Wavefront Manipulation Research Articles

On-chip non-uniform geometric metasurface for multi-channel wavefront manipulations.

Metasurfaces integrated with waveguides have been recently explored as a means to control the conversion between guided modes and radiation modes for versatile functionalities. However, most efforts have been limited to constructing a single free-space wavefront using guided waves, which hinders the functional diversity and requires a complex configuration. Here, a new, to the best of our knowledge, type of non-uniformly arranged geometric metasurface enabling independent multi-channel wavefront engineering of guided wave radiation is ingeniously proposed. By endowing three structural degrees of freedom into a meta-atom, two mechanisms (the Pancharatnam-Berry phase and the detour phase) of the metasurface are perfectly joined together, giving rise to three phase degrees of freedom to manipulate. Therefore, an on-chip polarization demultiplexed metalens, a wavelength-multiplexed metalens, and RGB-colored holography with an improved information capacity are successively demonstrated. Our results enrich the functionalities of an on-chip metasurface and imply the prospect of advancements in multiplexing optical imaging, augmented reality (AR) holographic displays, and information encryption.

Terahertz Beam Steering with Curved Metasurfaces

Considerable recent research interest has focused on the possibility of using metasurfaces for manipulation of terahertz wavefronts. For example, metasurfaces allow a beam to be targeted in any desired direction using strategically placed meta-elements. With rapid prototyping techniques, metasurfaces can be fabricated quickly and at a low cost. These techniques also permit the fabrication of metasurfaces on flexible substrates which can be bent easily. This opens the possibility of employing such devices as conformable arrays on non-flat surfaces. To explore this idea, we experimentally and numerically analyze the performance of a terahertz metasurface printed on paper, as a function of its radius of curvature. We observe that when the metasurface is bent, the direction of the refracted beam is minimally impacted and the performance of the metasurface remains very similar to when it is flat. This conclusion will simplify the design and modeling criteria for conformable metasurfaces.

Three-Channel Near-Field Display and Encryption Based on a Polarization Multiplexed Metasurface.

Multichannel metasurfaces are becoming a significant trend in the field of optical encryption due to their excellent manipulation of optical wavefronts. However, existent multichannel metasurfaces for optical encryption mostly implement only two channels in the near-field, or three channels by combining the near- and far-field. In this paper, we propose and simulate a three-channel metasurface that works entirely in the near-field and uses the polarization state of the incident light, left circularly polarized (LCP) light, right circularly polarized (RCP) light, and linearly polarized (LP) light as the security key. The metasurface consists of two types of nanostructures that work as a polarizer and a quarter-wave plate, providing an additional degree of freedom for encoding that enables independent near-field display at 633 nm wavelength incident light. The proposed three-channel metasurface has the advantages of high information density and high security, which will pave the way for multi-channel applications such as ultracompact displays, optical encryption, and information storage.

Fickian yet non-Gaussian diffusion of a quasi-2D colloidal system in an optical speckle field: experiment and simulations

We investigate a quasi-2D suspension of Brownian particles in an optical speckle field produced by holographic manipulation of a laser wavefront. This system was developed to study, in a systematic and controllable way, a distinctive instance of diffusion, called Fickian yet Non Gaussian diffusion (FnGD), observed, during the last decade, for colloidal particles in a variety of complex and biological fluids. Our setup generates an optical speckle field that behaves like a disordered set of optical traps. First, we describe the experimental setup and the dynamics of the particles, focusing on mean square displacements, displacement distributions and kurtosis. Then, we present Brownian Dynamics simulations of point-like particles in a complex energy landscape, mimicking that generated by the optical speckle field. We show that our simulations can capture the salient features of the experimental results, including the emergence of FnGD, also covering times longer than the ones so far achieved in experiments. Some deviations are observed at long time only, with the Gaussian restoring being slower in simulations than in experiments. Overall, the introduced numerical model might be exploited to guide the design of upcoming experiments targeted, for example, to fully monitor the recovery of Gaussianity.

