Showing papers on "Wavefront published in 2021"

Phyllotaxis-inspired nanosieves with multiplexed orbital angular momentum

Abstract: Nanophotonic platforms such as metasurfaces, achieving arbitrary phase profiles within ultrathin thickness, emerge as miniaturized, ultracompact and kaleidoscopic optical vortex generators. However, it is often required to segment or interleave independent sub-array metasurfaces to multiplex optical vortices in a single nano-device, which in turn affects the device’s compactness and channel capacity. Here, inspired by phyllotaxis patterns in pine cones and sunflowers, we theoretically prove and experimentally report that multiple optical vortices can be produced in a single compact phyllotaxis nanosieve, both in free space and on a chip, where one meta-atom may contribute to many vortices simultaneously. The time-resolved dynamics of on-chip interference wavefronts between multiple plasmonic vortices was revealed by ultrafast time-resolved photoemission electron microscopy. Our nature-inspired optical vortex generator would facilitate various vortex-related optical applications, including structured wavefront shaping, free-space and plasmonic vortices, and high-capacity information metaphotonics.

Dynamically controlling terahertz wavefronts with cascaded metasurfaces

Abstract: Dynamically controlling terahertz (THz) wavefronts in a designable fashion is highly desired in practice. However, available methods working at microwave frequencies do not work well in the THz regime due to lacking suitable tunable elements with submicrometer sizes. Here, instead of locally controlling individual meta-atoms in a THz metasurface, we show that rotating different layers (each exhibiting a particular phase profile) in a cascaded metadevice at different speeds can dynamically change the effective Jones-matrix property of the whole device, thus enabling extraordinary manipulations on the wavefront and polarization characteristics of a THz beam impinging on the device. After illustrating our strategy based on model calculations, we experimentally demonstrate two proof-of-concept metadevices, each consisting of two carefully designed all-silicon transmissive metasurfaces exhibiting different phase profiles. Rotating two metasurfaces inside the fabricated devices at different speeds, we experimentally demonstrate that the first metadevice can efficiently redirect a normally incident THz beam to scan over a wide solid-angle range, while the second one can dynamically manipulate both the wavefront and polarization of a THz beam. Our results pave the way to achieving dynamic control of THz beams, which is useful in many applications, such as THz radar, and bio- and chemical sensing and imaging.

A Multi-Functional Reconfigurable Metasurface: Electromagnetic Design Accounting for Fabrication Aspects

Abstract: In this article, we present the theoretical considerations and the design evolution of a proof-of-concept reconfigurable metasurface, primarily used as a tunable microwave absorber, but also as a wavefront manipulation and polarization conversion device in reflection. We outline the design evolution and all considerations taken into account, from the selection of patch shape, unit cell size, and substrate to the topology of the structure that realizes the desired tunability. The presented design conforms to fabrication restrictions and is codesigned to work with an integrated circuit (IC) chip for providing tunable complex loads to the metasurface, using a commercially available semiconductor process. The proposed structure can perform multiple tunable functionalities by appropriately biasing the IC. Perfect absorption for a wide range of incidence angles of both linear polarization states, accommodating a spectral range in the vicinity of 5 GHz, with potential also for wavefront control, exemplified via anomalous reflection and polarization conversion. The end vision is for such a design to be scalable and deployable as a practical HyperSurface, i.e., an intelligent multifunctional metasurface capable of concurrent reconfigurable functionalities: absorption, beam steering, polarization conversion, wavefront shaping, holography, and sensing.

Polarization-insensitive 3D conformal-skin metasurface cloak.

