Flat Optics Research Articles

Near-infrared double-layer cascaded metasurface for beam shaping

The vast applicability of collimated flat-topped beam shapers, predominantly constructed from traditional lens elements, is met with challenges when the scale is less than wavelength. Metasurfaces have an excellent ability for optical manipulation, which can provide a promising approach to flat optics. Here, a metasurface-based Gaussian beam shaper is designed to combine the transmission phase principle with geometric transformation methods, which can reshape a 1550 nm Gaussian beam into a flat-topped beam with a uniformity of 84.39%. Furthermore, a cascaded metasurface beam shaper design is proposed to address the significant divergence in the flat-topped beam obtained from the single-layer metasurface. Simulation results indicate the output beam exhibits both uniform intensity and phase distributions over a considerable transmission distance, effectively minimizing the divergence of the output beam. This research has potential applications in various fields, such as optical antennas, fiber optics, and other optical systems.

Spatially varying nanophotonic neural networks.

The explosive growth in computation and energy cost of artificial intelligence has spurred interest in alternative computing modalities to conventional electronic processors. Photonic processors, which use photons instead of electrons, promise optical neural networks with ultralow latency and power consumption. However, existing optical neural networks, limited by their designs, have not achieved the recognition accuracy of modern electronic neural networks. In this work, we bridge this gap by embedding parallelized optical computation into flat camera optics that perform neural network computations during capture, before recording on the sensor. We leverage large kernels and propose a spatially varying convolutional network learned through a low-dimensional reparameterization. We instantiate this network inside the camera lens with a nanophotonic array with angle-dependent responses. Combined with a lightweight electronic back-end of about 2K parameters, our reconfigurable nanophotonic neural network achieves 72.76% accuracy on CIFAR-10, surpassing AlexNet (72.64%), and advancing optical neural networks into the deep learning era.

Large‐Area Floating Display with Wafer‐Scale Manufactured Metalens Arrays

AbstractMetasurface‐based flat optics has a great potential to replace conventional bulky and heavy optical elements, consistent with the trend of miniaturizing optical elements. One of the trends is to broaden the operating area of the metasurface. The previous approaches are focused on expanding the metasurface area, which has intrinsic manufacturing and optical limitations. Here, this work presents the fabrication of wafer‐scale metalens arrays, and demonstrates the Gabor superlens composed of the metalens arrays, which behaves optically like a large lens system. A pair of fabricated 8‐inch‐sized metalens arrays are used to float the large‐area display, producing a real image in the air. This superlens is easily manufactured in a high‐throughput and cost‐effective manner using an argon fluoride dry scanner and a single reticle. Their capability for diffraction‐limited focusing and imaging is demonstrated. Considering the groundbreaking nature of imaging a large‐area display through the metalens arrays, this work shows a great potential for scaling up the optical display systems in a simple manner.

Substrate‐Induced Maximum Optical Chirality of Planar Dielectric Structures

AbstractResonant dielectric planar structures can interact selectively with light of particular helicity thus providing an attractive platform for “chiral flat optics”. The absence of mirror‐symmetry planes defines geometric chirality, and it remains the main condition for achieving strong circular dichroism. For planar optical structures such as photonic‐crystal slabs and metasurfaces, breaking the out‐of‐plane mirror symmetry is especially challenging, as it requires to fabricate meta‐atoms with a tilt, variable height, or vertically shifted positions. Although transparent substrates formally break out‐of‐plane mirror symmetries, their optical effect is typically subtle being rarely considered for enhancing optical chirality. Here it is revealed that low‐refractive‐index substrates can induce up to maximum intrinsic optical chirality in otherwise achiral metastructures so that the transparency to waves of one helicity is combined with resonant blocking of waves of the opposite helicity. This effect originates from engineering twisted photonic eigenstates of different parities. The perturbation analysis developed in terms of the resonant‐state expansion reveals how the eigenstate coupling induced by a substrate gives rise to a pair of chiral resonances of opposite handedness. The general theory is confirmed by the specific examples of light transmission in the normal and oblique directions by a rotation‐symmetric photonic‐crystal slab placed on different transparent substrates.

