Phase Manipulation Research Articles

Freeform surface for light shaping by iterative design via Fourier domain.

We extend our previous work [Yang et al., Opt. Express29, 3621 (2021)10.1364/OE.415649] and propose an iterative algorithm to design a freeform surface for far-field light shaping. The algorithm alternately performs a wavefront phase design step and a freeform surface construction step. The smooth wavefront phase is designed by the mapping-type Fourier pair synthesis method, and the freeform surface is constructed by using the obtained wavefront phase. The algorithm provides a solid approach that ensures the introduction of the required wavefront phase manipulation for light shaping. Moreover, the related physical effects such as the Fresnel effect and polarization effect are included in the algorithm. We demonstrate the flexibility of the algorithm by examples.

Phase modulation of metasurfaces for polarization conversion and RCS reduction

Metasurface (MS) can manipulate the transmission of electromagnetic (EM) wave with more degrees of freedom than other conventional radar stealth material, enabling numerous potential applications in both military and civilian fields. In this paper, we propose two kinds of polarization converters to reveal the relation between phase gradient and linear/circular polarization conversion (LPC/CPC). The proposed split square-ring (SSR) and split circle-ring (SCR) converters both can realize the CPC with different mechanisms. Further, four kinds of digital MSs are designed to explore the energy scattering characteristics. We find that the efficient radar cross section (RCS) reduction of the 01/01-typed SSR converter is due to the 180° phase gradient of reflected x - and y -polarized waves; the RCS reduction of the 01/10-typed SCR converter depends on the phase difference of u - and v -polarized waves and the amplitudes of x - and y -polarized waves. The calculation and the simulation results verify the relevance between RCS reduction and reflection amplitude, the reason of insufficient RCS reduction is fully explained. The experiments match well with the simulations. This work can be useful for the exploration of polarization conversion MSs and build the connection between reflection loss and RCS reduction. • The relationship between phase modulation and linear/circular polarization conversion. • The method to achieve circular polarization conversion with different mechanisms. • Optimizing the defects of RCS reduction. • Associating the relation between reflection loss and RCS reduction based on phase manipulation.

Author Correction: High-precision digital terahertz phase manipulation within a multichannel field perturbation coding chip

Author Correction: High-precision digital terahertz phase manipulation within a multichannel field perturbation coding chip

Multiplexing meta-hologram with separate control of amplitude and phase.

Metasurfaces have shown their unique capabilities to manipulate the phase and/or amplitude properties of incident light at the subwavelength scale, which provides an effective approach for constructing amplitude-only, phase-only or even complexed amplitude meta-devices with high resolution. Most of meta-devices control the amplitude and/or phase of the incident light with the same polarization state; however, separately controlling of amplitude and phase of the incident light with different polarization states can provide a new degree of freedom for improving the information capacity of metasurfaces and designing multifunctional meta-devices. Herein, we combine the amplitude manipulation and geometric phase manipulation by only reconfiguring the orientation angle of the nanostructure and present a single-sized design strategy for a multiplexing meta-hologram which plays the dual roles: a continuous amplitude-only meta-device and a two-step phase-only meta-device. Two different modulation types can be readily switched merely by polarization controls. Our approach opens up the possibilities for separately and independently controlling of amplitude and phase of light to construct a multiplexing meta-hologram with a single-sized metasurface, which can contribute to the advanced research and applications in multi-folded optical anti-counterfeiting, optical information hiding and optical information encoding.

High-precision digital terahertz phase manipulation within a multichannel field perturbation coding chip

Direct phase modulation is one of the most urgent and difficult issues in the terahertz research area. Here, we propose a new method employing a two-dimensional electron gas (2DEG) perturbation microstructure unit coupled to a transmission line to realize high-precision digital terahertz phase manipulation. We induce local perturbation resonances to manipulate the phase of guided terahertz waves. By controlling the electronic transport characteristics of the 2DEG using an external voltage, the strength of the perturbation can be manipulated, which affects the phase of the guided waves. This external control permits electronic manipulation of the phase of terahertz waves with high precision, as high as 2−5° in the frequency range 0.26–0.27 THz, with an average phase error of only 0.36°, corresponding to a timing error of only 4 fs. Critically, the average insertion loss is as low as 6.14 dB at 0.265 THz, with a low amplitude fluctuation of 0.5 dB, so the device offers near-ideal phase-only modulation. A terahertz phase modulator based on the switchable perturbation resonance in two-dimensional electron gas is demonstrated. Phase manipulation with precision ranging from 2° to 5° is obtained at frequencies in the range from 0.26 to 0.27 THz.