Abstract: Electromagnetic metasurface cloaks provide an alternative paradigm toward rendering arbitrarily shaped scatterers invisible. Most transformation-optics (TO) cloaks intrinsically need wavelength-scale volume/thickness, such that the incoming waves could have enough long paths to interact with structured meta-atoms in the cloak region and consequently restore the wavefront. Other challenges of TO cloaks include the polarization-dependent operation to avoid singular parameters of composite cloaking materials and limitations of canonical geometries, e.g., circular, elliptical, trapezoidal, and triangular shapes. Here, we report for the first time a conformal-skin metasurface carpet cloak, enabling to work under arbitrary states of polarization (SOP) at Poincare sphere for the incident light and arbitrary conformal platform of the object to be cloaked. By exploiting the foundry three-dimensional (3D) printing techniques to fabricate judiciously designed meta-atoms on the external surface of a conformal object, the spatial distributions of intensity and polarization of its scattered lights can be reconstructed exactly the same as if the scattering wavefront were deflected from a flat ground at any SOP, concealing targets under polarization-scanning detections. Two conformal-skin carpet cloaks working for partial- and full-azimuth plane operation are respectively fabricated on trapezoid and pyramid platforms via 3D printing. Experimental results are in good agreement with numerical simulations and both demonstrate the polarization-insensitive cloaking within a desirable bandwidth. Our approach paves a deterministic and robust step forward to the realization of interfacial, free-form, and full-polarization cloaking for a realistic arbitrary-shape target in real-world applications.

Radio Positioning With EM Processing of the Spherical Wavefront

Abstract: Next 5G and beyond applications have attracted a tremendous interest towards systems using antenna arrays with an extremely large number of antennas where the technology conceived for communication might also be exploited for high-accuracy positioning applications. In this paper, we investigate the possibility to infer the position of a single antenna transmitter using a single asynchronous receiving node by retrieving information from the incident spherical wavefront. To this end, we consider the adoption of a suitable mix of processing at electromagnetic (EM) and signal levels, as a lower complexity alternative to classical massive array systems where the processing is done entirely at signal level. Thus, we first introduce a dedicated general model for different EM processing architectures, entailing the use or not of a lens that can have either a reconfigurable or a fixed phase profile, and successively we investigate their attainable positioning performance. The effect of the interference is also investigated to evaluate the robustness of the considered system to the presence of multiple simultaneous transmitting sources. Results, obtained for different apertures of the exploited lens/array, confirm the possibility to achieve interesting positioning performance using a single antenna array with a limited aperture.

High-Efficiency ultrathin terahertz geometric metasurface for full-space wavefront manipulation at two frequencies

Abstract: It has been demonstrated that metasurfaces have the ability to manipulate the wavefront. However, most multifunctional metasurfaces reported to date only operate in either reflection or transmission mode. In this paper, a bilayer metasurface based on geometric phase is proposed to independently tailor the wavefronts of transmitted and reflected circularly polarized (CP) waves at two different terahertz frequencies. More specifically, the metasurface can transform the incident CP wave to its cross-polarization component with a high conversion coefficient of about 0.87 (0.92) after refraction (reflection) at 0.6 (1.67) THz. The full 2π phase shift can be obtained independently by varying the geometrical parameters of the unit-cell structure at two different operation modes. As proofs of concept, anomalous refraction and reflection, dual-band full-space cylindrical focusing metalens and vortex beam generation with different modes are numerically demonstrated. Our work provides an effective method to integrate two or more different functionalities into a simple metasurface-based device, and the independent phase modulation characteristic of our proposed metasurface also shows infinite potential in wavefront control of full space.

High-dimensional orbital angular momentum multiplexing nonlinear holography

Abstract: Nonlinear holography has been identified as a vital platform for optical multiplexing holography because of the appearance of new optical frequencies. However, due to nonlinear wave coupling in nonlinear optical processes, the nonlinear harmonic field is coupled with the input field, laying a fundamental barrier to independent control of the interacting fields for holography. We propose and experimentally demonstrate high-dimensional orbital angular momentum (OAM) multiplexing nonlinear holography to overcome this problem. By dividing the wavefront of the fundamental wave into different orthogonal OAM channels, multiple OAM and polarization-dependent holographic images in both the fundamental wave and second-harmonic wave have been reconstructed independently in the spatial frequency domain through a type-II second harmonic generation process. Moreover, this method can be easily extended to cascaded χ2 nonlinear optical processes for multiplexing in more wavelength channels, leading to potential applications in multicasting in optical communications, multiwavelength display, multidimensional optical storage, anticounterfeiting, and optical encryption.