Low-cost, high-volume, manufacturable 0.88 NA multi-wavelength diffractive lens array for optical document security

We design, manufacture, and characterize a high-numerical-aperture (NA=0.88, f/0.27), multi-wavelength (480 nm, 550 nm, and 650 nm) multilevel diffractive microlens array (MLA). This MLA achieves multi-wavelength focusing with a depth of focus (DoF) twice the diffraction-limited value. Each microlens in the array is closely packed with a diameter of 70 µm and a focal length of 19 µm in air. The MLA is patterned on one surface of a polymer film via UV casting, positioning the focal plane at the distal end of the polymer film. Each microlens focuses light at three design wavelengths into a focal spot with an estimated FWHM of ∼310nm. By placing this MLA directly on a standard high-resolution banknote print (minimum feature width of 10–15 µm), we demonstrate color-integral imaging for anti-counterfeiting. In contrast, refractive MLAs cannot achieve high-NA, multi-wavelength focusing or extended DoF. The extended DoF of our MLA ensures reliable performance despite variations in the polymer film’s thickness. Our MLA, produced via UV casting, enables extremely low-cost, high-volume production, making it ideal for flat optics in banknotes and document security.

Amorphous to Crystalline Transition in Nanoimprinted Sol-Gel Titanium Oxide Metasurfaces.

Crystalline titanium dioxide (TiO2) (anatase and rutile) possesses a higher refractive index than amorphous TiO2 and near-zero absorption in the visible region, making them an ideal material for visible metasurfaces. However, fabrication limitations hinder their implementation into flat optics. In this work, a wafer-scale manufacturing platform is proposed for crystalline TiO2 metasurfaces. Sol-gel TiO2 is developed as a printable material in which its material phase can be precisely controlled to produce amorphous, anatase, or rutile, depending on the crystallization temperature. Therefore, anatase or rutile metalenses can be fabricated on a wafer scale using thermal nanoimprint lithography and sintering process. The high refractive index of the crystalline TiO2 contributes to the enhanced conversion efficiency of the fabricated metalenses. The fabricated metalenses exhibit diffraction-limited focusing and imaging capabilities, comparable to the theoretically ideal lenses.

Higher-Dimensional Communications Using Multimode Fibers and Compact Components to Enable a Dense Set of Communicating Channels

Higher-dimensional communications are of interest for multiple reasons, including increasing the classical transmission capacity and, more recently, the quantum state transfer through fibers using the many modes within the fiber. For quantum communications, this enables an increase in the number of bits per photon, increasing quantum fidelity, increasing error thresholds and enabling hyperentanglement transfer, among other possibilities. A high-dimensional quantum state transfer can be transported through multimode fiber using the many modes available. However, this transfer of information through multimode optical fiber is limited by attenuation and mode coupling among the various spatial and polarization modes. Here, we consider how this mode coupling impacts the transfer process. We consider the fiber’s modal properties, including orbital angular momentum, modal group numbers, and principal modes. We also investigate and propose input and output optical components, as well as fiber properties, which better mitigate the deleterious effects of mode coupling. We use the WKB approximation to the scaler wave equation as a guidance to quantify this coupling and then implement corrections to this approximation using exact solutions to the scaler wave equation. We consider methods to circumvent this mode coupling using optical fiber designs, holographic optical components and devices that are commercially available today. Some of these components, such as the holographic gratings and lenses, could be implemented using flat optics.

PyMOE: Mask design and modeling for micro optical elements and flat optics

We introduce a new open-source software package written in Python to design and model micro optical elements, such as diffractive lenses, holograms, as well as other components within the broad area of flat optics, and generate their corresponding (production-ready) lithography mask files. To this aim, the package provides functions to design a multitude of kinoform lenses, phase masks and holograms, but is versatile and the user can implement any arbitrary numerical or analytical optical component designs. For validating the designs, this package provides scalar diffraction propagation to simulate optical field propagation in different regimes covering near- and far-field regions (Fresnel, Fraunhofer and Rayleigh-Sommerfeld). Particularly, by implementing Rayleigh-Sommerfeld propagation, we demonstrate accurate field propagation within near- and far-field ranges, providing versatility and accuracy. Importantly, the package allows to directly export production-ready multilevel/binary lithography mask files of the designed optical components. Additionally, metasurface masks can conveniently be generated for any user-defined meta-element library given as input. Finally, the software package capabilities are illustrated with examples of mask design and modeling of diffractive lenses, holograms, and metasurfaces susceptible of being fabricated via lithography techniques. Beyond lithography, the package can also straightforwardly be used in other applications requiring mask generation, such as beam shaping, optical trapping and digital holography.

