Wide-angular-range and high-resolution beam steering by a metasurface-coupled phased array

Electrically switchable 2N-channel wave-front control for certain functionalities with N cascaded polarization-dependent metasurfaces

Metasurfaces with tunable functionalities are greatly desired for modern optical system and various applications. To increase the operating channels of polarization-multiplexed metasurfaces, we proposed a structure of N cascaded dual-channel metasurfaces to achieve 2N electrically switchable channels without intrinsic loss or cross-talk for certain functionalities, including beam steering, vortex beam generation, lens, etc. As proof of principles, we have implemented a 3-layer setup to achieve 8 channels. In success, we have demonstrated two typical functionalities of vortex beam generation with switchable topological charge of l = -3 ~ +4 or l = -1 ~ -8, and beam steering with the deflection direction switchable in an 8×1 line or a 4×2 grid. We believe that our proposal would provide a practical way to significantly increase the scalability and extend the functionality of polarization-multiplexed metasurfaces. Although this method is not universal, it is potential for the applications of LiDAR, glasses-free 3D display, OAM (de)multiplexing, and varifocal meta-lens.

Wavelength selective beam-steering in a dual-mode multi-layer plasmonic laser

Due to its improved localization and confinement of light in single or multiple wavelength modes, nanolasers based on plasmonic crystals have grown in popularity in recent years. However, the lasing modes are not spatially separated, making applying different modes to different applications difficult. This work demonstrates an effective technique for spatially separating the two modes of a merged lattice metal nanohole array-based dual-mode plasmonic laser. A flat dielectric metasurface-based beam-splitter that exploits phase gradient profiles on the interfaces has been added to the laser to separate the modes into distinct spatial beams. The proposed structure successfully separates two modes by ∼23°, and the separation can be raised to ∼63° by tuning structural parameters such as the radius of the nanocylinders and the number of supercell rows. In addition, multiple beams can be generated, allowing for manual beam steering. This approach has a high emission output with a narrow linewidth, clarity, and a substantial degree of future tunability potential. The proposed integrated structure will provide a novel means of device miniaturization and may also serve advanced optical applications such as optical communication, quantum optics, interferometry, spectroscopy, and light detection and ranging (LiDAR).

Large-volume focus control at 10 MHz refresh rate via fast line-scanning amplitude-encoded scattering-assisted holography

The capability of focus control has been central to optical technologies that require both high temporal and spatial resolutions. However, existing varifocal lens schemes are commonly limited to the response time on the microsecond timescale and share the fundamental trade-off between the response time and the tuning power. Here, we propose an ultrafast holographic focusing method enabled by translating the speed of a fast 1D beam scanner into the speed of the complex wavefront modulation of a relatively slow 2D spatial light modulator. Using a pair of a digital micromirror device and a resonant scanner, we demonstrate an unprecedented refresh rate of focus control of 31 MHz, which is more than 1,000 times faster than the switching rate of a digital micromirror device. We also show that multiple micrometer-sized focal spots can be independently addressed in a range of over 1 MHz within a large volume of 5 mm × 5 mm × 5.5 mm, validating the superior spatiotemporal characteristics of the proposed technique - high temporal and spatial precision, high tuning power, and random accessibility in a three-dimensional space. The demonstrated scheme offers a new route towards three-dimensional light manipulation in the 100 MHz regime.

Amorphous silicon carbide high contrast gratings as highly efficient spectrally selective visible reflectors

We report spectrally selective visible wavelength reflectors using hydrogenated amorphous silicon carbide (a-SiC:H) as a high index contrast material. Beyond 610nm and through the near infrared spectrum, a-SiC:H exhibits very low loss and exhibits an wavelength averaged index of refraction of n = 3.1. Here we design, fabricate, and characterize such visible reflectors using a hexagonal array of a-SiC:H nanopillars as wavelength-selective mirrors with a stop-band of approximately 40 nm full-width at half maximum. The fabricated high contrast grating exhibits reflectivity R >94% at a resonance wavelength of 642nm with a single layer of a-SiC:H nanopillars. The resonance wavelength is tunable by adjusting the geometrical parameters of the a-SiC:H nanopillar array, and we observe a stop-band spectral center shift from 635 nm up to 642 nm. High contrast gratings formed from a-SiC:H nanopillars are a promising platform for various visible wavelength nanophotonics applications.