Non-orthogonal polarization multiplexed metasurfaces for tri-channel polychromatic image displays and information encryption

Abstract Interference usually occurs between two non-orthogonally polarized light beams. Hence, metasurface enabled polarization multiplexing is generally conducted under two orthogonal polarization states to realize independent intensity and/or phase modulations. Herein, we show that polarization multiplexed metasurfaces can work under three non-orthogonal polarization states to realize tri-channel image displays with independent information encoding. Specifically, enabled by orientation degeneracy, each nanostructure of the metasurface operates with triple-manipulations of light, i.e., two channels for independent intensity manipulation under π/4 and 3π/8 linearly polarized (LP) light, respectively, and one channel for phase manipulation without polarization control. We experimentally demonstrate this concept by recording one continuous-brightness polychromatic image and one binary-brightness polychromatic image right at the metasurface plane, while a continuous-brightness polychromatic image is reconstructed in the far field, corresponding to three independent channels, respectively. More interestingly, in another design strategy with separated image encoding of two wavelengths, up to six independent image-display channels can be established and information delivery becomes safer by utilizing encryption algorithms. With the features of high information capacity and high security, the proposed meta-devices can empower advanced research and applications in multi-channel image displays, orbital angular momentum multiplexing communication, information encryption, anti-counterfeiting, multifunctional integrated nano-optoelectronics, etc.

Invisible surfaces enabled by the coalescence of anti-reflection and wavefront controllability in ultrathin metasurfaces

Reflection inherently occurs on the interfaces between different media. In order to perfectly manipulate waves on the interfaces, integration of antireflection function in metasurfaces is highly desired. In this work, we demonstrate an approach to realize exceptional metasurfaces that combine the two vital functionalities of antireflection and arbitrary phase manipulation in the deep subwavelength scale. Such ultrathin devices confer reflection-less transmission through impedance-mismatched interfaces with arbitrary wavefront shapes. Theoretically and experimentally, we demonstrate a three-layer antireflection metasurface that achieves an intriguing phenomenon: the simultaneous elimination of the reflection and refraction effects on a dielectric surface. Incident waves transmit straightly through the dielectric surface as if the surface turns invisible. We further demonstrate a wide variety of applications such as invisible curved surfaces, “cloaking” of dielectric objects, reflection-less negative refraction and flat axicons on dielectric-air interfaces, etc. The coalescence of antireflection and wavefront controllability in the deep subwavelength scale brings new opportunities for advanced interface optics with high efficiency and great flexibility.

Microscopic metavehicles powered and steered by embedded optical metasurfaces.

Nanostructured dielectric metasurfaces offer unprecedented opportunities to manipulate light by imprinting an arbitrary phase gradient on an impinging wavefront1. This has resulted in the realization of a range of flat analogues to classical optical components, such as lenses, waveplates and axicons2-6. However, the change in linear and angular optical momentum7 associated with phase manipulation also results in previously unexploited forces and torques that act on the metasurface itself. Here we show that these optomechanical effects can be utilized to construct optical metavehicles-microscopic particles that can travel long distances under low-intensity plane-wave illumination while being steered by the polarization of the incident light. We demonstrate movement in complex patterns, self-correcting motion and an application as transport vehicles for microscopic cargoes, which include unicellular organisms. The abundance of possible optical metasurfaces attests to the prospect of developing a wide variety of metavehicles with specialized functional behaviours.