Circularly polarized plane terahertz waves radiated from a resonant tunneling diode integrated with a collimating metalens and quarter-wave plate

Terahertz flat optics based on metasurfaces can replace massive optical components with optically thin components. However, metasurfaces with unprecedented material properties frequently produce a specified function, and terahertz flat optics has yet to be commonly adopted in terahertz devices that require multiple functions. Here, we present a two-layer component composed of a collimating metalens and a quarter-wave plate that convert linearly polarized terahertz wide-angle radiation waves from a resonant tunneling diode to circularly polarized plane waves. Our findings would be applied to laminate structures with optical vortices, ultrahigh directivity and arbitrary wavefront control in 6 G wireless communications.

Bio-inspired flat optics for directional 3D light detection and ranging

The eyes of arthropods, such as those found in bees and dragonflies, are sophisticated 3D vision tools that are composed of an array of directional microlenses. Despite the attempts in achieving artificial panoramic vision by mimicking arthropod eyes with curved microlens arrays, a wealth of issues related to optical aberrations and fabrication complexity have been reported. However, achieving such a wide-angle 3D imaging is becoming essential nowadays for autonomous robotic systems, yet most of the available solutions fail to simultaneously meet the requirements in terms of field of view, frame rate, or resistance to mechanical wear. Metasurfaces, or planar nanostructured optical surfaces, can overcome the limitation of curved optics, achieving panoramic vision and selective focusing of the light on a plane. On-chip vertical integration of directional metalenses on the top of a planar array of detectors enables a powerful bio-inspired LiDAR that is capable of 3D imaging over a wide field of view without using any mechanical parts. Implementation of metasurface arrays on imaging sensors is shown to have relevant industrial applications in 3D sensing that goes beyond the basic usage of metalenses for imaging.

All‐Dielectric Meta‐Waveguides for Flexible Polarization Control of Guided Light

AbstractGuided waves are the perfect carriers of electromagnetic signals in planar miniaturized devices due to their high localization and controlled propagation direction. However, it is still a challenge to control the polarization of propagating guided waves. In this work, the broadband polarization TE‐TM degeneracy of highly localized guided waves propagating along an all‐dielectric metasurface and a subwavelength chain of dielectric high‐index cylinders is discovered, both theoretically and experimentally. Using the discovered near‐field polarization degree of freedom, the polarization transformation for guided waves propagating along a subwavelength chain of dielectric cylinders at any frequency within the finite spectral range is demonstrated experimentally. Namely, the simplest planar near‐field polarization device – the quarter‐wave‐retardation (linear‐to‐circular) polarization transformer of guided waves is implemented. The results obtained discover the polarization degree of freedom for guided waves paving the way toward numerous applications in flat optics and planar photonics.

Performance analysis of free space optical communications with FOA-WFS.

Adaptive optics (AO) technology can correct wavefront distortion in coherent free space optical communication (FSOC), with wavefront sensors playing a vital role in this process. However, traditional wavefront sensors are large and expensive. Therefore, we propose using the inexpensive and easy-to-deploy flat optics angle-based wavefront sensor (FOA-WFS) to measure the wavefront aberration. It aims to meet the needs of various FSOC applications. We first establish the relationship between the energy ratio and the Zernike coefficient through theoretical studies and analyze the feasibility of applying the FOA-WFS to the FSOC. We then generate experimental datasets based on the relevant principles. Through numerical simulation, we verify that it can reconstruct wavefront aberration accurately and improve system performance. Finally, we analyze the mixing efficiency and bit error rate based on the collected aberration data by the experimental platform. The results indicate that the AO system based on the FOA-WFS can efficiently improve the performance of the FSOC. This study provides a novel wavefront aberration detection method for designing the AO systems in the FSOC.