Data-driven concurrent nanostructure optimization based on conditional generative adversarial networks

Iterative numerical optimization is a ubiquitous tool to design optical nanostructures. However, there can be a significant performance gap between the numerically simulated results, with pristine shapes, and the experimentally measured values, with deformed profiles. We introduce conditional generative adversarial networks (CGAN) into the standard iterative optimization loop to learn process-structure relationships and produce realistic simulation designs based on the fabrication conditions. This ensures that the process-structure mapping is accurate for the specific available equipment and moves the optimization space from the structural parameters (e.g. width, height, and period) to process parameters (e.g. deposition rate and annealing time). We demonstrate this model agnostic optimization platform on the design of a red, green, and blue color filter based on metallic gratings. The generative network can learn complex M-to-N nonlinear process-structure relations, thereby generating simulation profiles similar to the training data over a wide range of fabrication conditions. The CGAN-based optimization resulted in fabrication parameters leading to a realistic design with a higher figure of merit than a standard optimization using pristine structures. This data-driven approach can expedite the design process both by limiting the design search space to a fabrication-accurate subspace and by returning the optimal process parameters automatically upon obtaining the optimal structure design.

High resolution 2D beam steerer made from cascaded 1D liquid crystal phase gratings

Optical beam steering (BS) has multiple applications in fields like target seeking and tracking, optical tweezers, billboard displays and many others. In this work, a two-dimensional beam deflector based on blaze gratings is presented. Phase-only 1D blaze gratings have been prepared using maskless Direct Laser Writing (DLW) resulting in high-resolution structures in indium-tin oxide (ITO) coated glass wafers. The device is composed of two identical 1D liquid crystal (LC) cells cascaded orthogonally back-to-back, with a resultant active area of 1.1 × 1.1 mm². The 1D cells have been prepared with 144 pixels each with a 7.5 µm pitch. The total 288 pixels are driven by a custom made 12-bit Pulse Width Modulation (PWM) electronic driver, allowing for an arbitrarily high resolution. The system performance is documented, and the efficiency of the system has been tested. A maximum diagonal steering angle of ± 3.42° was achieved.

A Semisolid Micromechanical Beam Steering System Based on Micrometa-Lens Arrays

Optical beam steerers have been widely employed for information acquisitions. Numerous beam steering schemes have been developed, and each of them can satisfy practical requirements for certain scenarios. However, there is still a lack of a comprehensive approach that is able to balance all of the critical technical parameters for wide range of applications. Here, a semisolid micromechanical beam steering system based on micrometa-lens arrays (MMLAs) is demonstrated. It is operated by manipulating the probe beam over two sets of decentered MMLAs potentially driven by high-speed piezo-electric motors. Small f-numbers, well-corrected aberration, and easy lateral reproduction of micrometa-lenses optimize the overall technical parameters. As a proof-of-concept, we implement such a device exhibiting diffraction-limited resolution within a large field of view of 30° × 30°. A three-dimensional depth sensing is also performed to demonstrate its potential in light detection and ranging applications.

Near unity transmission and full phase control with asymmetric Huygens' dielectric metasurfaces for holographic projections

Huygens' metasurfaces are transparent arrays of nanostructures that enable phase-front manipulation. This is achieved by simultaneous excitation of electric dipole (ED) and magnetic dipole (MD) resonances with equal amplitudes and phases in the constituent meta-atoms. In usual designs, the size changes of the meta-atoms, necessary to map the phase front, can detune the overlapping of ED and MD resonances, decreasing the transmission and limiting the operating bandwidth. In this report, we demonstrate that ED and MD resonances can be almost perfectly tuned together over a large wavelength range, keeping their spectral overlap, in a silicon metasurface by using anisotropic meta-atoms. In particular, we show near-unity transmission (>95% in simulations) and 2π phase control in a wavelength range from 760 to 815 nm using cuboidal nanoantennas. Using this concept, we also experimentally demonstrate clear reconstruction from holograms of a single metasurface spanning the near infrared and the whole visible spectral range.