On‐Chip Multidimensional Manipulation of Far‐Field Radiation with Guided Wave‐Driven Metasurfaces

AbstractMiniaturization and integration of multiple optical components in one chip are important trends in the future development of optical chips. Despite various on‐chip multifunctional metasurfaces having been proposed, simultaneously manipulating the polarization and wavefront of far‐field radiation still remains a challenge. Here, a novel approach to simultaneously realize on‐chip polarization and wavefront manipulation of far‐field radiation with guided wave‐driven geometric metasurfaces is theoretically proposed and experimentally demonstrated. Various polarized beams with controllable radiation angles can be generated by maneuvering phase gradients and far‐field interferences of meta‐atoms. The phase and amplitude of off‐chip radiation can be tailored all at once by altering the rotation angles and positions of each meta‐atom. As a proof of concept, a guided wave‐driven metalens and four different on‐chip polarization generators are fabricated and characterized. These on‐chip devices work at broadband and can realize mode‐dependent beam deflection. This study provides a practical route toward on‐chip multidimensional control of far‐field radiation, which could support a further step in the development of photonic integrated circuits, wearable near‐eye displays, and optical information processing devices, just to name a few.

Generating a Superoscillation Three-Dimensional Hollow Spot by Polarization Manipulation

A wide variety of practical applications utilize three-dimensional (3D) hollow spots. To improve their performance, it is critical to independently control the transverse and longitudinal size of a 3D hollow spot. Here, we propose a method of creating a superoscillation 3D hollow spot by properly engineering the spatial polarization distribution. A cylindrical vector wave can be generated and the transverse and longitudinal sizes of the 3D hollow spot can be independently controlled by individually optimizing the wave fronts of the radial and azimuthal components in the cylindrical vector wave. A cylindrical vector metalens integrated with the functions of polarization conversion and wave-front manipulation is optimized and fabricated. The generated 3D hollow spot is experimentally demonstrated with a transverse size of (0.336 \ifmmode\pm\else\textpm\fi{} 0.005)\ensuremath{\lambda} and a longitudinal size of (1.170 \ifmmode\pm\else\textpm\fi{} 0.005)\ensuremath{\lambda}. Our method provides an easy and practical approach to generate a 3D hollow spot with a subwavelength transverse size, which can be intriguing for nanoparticle manipulation and in nanomicroscopy.

Flexible terahertz phase modulation profile via all-silicon metasurfaces

Metasurfaces represent a group of artificially designed structures capable of efficiently manipulating the transmission properties of incident waves, have attracted considerable attention in terahertz (THz) radiation, and gradually overcome the complexity of bulky optical systems. Although the introduction of meta-optics has made great progress in full-span phase modulation schemes within orthogonal polarization channels, flexible phase functions can still effectively alleviate the design complexity of wavefront engineering. Here, we experimentally demonstrate several designs of general-purpose all-silicon THz metasurfaces applied in different scenarios. By combining geometric phase and propagation phase, switchable multiple phase modulation in orthogonal circularly polarized (CP) channels can be realized by switching the handedness of the incident wave. The advantage of the proposed scheme is that not only a locked phase modulation profile can be generated within the cross-polarized channels with opposite handedness, but also the phase distribution within each channel can be arbitrarily tailored to break the locking, demonstrating the flexible wavefront manipulation capability of the spin-decoupled phase control. Two different metalens are experimentally demonstrated to verify the feasibility of the multiple phase modulation scheme, mainly exploiting the topological charge and type of the focused beams. This work allows the metasurface to show a higher degree of freedom in wavefront manipulation by using a simpler phase-composite algorithm.

Broadband electrically controlled reflective metasurface for reconfigurable circularly polarized wavefront manipulation.