Highly Absorbing Monolayer MoS2 for a Large Reflection Phase Modulation

AbstractManipulation of wavefront lies at the core of next‐generation information technologies. Compared to metal and dielectric metasurfaces, atomic 2D materials exhibit excellent prospects toward fulfilling ultra‐thin thickness requirements in flat optics in wavefront shaping, with thickness much smaller than those of traditional bulky devices. However, phase manipulation by light propagating through atomic 2D materials is suppressed due to its sub‐nanometer thickness. Here, an approach is reported to realize reflection phase singularities by establishing a zero‐reflection point in a monolayer MoS2‐based multilayer system, which broadens topological study beyond polarization singularity. This is achieved through the creation of a multilayer Fabry‐Perot‐type interference, and a pronounced phase change in the reflected light is realized due to the high absorption of monolayer MoS2 in the studied wavelength range. As an application, a rapid, sensitive, and label‐free detection of SARS‐CoV‐2 (2019‐nCov) antigen is demonstrated with a detection limit of 10−12 M L−1 (62 pg ml−1) by using monolayer MoS2 based optical biosensor. In addition to offering a comprehensive study in phase singularity, efficient wavefront engineering based on the reflective system using materials is presented with atomic thickness which may greatly simplify optical architecture in flat optics, and promote its development toward compactness and integrated functions.

An Electrochemically Programmable Metasurface with Independently Controlled Metasurface Pixels at Optical Frequencies.

Metasurfaces have revolutionized optical technologies by offering powerful, compact, and versatile solutions to control light. Conducting polymers, characterized by their conjugated molecular structures, facilitate charge transport and exhibit interesting electrical, optical, and mechanical properties. Integrating conducting polymers with optical metasurfaces can unlock new opportunities and functionalities in modern optics. In this work, we demonstrate an electrochemically programmable metasurface with independently controlled metasurface pixels at optical frequencies. Electrochemical modulation of locally conjugated polyaniline on gold nanorods, which are arranged on addressable electrodes according to the Pancharatnam-Berry phase design, enables dynamic control over the metasurface pixels into programmable configurations. With the same metasurface device, we showcase diverse optical functions, including dynamic beam diffraction and varifocal lensing along and off the optical axis. The synergy between flat optics and conducting polymer science holds immense potential to enhance the performance and function versatility of metasurfaces, paving the way for innovative optical applications.

Flat optics of nonuniform phase gradient metasurfaces

Flat optics of uniform phase gradient metasurfaces based on the generalized Snell’s law has been extensively studied. The optics of nonuniform phase gradient metasurfaces (NPGMs) is less clear. Here based on Huygens’ principle and finite-difference time-domain (FDTD) simulation, we explore the optical properties of NPGMs made of nanorods (meta-atoms), which can be tuned by modulation of propagation phase and geometric phase. It is found that the nonuniformity of phase gradient may lead to multichannel anomalous reflection/refraction. The multiple beam splitters with arbitrary intensity ratio can be achieved by using the amplitude/phase modulation. Based on our design principle, we can obtain different anomalous reflection patterns (with channels ±1, ±2, both ±1 and ±2, or no reflection at anomalous angle) by different arrangement of just two types of meta-atoms. In addition, we are able to achieve chiral anomalous reflections for designed NPGMs made of nanorods and L-shaped nanoparticles. Our formulism provides general design guidance for NPGMs and can be used to realize the beam splitting function with adjustable angle and intensity ratio.

Flat Optics: feature issue introduction.

This feature issue of Optics Express was created in conjunction with a topical meeting that took place during the 2023 Optica Imaging Congress and includes 17 state-of-the art articles. This introduction provides a summary of these articles that cover various aspects of metasurfaces from fundamental mechanisms, design methods, novel materials and processes to applications.