Phase shifter based on voltage-controlled magnetic domain walls

A controllable phase shifter is an essential part of spin-wave (SW) logic devices. Magnetic domain walls (DWs) as magnonic waveguides have been used to study SW propagation in ultrathin ferromagnetic films. In this study, we present a channel for SW propagation that relies on magnetic DWs as natural waveguides and realize phase manipulation by voltage-controlled magnetic anisotropy (VCMA) at a lower excitation frequency with micromagnetic simulations. The system model is a double-layer magnetic film structure, containing two oppositely magnetized domains and an upper layer (Co20Fe60B20) coupled with the lower layer ([Co/Pd]) through exchange interaction. The pinning structure can effectively maintain the state of the waveguide during later operation without an additional magnetic field. By numerical micromagnetic simulation, we can clearly see that the exchange-dominated isotropic magnetostatic forward volume like SWs are confined in the Bloch-type DW. The phase accumulation by a propagating SW depends on the accumulated phase wave vector (k) and the distance traveled. Therefore, SWs accumulate more phase through DW waveguides covered by electrodes with different lengths. The phase of the SW is shifted after the SW passes through the voltage region, compared with the waveguide where a voltage is unapplied. As a result, the established phase shifter shows a phase difference of 2π by applying a voltage (E = 1 V/nm), and the length of the applied electrode is about 644 nm.

Construction of Optical Topological Cavities Using Photonic Crystals

A novel design of the Fabry–Pérot optical cavity is proposed, utilizing both the topological interface state structures and photonic bandgap materials with a controllable reflection phase. A one-to-one correspondence between the traditional Fabry–Pérot cavity and optical topological cavity is found, while the tunable reflection phase of the photonic crystal mirrors provides an extra degree of freedom on cavity mode selection. The relationship between the Zak phase and photonic bandgap provides theoretical guidance to the manipulation of the reflection phase of photonic crystals. The dispersions of interface states with different topology origins are explored. Linear interfacial dispersion emerging in photonic crystals with the valley–spin Hall effect leads to an extra n = 0 cavity mode compared to the Zak phase–induced deterministic interface states with quadratic dispersion. The frequency of the n = 0 cavity mode is not affected by the cavity length, whose quality factor can also be tuned by the thickness of the photonic crystal mirrors. With the recent help of topology photonics in the tuning reflection phase and dispersion relationship, we hope our results can provide more intriguing ideas to construct topological optical devices.

Lossless Complex-Valued Optical-Field Control with Compound Metaoptics

Forming a desired optical field distribution from a given source requires precise spatial control of a field's amplitude and phase. Low-loss metasurfaces that allow extreme phase and polarization control of optical fields have been demonstrated over the past few years. However, metasurfaces that provide amplitude control have remained lossy, utilizing mechanisms such as reflection, absorption, or polarization loss to control amplitude. Here, we describe the amplitude and phase manipulation of optical fields without loss, by using two lossless phase-only metasurfaces separated by a distance. We first demonstrate a combined beam-former and splitter optical component using this approach. Next, we show a high-quality computer-generated three-dimensional hologram. The proposed metaoptic platform combines the advantage of lossless, complex-valued field control with a physically small thickness. This approach could lead to low-profile, three-dimensional holographic displays, compact optical components, and high precision optical tweezers for micro-particle manipulation.

Modeling of plasticity-induced martensitic transformation to achieve hierarchical, heterogeneous, and tailored microstructures in stainless steels

While stainless steels (SS) have excellent corrosion resistance for use in industries such as chemical and food processing, medical implants made of such steels require more stringent specifications, e.g., high strength while maintaining a low weight. A way to design and manufacture such behavior of SS is through the intentional deformation induced manipulation of constituent phases to achieve heterogeneous and hierarchical microstructures. In this paper, an elasto-plastic self-consistent modeling framework incorporating a strain-induced austenite-to-martensite transformation kinetic sub-model is calibrated using a set of SS304L data from the literature to capture stress-strain response and volume fraction of phases. The model is then validated by predicting the mechanical responses of SS316L using a new data set recorded as a function of strain-rate and temperature. By accurately predicting the material behavior, the modeling results can guide the manufacturing process to achieve the desired final part properties.