A broadband, electrically controlled, reconfigurable, circularly polarized reflective metasurface is presented. The chirality of the metasurface structure is changed by switching active elements, which benefits from the tunable current distributions generated by the elaborately designed structure under x-polarized and y-polarized waves. Notably, the proposed metasurface unit cell maintains a good circular-polarization efficiency in a broadband range of 6.82-9.96 GHz (fractional bandwidth of 37%) with a phase difference of π between the two states. As a demonstration, a reconfigurable circularly polarized metasurface containing 8 × 8 elements was simulated and measured. The results verify that the proposed metasurface can flexibly control circularly polarized waves in a broadband, realizing beam splitting, mirror reflection, and other beam manipulations from 7.4 GHz to 9.9 GHz (fractional bandwidth of 28.9%) by simply adjusting the loaded active elements. The proposed reconfigurable metasurface may offer a promising approach to electromagnetic wave manipulation or communication systems.

Frequency‐Multiplexed Holographic‐Reflective Coding Metasurface for Independent Controls of Surface Wave and Spatially Propagating Wave

AbstractIn recent years, frequency‐multiplexed metasurfaces have received extensive attention due to the increasing demands for device integration and communication capacity. However, most of previously reported works can only manipulate either surface waves (SW) or spatially propagating waves (SPW). In this paper, a frequency‐multiplexed holographic‐reflective‐integration (HRI) coding metasurface is proposed, which can independently manipulate SW and SPW at different frequencies with a shared aperture. To be specific, the metasurface can act as a holographic metasurface to manipulate SW at low frequency and a reflective metasurface to manipulate SPW at high frequency. The performance of the frequency‐multiplexed HRI coding metasurface is validated by matched simulation and measurement results of versatile wavefront and polarization manipulations. Due to the advantages of low profile, high aperture usage, and multifunction integration, the proposed frequency‐multiplexed HRI coding metasurface is promising to be applied in wireless communications, radar detections, and holographic technology.

Multi-channel and multi-function terahertz metasurface

Generally, one of the disadvantages of the reported metasurface is single-channel, which affects its practical application in multiplexing system. To overcome this shortage, a reflected shared-aperture metasurface is proposed to produce multi-channel and multifunction. The unit cell consists of a square metal strip silicon dioxide substrate and a metal plate bottom layer. Based on geometrical phase theory, 0∼2π phase shift can be covered by rotating orientation angles of the top metal strip. The designed metasurface realizes multi-channel and multi-function including four-channel deflected beams with different angles, dual-channel vortex beams with different topological charges, and dual-channel vortex focusing beams under right-handed circularly polarized (RCP) wave incidents. Our investigation offers a simple and effective approach to design high-efficiency terahertz wavefront manipulation devices.

Ultra-compact active induced-magnetism Huygens’ metasurfaces: design and application

Recently, a new type of metasurface, the induced-magnetism Huygens’ metasurface, has attracted considerable attention due to its capability to manipulate electromagnetic waves. However, these studies mainly work in a passive way such that the functionalities are fixed once fabricated, limiting the device’s adaptability for practical applications and future commercialization. To comply with potential applications, further research endeavors need to be exerted to advance the reconfigurability and practicality. Herein, an ultra-compact and dynamically tunable design strategy of an induced-magnetism Huygens’ metasurface is proposed, whose basic unit is a pair of electric dipoles separated by a dielectric spacer and integrated with a varactor in each electric dipole. By simply changing the driven voltage of each unit, the transmission phase can be individually tuned in the near 2π range; meanwhile, the transmission amplitude remains very high. As a proof-of-concept demonstration, an active magnetism induced Huygens’ meta-lens is proposed to exhibit excellent dynamic wavefront manipulation performance. The proposed method may open an avenue toward planar lightweight low-cost dynamic wavefront manipulation devices.

Ultrabroadband and Reconfigurable Transmissive Acoustic Metascreen

AbstractPassive acoustic wave manipulations are severely constrained by the narrow frequency bandwidth of acoustic metastructures. In this research, an unprecedented type of reconfigurable acoustic metascreen is proposed for broadband manipulations of transmitted acoustic waves. The conceived structure is composed of uniquely designed unit cells producing the modulation of the transmitted phase shift within the full 2π range with an excellent impedance matching with the background medium. By rationally arranging the reconfigurable elements within the metascreen based on the corresponding parameter profile, different phenomena and functionalities can be easily realized. As examples, acoustic focusing and acoustic bending are presented to showcase the performance of the proposed metascreen. We indeed numerically and experimentally demonstrate the ultra‐broadband and reconfigurable features of our concept over an astonishing frequency range extending from 3 to 17 kHz, which covers the majority spectrum of the audible range of human hearing. Our work provides a unique and remarkable conceptual design of acoustic metascreen opening a promising and pragmatic route to conceive compact broadband acoustic devices, where wavefront manipulations on broadband sound signals or pulsed signals are required.