Precise Traveling Wave-Based Transmission Line Fault Location Method Using Single-Ended Data

Abstract: In this article, an accurate single-ended traveling wave-based fault location method is proposed for transmission lines with the inputs as the first three arrival times of traveling wave wavefronts and wave velocity of the line. In the calculation process, two fault distances are obtained considering the fault in the first or second half segment of the line by using the first (incident) and second (reflected) wavefronts. The third wavefront is then used to determine the faulted half segment and thereby select the correct fault distance out of the two. The technique is economical, and it does not require communication. The proposed technique is verified on a 400 kV, 50 Hz, 150 km two-terminal line using EMTDC simulations with a realistic line model. The performance of the method is evaluated for different fault locations, fault inception angles, and fault resistances. The test results obtained by the proposed method are compared with a practically proven two-terminal communication-based method and are found to be accurate.

Efficient generation of complex vectorial optical fields with metasurfaces.

Abstract: Vectorial optical fields (VOFs) exhibiting arbitrarily designed wavefronts and polarization distributions are highly desired in photonics. However, current methods to generate them either require complicated setups or exhibit limited functionalities, which is unfavorable for integration-optics applications. Here, we propose a generic approach to efficiently generate arbitrary VOFs based on metasurfaces exhibiting full-matrix yet inhomogeneous Jones-matrix distributions. We illustrate our strategy with analytical calculations on a model system and an experimental demonstration of a meta-device that can simultaneously deflect light and manipulate its polarization. Based on these benchmark results, we next experimentally demonstrate the generation of a far-field VOF exhibiting both a vortex wavefront and an inhomogeneous polarization distribution. Finally, we design/fabricate a meta-device and experimentally demonstrate that it can generate a complex near-field VOF-a cylindrically polarized surface plasmon wave possessing orbital angular momentum-with an efficiency of ~34%. Our results establish an efficient and ultracompact platform for generating arbitrary predesigned VOFs in both the near- and far-fields, which may find many applications in optical manipulation and communications.

Dynamic piezoelectric MEMS-based optical metasurfaces

Abstract: Optical metasurfaces (OMSs) have shown unprecedented capabilities for versatile wavefront manipulations at the subwavelength scale. However, most well-established OMSs are static, featuring well-defined optical responses determined by OMS configurations set during their fabrication, whereas dynamic OMS configurations investigated so far often exhibit specific limitations and reduced reconfigurability. Here, by combining a thin-film piezoelectric microelectromechanical system (MEMS) with a gap-surface plasmon-based OMS, we develop an electrically driven dynamic MEMS-OMS platform that offers controllable phase and amplitude modulation of the reflected light by finely actuating the MEMS mirror. Using this platform, we demonstrate MEMS-OMS components for polarization-independent beam steering and two-dimensional (2D) focusing with high modulation efficiencies (~50%), broadband operation (~20% near the operating wavelength of 800 nanometers), and fast responses (<0.4 milliseconds). The developed MEMS-OMS platform offers flexible solutions for realizing complex dynamic 2D wavefront manipulations that could be used in reconfigurable and adaptive optical networks and systems.

Structural designs, principles, and applications of thin-walled membrane and plate-type acoustic/elastic metamaterials

Abstract: Many advanced physical properties can be realized by using well-designed acoustic metamaterial (AM) structures, which have significant application value in engineering. In particular, thin-walled membrane, plate, and shell-type structures with deep subwavelength thicknesses that can meet light weight requirements have attracted the attention of many researchers and engineers from various specialized fields. This Tutorial systematically introduced the structural design methods, acoustic/elastic wave attenuation and regulation principles, and engineering applications of thin-walled AMs for low-frequency sound insulation, sound absorption, and vibration reduction. In particular, the design methods and sound insulation/absorption properties of thin-walled AMs for realizing narrow-band and broadband sound attenuation were explored. Furthermore, the local resonance bandgap characteristics, quantitative extraction method for the bending wave bandgap, vibration suppression properties, and the design method for local resonance vibration dampers for elastic wave regulation by thin-walled elastic metamaterials were summarized successively. Moreover, other thin-walled AM applications, such as the wavefront steering performance of thin-walled acoustic/elastic metasurfaces, and the active thin-walled AMs, were introduced as well.