Interferometric Reduction of Induced Phase Variation in Stimulated Brillouin Processing of Light Waves

Phase manipulation is crucial for radio frequency (RF) and optical signal processing. In this work, we introduce an interferometric phase-reduction scheme to flexibly suppress the phase change while simultaneously amplifying the signal. We develop a numerical model to evaluate the characteristics of this processing scheme. The concept is further demonstrated with a proof-of-principle experiment that modifies the phase and gain profiles induced by stimulated Brillouin scattering in an optical fiber. This scheme provides an additional freedom to control the signal phase and amplitude in RF/optical processing functionalities.

Polarization and Holography Recording in Real‐ and k‐Space Based on Dielectric Metasurface

AbstractThe diverse design freedom and mechanisms of metasurfaces motivate the manipulation of polarization in an ultrashort distance with subwavelength resolution and make metasurfaces outperform conventional polarization optical elements. However, in order to enhance the information capability and encryption security of metasurface holograms, polarization manipulation together with multiplexing technologies are still highly desired. Here, a birefringent dielectric metasurface with the capability of encoding a grayscale image in real‐space based on Malus's law by utilizing the inhomogeneous polarization distribution and realizing the reconstruction of a vectorial holographic image in k‐space with the help of the phase profiles of different polarization components of output light is demonstrated. This novel functionality is realized by exploiting the manipulation of polarization and phase of output light simultaneously offered by the dielectric metasurface. The proposed method may enhance the information capability and security level of applications such as the anticounterfeiting and encryption.

A Circularly Polarized 1 Bit Electronically Reconfigurable Reflectarray Based on Electromagnetic Element Rotation

A circularly polarized (CP) reflectarray with 2-D electronically steerable beam is reported. The reconfigurable CP reflectarray element is realized using a circular-patch-based structure with p-i-n diodes incorporated. A new electronical phase manipulation method is proposed based on the element rotation technique and by additionally exploiting symmetries in the field distribution of the element's fundamental resonant mode. Element rotation can be equivalently achieved by altering the biasing voltage and changing the electromagnetic behavior of the reflective phasing cell. The proposed design approach brings about simplification in element configuration, leading to a reduction in the number of required diodes by a factor of one half. Practical concerns of the element and biasing complexity as well as the insertion loss are, therefore, well-handled. A fully functional 16×16 reconfigurable CP reflectarray is designed, fabricated, and tested to validate the concept. Good beam focusing and 2-D dynamic beam steering capabilities are confirmed through experiments, along with decent polarization purity and comparatively high aperture efficiency. The proposed simple and robust design could be attractive for implementing lightweight, low-cost, and large-scale 2-D reconfigurable CP reflectarrays.

Circular dichroism-like response of terahertz wave caused by phase manipulation via all-silicon metasurface

Chiral metasurfaces based on asymmetric meta-atoms have achieved artificial circular dichroism (CD), spin-dependent wavefront control, near-field imaging, and other spin-related electromagnetic control. In this paper, we propose and experimentally verify a scheme for achieving high-efficiency chiral response similar to CD of terahertz (THz) wave via phase manipulation. By introducing the geometric phase and dynamic phase in an all-silicon metasurface, the spin-decoupled terahertz transmission is obtained. The giant circular dichroism-like effect in the transmission spectrum is observed by using a random phase distribution for one of the circular polarization components. More importantly, the effect can be adjusted when we change the area of the metasurface illuminated by an incident terahertz beam. In addition, we also demonstrate the spin-dependent arbitrary wavefront control of the transmitted terahertz wave, in which one of the circularly polarized components is scattered, while the other forms a focused vortex beam. Simulated and experimental results show that this method provides a new idea for spin selective control of THz waves.

Non-linear interferometry with infrared metasurfaces

AbstractThe optical elements comprised of sub-diffractive light scatterers, or metasurfaces, hold a promise to reduce the footprint and unfold new functionalities of optical devices. A particular interest is focused on metasurfaces for manipulation of phase and amplitude of light beams. Characterisation of metasurfaces can be performed using interferometry, which, however, may be cumbersome, specifically in the infrared (IR) range. Here, we realise a new method for characterising metasurfaces operating in the telecom IR range using accessible components for visible light. Correlated IR and visible photons are launched into a non-linear interferometer so that the phase profile, imposed by the metasurface on the IR photons, modifies the interference at the visible photon wavelength. Furthermore, we show that this concept can be used for broadband manipulation of the intensity profile of a visible beam using a single IR metasurface. Our method unfolds the potential of quantum interferometry for the characterization of advanced optical elements.