Flat telescope based on an all-dielectric metasurface doublet enabling polarization-controllable enhanced beam steering

A flat telescope (FTS), which incorporates an all-dielectric metasurface doublet (MD) based on hydrogenated amorphous silicon nanoposts, is proposed and demonstrated to achieve flexibly magnified angular beam steering that is sensitive to both light polarization and deflection direction. Specifically, for transverse-electric-polarized incident beams, the MD exhibits deflection magnification factors of +5 and +2, while for transverse magnetic polarization, the beam is steered in reverse to yield magnification factors of -5 and -2 in the horizontal and vertical directions, respectively. The proposed MD comprises cascaded metalenses, which can invoke polarization-selective transmission phases. The MD which emulates a set of convex and concave lenses renders positively increased beam deflection, whereas the case corresponding to a pair of convex lenses facilitates negatively amplified beam deflection. The essential phase profiles required for embodying the MD are efficiently extracted from its geometric lens counterpart. Furthermore, the implemented FTS, operating in the vicinity of a 1550 nm wavelength, can successfully enable enhanced beam steering by facilitating polarization-sensitive bidirectional deflection amplifications. The proposed FTS can be applied in the development of a miniaturized light detection and ranging system, where the beam scanning range can be effectively expanded in two dimensions.

Ultralow sidelobe midinfrared optical phased array based on a broadband metasurface

In this paper, we propose an all-solid-state and ultralow sidelobe optical phased array (OPA) through designing a broadband angle-insensitive reflective metasurface in the midinfrared. The simulation results show that the metasurface can realize the wide-frequency metareflection characteristics in the range of 4.3∼5.0µm. Notably, the metasurface array can almost generate a continuous sweep between 0° and 342°, while the variation of reflectivity amplitude is only 10.2%, by changing the corresponding structural parameters. Then, we design and simulate an OPA based on these excellent characteristics of the broadband metasurface. By simply changing the periodicity of the OPA structure, the continuous deflection angles can be achieved within 29.41°, which can increase to 44.06° by changing the angle of the incident beam. A key feature of our design is that the sidelobe energy is less than 3.10% of the main lobe energy.

Integrated Optical Phased Arrays for Beam Forming and Steering

Integrated optical phased arrays can be used for beam shaping and steering with a small footprint, lightweight, high mechanical stability, low price, and high-yield, benefiting from the mature CMOS-compatible fabrication. This paper reviews the development of integrated optical phased arrays in recent years. The principles, building blocks, and configurations of integrated optical phased arrays for beam forming and steering are presented. Various material platforms can be used to build integrated optical phased arrays, e.g., silicon photonics platforms, III/V platforms, and III–V/silicon hybrid platforms. Integrated optical phased arrays can be implemented in the visible, near-infrared, and mid-infrared spectral ranges. The main performance parameters, such as field of view, beamwidth, sidelobe suppression, modulation speed, power consumption, scalability, and so on, are discussed in detail. Some of the typical applications of integrated optical phased arrays, such as free-space communication, light detection and ranging, imaging, and biological sensing, are shown, with future perspectives provided at the end.

Achromatic and wide-field metalens in the visible region

Optical metalens has been attracting more and more attention in recent years. To date, it is still difficult to simultaneously achieve wide field and broadband imaging in the visible region, which is very important in many applications, such as cameras, microscopy, and other imaging devices. In this paper, we design a double-layer metalens to achieve achromatic imaging over a field of view (FOV) of 60° in the visible light range of 470 nm to 650 nm, and its performance is verified by numerical simulations. The numerical aperture (NA) of the metalens is 0.61 and the average focusing efficiency is > 50% at normal incidence. The metalens has an additional advantage of polarization insensitivity.

Lidar system based on lens assisted integrated beam steering

We present a demonstration of solid-state light detection and ranging (Lidar) at 1550 nm by applying integrated two-dimensional (2D) lens assisted beam-steering (LABS) technology. LABS has O(logN) power consumption for N antennas and allows a simple control complexity with digital signal input. A time-of-flight coaxial Lidar is demonstrated with this beam-steering technology. The integrated beam-steering chip and lens both transmit and receive the light. The Lidar has 16 scanning angles, 19.5 m ranging distance, and a 3 cm ranging error. This Letter proves the potential application of 2D LABS in Lidar and paves the way for a fully integrated Lidar system.