Bandwidth-unlimited polarization-maintaining metasurfaces.

Abstract: Any arbitrary state of polarization of light beam can be decomposed into a linear superposition of two orthogonal oscillations, each of which has a specific amplitude of the electric field. The dispersive nature of diffractive and refractive optical components generally affects these amplitude responses over a small wavelength range, tumbling the light polarization properties. Although recent works suggest the realization of broadband nanophotonic interfaces that can mitigate frequency dispersion, their usage for arbitrary polarization control remains elusively chromatic. Here, we present a general method to address broadband full-polarization properties of diffracted fields using an original superposition of circular polarization beams transmitted through metasurfaces. The polarization-maintaining metasurfaces are applied for complex broadband wavefront shaping, including beam deflectors and white-light holograms. Eliminating chromatic dispersion and dispersive polarization response of conventional diffractive elements lead to broadband polarization-maintaining devices of interest for applications in polarization imaging, broadband-polarimetry, augmented/virtual reality imaging, full color display, etc.

Arbitrary Wave Transformations With Huygens’ Metasurfaces Through Surface-Wave Optimization

Abstract: Huygens’ metasurfaces (HMSs) have demonstrated the ability to tailor electromagnetic wavefronts with passive low-profile structures. The fundamental constraint enabling passive and ideally lossless solutions is the conservation of the normal real power locally along the metasurface. In this letter, we examine the use of auxiliary surface waves to design HMSs for wave transformations with different incident and output power density profiles. The developed method relies on the optimization of a surface-wave distribution that is utilized to redistribute the power at the input side of the metasurface without incurring any reflections. A full design example is presented with a linear patch array along the $H$ -plane illuminating a metasurface that produces uniform output fields along the $E$ -plane. A high aperture illumination efficiency of 92% is obtained despite the small distance between the source and the metasurface. Moreover, the effects of the evanescent spectrum to the losses and the bandwidth of the structure are discussed.

Single-Pixel Imaging Using Multimode Fiber and Silicon Photonic Phased Array

Abstract: Due to the unique nature of offering minimal invasiveness and high spatial resolution simultaneously, multimode fibers (MMFs) are receiving significant attention in bio-imaging applications. While a spatial light modulator is typically used for controlling the wavefront of the light emitted from the MMF, it makes the system slow, bulky, and expensive. To solve this problem, in this work, we demonstrate the use of a chip-scale integrated optical phased array (OPA) for imaging through an MMF. A silicon OPA with 128 independent phase shifters is fabricated and combined with a 3D waveguide interface to generate speckle patterns at the MMF output. Using the generated speckles and the optical power detected by a bucket detector, we experimentally obtain fine 2D images of the target. From the point spread function analysis of the system, the number of resolvable points is derived to be 1007 points, which is much larger than the number of phase shifters.

Deterministic Approach to Achieve Full-Polarization Cloak.