From Lingering to Rift: Metasurface Decoupling for Near- and Far-Field Functionalization.

Metasurfaces, simultaneously operating in near- and far-fields, can be employed as a promising candidate to implement different functions, thus significantly improving the information density, security, and system integration. Recent works have showcased some approaches for decoupling-at-large between near- and far-field functionalities, but unfortunately, their coupling effects are just reduced and mitigated to some extent rather than eradicated, which in turn leads to the performance limitation of metadevices. Herein, we propose a general platform for the complete rift between near- and far-field functionalities, enabled by strictly decoupled manipulation of optical amplitude and phase, leading to their distinct functions in the near- and far-fields, respectively. This concept is experimentally demonstrated by integrating the functions of a phase-only metalens and an amplitude-only grayscale-imaging nanoprint into a single-cell metasurface. Because of their completely decoupled functions, both meta-elements show high-performance characteristics, i.e., imaging quality close to the diffraction limit and high-definition grayscale-imaging with resolution as high as 63500 dots per inch (dpi). The validated recipe may empower advanced explorations and applications in highly integrated nano-optoelectronics requiring high performance and less crosstalk.

On the Speech Properties and Feature Extraction Methods in Speech Emotion Recognition.

Many speech emotion recognition systems have been designed using different features and classification methods. Still, there is a lack of knowledge and reasoning regarding the underlying speech characteristics and processing, i.e., how basic characteristics, methods, and settings affect the accuracy, to what extent, etc. This study is to extend physical perspective on speech emotion recognition by analyzing basic speech characteristics and modeling methods, e.g., time characteristics (segmentation, window types, and classification regions—lengths and overlaps), frequency ranges, frequency scales, processing of whole speech (spectrograms), vocal tract (filter banks, linear prediction coefficient (LPC) modeling), and excitation (inverse LPC filtering) signals, magnitude and phase manipulations, cepstral features, etc. In the evaluation phase the state-of-the-art classification method and rigorous statistical tests were applied, namely N-fold cross validation, paired t-test, rank, and Pearson correlations. The results revealed several settings in a 75% accuracy range (seven emotions). The most successful methods were based on vocal tract features using psychoacoustic filter banks covering the 0–8 kHz frequency range. Well scoring are also spectrograms carrying vocal tract and excitation information. It was found that even basic processing like pre-emphasis, segmentation, magnitude modifications, etc., can dramatically affect the results. Most findings are robust by exhibiting strong correlations across tested databases.

Use of Dielectric Metasurfaces to Generate Deep‐Subwavelength Nondiffractive Bessel‐Like Beams with Arbitrary Trajectories and Ultralarge Deflection

AbstractFor decades, accelerating beams have attracted considerable interest in fundamental physics and in various emerging applications. However, not only are conventional accelerating beam generators (such as spatial light modulators) bulky and diffraction‐inefficient, they also have a poorly resolved ability of phase manipulation that limits the accelerating beam's minimum size and maximum degree of curvature. In this study, a dielectric metasurface is used to generate highly focused nondiffractive Bessel‐like accelerating beams with predefined arbitrary trajectories within a broadband spectral range of 550–710 nm. In particular, a similar metasurface with a combined phase profile allows the generation of a Bessel‐like vortex beam with an ultrahigh numerical aperture of 0.79, resulting in a subwavelength beamwidth of 234 nm (≈0.43λ) down to the diffraction limit. In addition, various accelerating Bessel‐like beams and beam arrays with different predefined trajectories and characteristic beam parameters that are not otherwise achievable with conventional spatial light modulators are demonstrated with synthetic‐phase metasurfaces. This study promotes the diversity of Bessel‐like accelerating beams for practical applications in fields such as optical manipulation, optical storage, biomedical imaging, and material processing.