl times/8 switches have been applied to NTT's optical communication systems. We fabricated a compact and cost-effective wavelength switch by installing microfilter chips in the 1/spl times/2 switch and confirmed stable latching-type wavelength switching between 1.3- and 1.55-/spl mu/m optical signals. We also designed and fabricated a polarization-maintaining switch using polarization maintaining and absorption reducing (PANDA) fibers and confirmed stable polarization maintaining switching operations. These switches are applicable not only to optical fiber communication systems, but also to optical testing and sensing systems.