Thermo‐Optical Bistability Enabled by Bound States in The Continuum in Silicon Metasurfaces

AbstractThe control of light through all‐optical means is a fundamental challenge in nanophotonics and a key effect in optical switching and logic. The optical bistability effect enables this control and can be observed in various planar photonic systems such as microdisk and photonic crystal cavities and waveguides. However, the recent advancements in flat optics with wavelength‐thin optical elements require nonlinear elements based on metastructures and metasurfaces. The performance of these systems can be enhanced with high‐Q bound states in the continuum (BIC), which leads to intense harmonic generation, improved light‐matter coupling, and pushes forward sensing limits. This study reports enhanced thermo‐optical nonlinearity and the observation of optical bistability in an all‐dielectric metasurface membrane with BIC. Unlike many other nanophotonic platforms, metasurfaces allow for fine control of the quality factor of the BIC resonance by managing the radiative losses. This provides an opportunity to control the parameters of the observed hysteresis loop and even switch from bistability to optical discrimination by varying the angle of incidence. Additionally, this work proposes a mechanism of nonlinear critical coupling that establishes the conditions for maximal hysteresis width and minimal switching power, which has not been reported before. The study suggests that all‐dielectric metasurfaces supporting BICs can serve as a flat‐optics platform for optical switching and modulation based on strong thermo‐optical nonlinearity.

Efficient characterizations of multiphoton states with an ultra-thin optical device

Metasurface enables the generation and manipulation of multiphoton entanglement with flat optics, providing a more efficient platform for large-scale photonic quantum information processing. Here, we show that a single metasurface optical device would allow more efficient characterizations of multiphoton entangled states, such as shadow tomography, which generally requires fast and complicated control of optical setups to perform information-complete measurements, a demanding task using conventional optics. The compact and stable device here allows implementations of general positive operator valued measures with a reduced sample complexity and significantly alleviates the experimental complexity to implement shadow tomography. Integrating self-learning and calibration algorithms, we observe notable advantages in the reconstruction of multiphoton entanglement, including using fewer measurements, having higher accuracy, and being robust against experimental imperfections. Our work unveils the feasibility of metasurface as a favorable integrated optical device for efficient characterization of multiphoton entanglement, and sheds light on scalable photonic quantum technologies with ultra-thin optical devices.

Design, fabrication, and test of bi-functional metalenses for the spin-dependent OAM shift of optical vortices

The ability to encode different operations into a single miniaturized optical device is required to reduce the complexity and size of optical paths for light manipulation, which usually employs dynamic optical components, interferometric setups, and/or multiple bulky elements in cascade. A very efficient solution is provided by metalenses, which are flat optical elements able to generate and manipulate structured light beams in a compact and efficient way, offering a powerful and attractive tool in many fields, such as life science and telecommunications. In this work, we present the design and test of transmission dielectric bi-functional metalenses that exploit both the dynamic and the geometric phases, to enable the spin-controlled manipulation of different focused orbital angular momentum (OAM) beams, depending on the circularly polarized state in input. In detail, we provide numerical algorithms for the design and simulation of the meta-optics in the telecom infrared, the fabrication processes, and the optical characterization under different impinging polarized optical vortices. This solution provides new integrated flat optics for applications in imaging, optical tweezing and trapping, optical computation, and high-capacity telecommunication and encryption.

Engineering Quantum Light Sources with Flat Optics.

Quantum light sources are essential building blocks for many quantum technologies, enabling secure communication, powerful computing, and precise sensing and imaging. Recent advancements have witnessed a significant shift toward the utilization of "flat" optics with thickness at subwavelength scales for the development of quantum light sources. This approach offers notable advantages over conventional bulky counterparts, including compactness, scalability, and improved efficiency, along with added functionalities. This review focuses on the recent advances in leveraging flat optics to generate quantum light sources. Specifically, the generation of entangled photon pairs through spontaneous parametric down-conversion in nonlinear metasurfaces, and single photon emission from quantum emitters including quantum dots and color centers in 3D and 2D materials are explored. The review covers theoretical principles, fabrication techniques, and properties of these sources, with particular emphasis on the enhanced generation and engineering of quantum light sources using optical resonances supported by nanostructures. The diverse application range of these sources is discussed and the current challenges and perspectives in the field arehighlighted.