Disordered Optics: Exploiting Multiple Light Scattering and Wavefront Shaping for Nonconventional Optical Elements

Advances in diverse areas such as inspection, imaging, manufacturing, telecommunications, and information processing have been stimulated by novel optical devices. Conventional material ingredients for these devices are typically made of homogeneous refractive or diffractive materials and require sophisticated design and fabrication, which results in practical limitations related to their form and functional figures of merit. To overcome such limitations, recent developments in the application of disordered materials as novel optical elements have indicated great potential in enabling functionalities that go beyond their conventional counterparts, while the materials exhibit potential advantages with respect to reduced form factors. Combined with wavefront shaping, disordered materials enable dynamic transitions between multiple functionalities in a single active optical device. Recent progress in this field is summarized to gain insight into the physical principles behind disordered optics with regard to their advantages in various applications as well as their limitations compared to conventional optics.

Metasurfaces-based imaging and applications: from miniaturized optical components to functional imaging platforms

This review focuses on the imaging applications of metasurfaces. These optical elements provide a unique platform to control light; not only do they have a reduced size and complexity compared to conventional imaging systems but they also enable novel imaging modalities, such as functional-imaging techniques. This review highlights the development of metalenses, from their basic principles, to the achievement of achromatic and tunable lenses, and metasurfaces implemented in functional optical imaging applications.

Lens-based integrated 2D beam-steering device with defocusing approach and broadband pulse operation for Lidar application

We propose an integrated two-dimensional beam-steering device based on an on-chip silicon-nitride switch/emitter structure and off-chip lens for light detection and ranging (Lidar) application at 1550 nm. In this device, light is guided by a 1 × 16 switch to one grating emitter in a 4 × 4 grating-emitter array. The beam from the grating emitter is collimated and steered by a fixed lens. By changing the grating emitter that emits light, different beam-steering angle can be achieved. A divergence angle of 0.06° and a field of view of 2.07° × 4.12° in the far field are achieved. The device has O(log₂N) power consumption for N emitters, allows digital control and achieves 18 dB background suppression. Blind-zone elimination and broadband operation are also achieved in our lens-based beam-steering device. Therefore, it is suitable for broadband solid-state Lidar application.

Liquid crystal-optical phased arrays (LC-OPA)-based optical beam steering with microradian resolution enabled by double gratings

We demonstrate an optical beam steering setup based on a liquid crystal-optical phased array (LC-OPA) with high angular resolution and low beam divergence, due to the use of double-grating configuration to enhance the corresponding angular resolution of LC-OPA. The introduction of two nonparallel blazed gratings with a special included angle can achieve multiple diffractions for the incident light, leading to the realization of angular compression. Numerical simulation results show the angular compression ratio can be optimized by selecting the appropriate grating constant and the included angle of double-grating. Experimental results verify that the steered angle of the incident light can be compressed so that the angular resolution of the steered beam can correspondingly be improved more than six times. In addition, when the beam is multiply diffracted within the double-grating configuration, the divergence angle becomes smaller as the beam size is enlarged. Therefore, both the microradian steering resolution and the narrow beam divergence can be simultaneously obtained with our proposed setup. The efficiency of the steering where the beam diffracted four times within the double-grating configuration is 80%.

Metalens-Based Miniaturized Optical Systems

Metasurfaces have been studied and widely applied to optical systems. A metasurface-based flat lens (metalens) holds promise in wave-front engineering for multiple applications. The metalens has become a breakthrough technology for miniaturized optical system development, due to its outstanding characteristics, such as ultrathinness and cost-effectiveness. Compared to conventional macro- or meso-scale optics manufacturing methods, the micro-machining process for metalenses is relatively straightforward and more suitable for mass production. Due to their remarkable abilities and superior optical performance, metalenses in refractive or diffractive mode could potentially replace traditional optics. In this review, we give a brief overview of the most recent studies on metalenses and their applications with a specific focus on miniaturized optical imaging and sensing systems. We discuss approaches for overcoming technical challenges in the bio-optics field, including a large field of view (FOV), chromatic aberration, and high-resolution imaging.