Abstract: Achieving full-polarization (σ) invisibility on an arbitrary three-dimensional (3D) platform is a long-held knotty issue yet extremely promising in real-world stealth applications. However, state-of-the-art invisibility cloaks typically work under a specific polarization because the anisotropy and orientation-selective resonant nature of artificial materials made the σ-immune operation elusive and terribly challenging. Here, we report a deterministic approach to engineer a metasurface skin cloak working under an arbitrary polarization state by theoretically synergizing two cloaking phase patterns required, respectively, at spin-up (σ+) and spin-down (σ-) states. Therein, the wavefront of any light impinging on the cloak can be well preserved since it is a superposition of σ+ and σ- wave. To demonstrate the effectiveness and applicability, several proof-of-concept metasurface cloaks are designed to wrap over a 3D triangle platform at microwave frequency. Results show that our cloaks are essentially capable of restoring the amplitude and phase of reflected beams as if light was incident on a flat mirror or an arbitrarily predesigned shape under full polarization states with a desirable bandwidth of ~17.9%, conceiving or deceiving an arbitrary object placed inside. Our approach, deterministic and robust in terms of accurate theoretical design, reconciles the milestone dilemma in stealth discipline and opens up an avenue for the extreme capability of ultrathin 3D cloaking of an arbitrary shape, paving up the road for real-world applications.

Near-Field Tracking With Large Antenna Arrays: Fundamental Limits and Practical Algorithms

Abstract: Applications towards 6G have brought a huge interest towards arrays with a high number of antennas and operating within the millimeter and sub-THz bandwidths for joint communication, sensing, and localization. With such large arrays, the plane wave approximation is often not accurate because the system may operate in the (radiating) near-field propagation region, namely the Fresnel region, where the electromagnetic field wavefront is spherical. In such a case, the curvature of arrival (CoA) is a measure of the spherical wavefront that can be used to infer the source position using only a single large antenna array. In this paper, we study a near-field tracking problem for inferring the position and the velocity of a moving source with an ad-hoc observation model that accounts for the phase-difference profile of a large receiving array. For this tracking problem, we derive the posterior Cramer-Rao Lower Bound (P-CRLB), and we provide insights on how the loss of positioning information outside the Fresnel region results from an increase of the ranging error rather than from inaccuracies of angular estimation. Then, we investigate the accuracy and complexity performance of different Bayesian tracking algorithms in the presence of model parameter mismatches and abrupt trajectory changes. Our results demonstrate the feasibility and high accuracy of most tracking approaches without the need for wideband signals and of any synchronization scheme.

Structuring total angular momentum of light along the propagation direction with polarization-controlled meta-optics.

Abstract: Recent advances in wavefront shaping have enabled complex classes of Structured Light which carry spin and orbital angular momentum, offering new tools for light-matter interaction, communications, and imaging. Controlling both components of angular momentum along the propagation direction can potentially extend such applications to 3D. However, beams of this kind have previously been realized using bench-top setups, requiring multiple interaction with light of a fixed input polarization, thus impeding their widespread applications. Here, we introduce two classes of metasurfaces that lift these constraints, namely: i) polarization-switchable plates that couple any pair of orthogonal polarizations to two vortices in which the magnitude and/or sense of vorticity vary locally with propagation, and ii) versatile plates that can structure both components of angular momentum, spin and orbital, independently, along the optical path while operating on incident light of any polarization. Compact and integrated devices of this type can advance light-matter interaction and imaging and may enable applications that are not accessible via other wavefront shaping tools. Creating complex forms of structured light typically requires bulky optics and multiple interactions with incident light. Here the authors demonstrate versatile control over light’s polarization and orbital angular momentum along the propagation direction with a single metasurface.

Wavefront-selective Fano resonant metasurfaces

Abstract: Fano resonances are conventionally understood as sharp spectral features with selectivity in the momentum-frequency domain, implying that they can be excited only by plane waves with specific frequencies and incident angles. We demonstrate that Fano resonances can be made generally selective in the space-frequency domain. They can be tailored to resonate only when excited by a frequency, polarization, and wavefront of choice. This generalization reveals that Fano systems are characterized by eigenwaves that scatter to their time-reversed image upon reflection. Although in conventional Fano systems this trivially occurs for normally incident plane waves, we show that, in general, the selected wavefront is locally retroreflected everywhere across the device. These results show that conventional Fano resonances are a subset of a broader dichroic phenomenon with spin, spatial, and spectral selectivity. We demonstrate these concepts with nonlocal metasurfaces whose governing principles are deeply rooted in the symmetry features of quasi-bound states in the continuum. Enhanced light–matter interactions and symmetry-protection make these phenomena uniquely suited for enriching applications in quantum optics, non-linear optics, augmented reality, and secure optical communications, laying the groundwork for a range of novel compact optical sources and devices.