Asymmetric acoustic metagrating enabled by parity-time symmetry

Passive parity-time (PT) symmetric metamaterials as a fundamental carrier to investigate non-Hermitian systems have attracted significant interest, but previous studies have rarely focused on the diffraction properties of the PT system, especially the diffraction effect in the metasurfaces with phase gradient modulation. Here, by merging two concepts of phase gradient and PT symmetry together, we design and study a non-Hermitian acoustic gradient metasurface (i.e., metagrating). We show that the PT symmetry breaks spatially the symmetry of reflected wave diffraction in the acoustic metagrating, making it an asymmetric mirror. In particular, at the PT-ensured exceptional point, for incident waves from one side, such metagrating behaves as a phase gradient metasurface, in which high-efficiency retroreflection governed generalized Snell's law occurs. For an incident wave from the opposite direction, such a metagrating is a common grating, whose diffraction in the reflection follows the common diffraction equation. Our work builds a bridge connecting the phase gradient and passive PT symmetric system, enriching the methods for wavefront manipulation and exhibiting potential applications in the design of multifunctional acoustic devices.

Wavefront shaping with nonlinear four-wave mixing

Wavefront manipulations have enabled wide applications across many interdisciplinary fields ranging from optics and microwaves to acoustics. However, the realizations of such functional surfaces heavily rely on micro/nanofabrication to define the structured surfaces, which are fixed and only work within a limited spectrum. To address these issues, previous attempts combining tunable materials like liquid crystal or phase-change ones onto the metasurfaces have permitted extra tunability and working spectra, however, these additional layers bring in inevitable loss and complicate the fabrication. Here we demonstrate a fabrication-free tunable flat slab using a nonlinear four-wave mixing process. By wavefront-shaping the pump onto the flat slab, we can successfully tune the effective nonlinear refraction angle of the emitting FWM beams according to the phase-matching condition. In this manner, a focusing and a defocusing nonlinear of FWM beam through the flat slab have been demonstrated with a converging and a diverging pump wavefronts, respectively. Furthermore, a beam steering scheme over a 20° angle has been realized through a non-degenerate four-wave mixing process by introducing a second pump. These features open up a door to manipulating light propagation in an all-optical manner, paving the way to more functional and tunable flat slab devices in the applications of imaging and all-optical information.

Diverse terahertz wavefront manipulations empowered by the spatially interleaved metasurfaces

Diverse terahertz wavefront manipulations empowered by the spatially interleaved metasurfaces

Simple acoustic metagrating for perfect two- and three-beam splitting

Acoustic metasurfaces have been widely explored and attracted great attention for their extraordinary wavefront manipulation abilities. In this paper, we propose a simple acoustic metagrating with periodic grooves that can split a normally incident beam into two or three reflected beams. The amplitudes and power flows of different reflected beams can be manipulated by changing the groove parameters. The mirror reflected wave is suppressed for equal two-beam splitting case and allowed for three-beam splitting case. Theoretical analysis and numerical simulations are performed to demonstrate the perfect two- and three-beam splitting performances based on local power conservation. Our research work provides a simple method for designing acoustic beam splitters and has extensive applications in acoustic sensing and communication.