Non-local metasurfaces for spectrally decoupled wavefront manipulation and eye tracking.

Abstract: Metasurface-based optical elements typically manipulate light waves by imparting space-variant changes in the amplitude and phase with a dense array of scattering nanostructures. The highly localized and low optical-quality-factor (Q) modes of nanostructures are beneficial for wavefront shaping as they afford quasi-local control over the electromagnetic fields. However, many emerging imaging, sensing, communication, display and nonlinear optics applications instead require flat, high-Q optical elements that provide substantial energy storage and a much higher degree of spectral control over the wavefront. Here, we demonstrate high-Q, non-local metasurfaces with atomically thin metasurface elements that offer notably enhanced light–matter interaction and fully decoupled optical functions at different wavelengths. We illustrate a possible use of such a flat optic in eye tracking for eyewear. Here, a metasurface patterned on a regular pair of eye glasses provides an unperturbed view of the world across the visible spectrum and redirects near-infrared light to a camera to allow imaging of the eye. High optical-quality-factor, non-local metasurfaces enable independent functions across different wavelength bands.

Domain multiplexed computer-generated holography by embedded wavevector filtering algorithm

Abstract: Computer-generated holography can obtain the wavefront required for constructing arbitrary intensity distributions in space. Currently, speckle noises in holography remain an issue for most computational methods. In addition, there lacks a multiplexing technology by which images from a single hologram and light source can be switched by a lens. In this work, we first come up with a new algorithm to generate holograms to project smoother images by wavevector filtering. Thereupon, we propose a unique multiplexing scheme enabled by a Fourier lens, as the incident light can be decomposed either by a superposition of spherical waves or plane waves. Different images are obtained experimentally in the spatial and wavevector domains, switchable by a lens. The embedded wavevector filtering algorithm provides a new prospective for speckle suppression without the need for postprocessing. The multiplexing technology can double the capacity of current holographic systems and exhibits potential for various interesting display applications.

Broadband Liquid Crystal Tunable Metasurfaces in the Visible: Liquid Crystal Inhomogeneities Across the Metasurface Parameter Space

Abstract: Optical metasurfaces—planar nanostructured devices that can arbitrarily tailor the wavefront of light—may be reconfigured by changing their dielectric environment. The application of external stimu...

Modern Types of Axicons: New Functions and Applications.

Abstract: 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.

Real-time multi-functional near-infrared wave manipulation with a 3-bit liquid crystal based coding metasurface.

Abstract: We propose a new generation of reprogrammable multi-functional bias encoded metasurfaces for dynamic wave manipulation using liquid crystals (LC). This metadevice is an array of unit-cells based on LCs to provide the desired phase steps based on its large birefringence property. The presented 3-bit coding metasurface (CM) use 8 states of "000"-"111" to control and manipulate the scattered wave at λ=1.4µm for several applications. The metasurface is introduced in detail and followed by several examples to show its versatility. Steered pencil, regular, and focused vortex beams with different topological charges are realized. The theoretical predictions are confirmed by numerical simulations. The proposed CM enables the realization of multifunctional optical wavefront manipulation and future intelligent optical devices.

Deep learning wavefront sensing and aberration correction in atmospheric turbulence

Abstract: Deep learning neural networks are used for wavefront sensing and aberration correction in atmospheric turbulence without any wavefront sensor (i.e. reconstruction of the wavefront aberration phase from the distorted image of the object). We compared and found the characteristics of the direct and indirect reconstruction ways: (i) directly reconstructing the aberration phase; (ii) reconstructing the Zernike coefficients and then calculating the aberration phase. We verified the generalization ability and performance of the network for a single object and multiple objects. What’s more, we verified the correction effect for a turbulence pool and the feasibility for a real atmospheric turbulence environment.

Laser Interference Lithography for Fabrication of Planar Scale Gratings for Optical Metrology

Abstract: Laser interference lithography is an attractive method for the fabrication of a large-area two-dimensional planar scale grating, which can be employed as a scale for multi-axis optical encoders or a diffractive optical element in many types of optical sensors. Especially, optical configurations such as Lloyd’s mirror interferometer based on the division of wavefront method can generate interference fringe fields for the patterning of grating pattern structures at a single exposure in a stable manner. For the fabrication of a two-dimensional scale grating to be used in a planar/surface encoder, an orthogonal two-axis Lloyd’s mirror interferometer, which has been realized through innovation to Lloyd’s mirror interferometer, has been developed. In addition, the concept of the patterning of the two-dimensional orthogonal pattern structure at a single exposure has been extended to the non-orthogonal two-axis Lloyd’s mirror interferometer. Furthermore, the optical setup for the non-orthogonal two-axis Lloyd’s mirror interferometer has been optimized for the fabrication of a large-area scale grating. In this review article, principles of generating interference fringe fields for the fabrication of a scale grating based on the interference lithography are reviewed, while focusing on the fabrication of a two-dimensional scale grating for planar/surface encoders. Verification of the pitch of the fabricated pattern structures, whose accuracy strongly affects the performance of planar/surface encoders, is also an important task to be addressed. In this paper, major methods for the evaluation of a grating pitch are also reviewed.

Elastic Metasurfaces for Full Wavefront Control and Low-Frequency Energy Harvesting

Abstract: Controlling and manipulating elastic/acoustic waves via artificially structured metamaterials, phononic crystals, and metasurfaces have gained an increasing research interest in the last decades. Unlike others, a metasurface is a single layer in the host medium with an array of subwavelength-scaled patterns introducing an abrupt phase shift in the wave propagation path. In this study, an elastic metasurface composed of an array of slender beam resonators is proposed to control the elastic wavefront of low-frequency flexural waves. The phase gradient based on Snell’s law is achieved by tailoring the thickness of thin beam resonators connecting two elastic host media. Through analytical and numerical models, the phase-modulated metasurfaces are designed and verified to accomplish three dynamic wave functions, namely, deflection, non-paraxial propagation, and focusing. An oblique incident wave is also demonstrated to show the versatility of the proposed design for focusing of wave energy incident from multiple directions. Experimentally measured focusing metasurface has nearly three times wave amplification at the designed focal point which validates the design and theoretical models. Furthermore, the focusing metasurface is exploited for low-frequency energy harvesting and the piezoelectric harvester is improved by almost nine times in terms of the harvested power output as compared to the baseline harvester on the pure plate without metasurface.

Anti-scattering light focusing by fast wavefront shaping based on multi-pixel encoded digital-micromirror device.

Abstract: Speed and enhancement are the two most important metrics for anti-scattering light focusing by wavefront shaping (WS), which requires a spatial light modulator with a large number of modulation modes and a fast speed of response. Among the commercial modulators, the digital-micromirror device (DMD) is the sole solution providing millions of modulation modes and a pattern rate higher than 20 kHz. Thus, it has the potential to accelerate the process of anti-scattering light focusing with a high enhancement. Nevertheless, modulating light in a binary mode by the DMD restricts both the speed and enhancement seriously. Here, we propose a multi-pixel encoded DMD-based WS method by combining multiple micromirrors into a single modulation unit to overcome the drawbacks of binary modulation. In addition, to efficiently optimize the wavefront, we adopted separable natural evolution strategies (SNES), which could carry out a global search against a noisy environment. Compared with the state-of-the-art DMD-based WS method, the proposed method increased the speed of optimization and enhancement of focus by a factor of 179 and 16, respectively. In our demonstration, we achieved 10 foci with homogeneous brightness at a high speed and formed W- and S-shape patterns against the scattering medium. The experimental results suggest that the proposed method will pave a new avenue for WS in the applications of biomedical imaging, photon therapy, optogenetics, dynamic holographic display, etc.