Adaptive control on flexural waves by a piezoelectric-based elastic metasurface with hybrid shunting circuits

In aspects of its simple configuration and tunability, piezoelectric-based metasurface has benefited over its mechanical counterpart in adaptive wavefront manipulation. By tuning the circuitry elements in the shunting circuits properly, the equivalent material properties of the metasurface can be modified to attain a complete 2 π phase shift and to maintain high transmission at desired working frequencies. In this paper, a piezoelectric-based elastic metasurface with hybrid shunting circuits ( NC – LC – NC shunts) are proposed to adaptively control the propagation of flexural waves. Through tuning of negative capacitance and inductance simultaneously, the proposed elastic metasurface can cover a phase shift ranging from 0 − 2 π while having a high transmission ( ≥ 0.7). It is discovered that any similar metasurface structural design with solely single shunting circuit presently available could not achieve the same results as the proposed elastic metasurface. In addition, analytical modeling based on equivalent Young’s modulus is developed and a finite element model is built to study the transmission and phase shift of the proposed metasurface. It is demonstrated numerically that the proposed piezoelectric-based elastic metasurface is capable of performing switchable functionalities, such as wave refraction and focusing, at different working frequencies (3500 Hz, 5000 Hz, etc.).

Transmissive metasurface for multi-channel and full-polarization modulation of electromagnetic wavefronts

Metasurfaces have great potential for flexible manipulation of electromagnetic wave polarizations and wavefronts. Here, we propose a general method for achieving independent wavefront manipulation in a single polarization-multiplexing transmissive metasurface. As a proof of concept, we design a transmission-type anisotropic metasurface for independent wavefront manipulation in full-polarization channels. An x -polarized wave transmitted through such a metasurface could be converted into four outgoing beams with delicately designed polarization states that converge to specific positions for holographic imaging. The measured results are in good agreements with simulated ones, verifying the independent wavefront manipulations with arbitrary polarization conversions. Compared with the existing traditional meta-devices with single-polarization modulation, we achieve polarization-multiplexed metasurfaces with mixed polarization and phase control, which can greatly improve the functional richness of the system.

Spin‐Decoupled Beam Steering with Active Optical Chirality Based on Terahertz Liquid Crystal Chiral Metadevice

AbstractBroadband terahertz (THz) chirality and wavefront manipulation play important roles in wireless communication, imaging, and radar systems. In this work, by integrating liquid crystal (LC) into a dielectric metasurface, the gradient phase distribution from the Si metasurface and the tunable birefringent phase shift from LC are engineered to perform an active spin‐decoupled beam steering with the strong spin asymmetric transmission. By using geometric symmetry analysis, the relationships between the symmetry breaking and the optical chirality of the metadevice are revealed under different LC orientations, and both the simulations and experiments further verify the dependence of this chirality on spin‐decoupled beam steering with different spin states. The results show that with the orientation changes of the LC by driving the different magnetic fields, the circular dichroism of the device at the 0° deflection angle is ≈100% dynamically modulated and even flipped in the broadband range of 0.8–1.3 THz. With these changes in THz chirality, a tunable spin‐decoupled beam steering occurs in the range of 25°‐45°. This work shows that tunable LC anisotropy leads to more complex symmetry breaking in this structure, and results in more functions than conventional spin‐decoupled wavefront manipulations, which expands the scope of THz chiral metadevice and its applications.

Full-space wavefront manipulation enabled by asymmetric photonic spin-orbit interactions.

Optical metasurfaces empower complete wavefront manipulation of electromagnetic waves and have been found in extensive applications, whereas most of them work in either transmission or reflection space. Here, we demonstrate that two independent and arbitrary phase profiles in transmission and reflection spaces could be produced by a monolayer all-dielectric metasurface based on the asymmetric photonic spin-orbit interactions, realizing full-space wavefront independent manipulation. Furthermore, the supercell-based non-local approach is employed to suppress crosstalk between adjacent nanopillars in one supercell for broadband and high-efficiency wavefront manipulation in full space. Compared with the conventional unit cell-based local approach, such a method could improve efficiency about 10%. As a proof of concept, two metadevices are designed, in which the maximum diffraction efficiencies are ∼95.53%/∼74.07% within the wavelength range of 1500-1600 nm in reflection/transmission space under circularly polarized light incidence. This configuration may offer an efficient way for 2π-space holographic imaging, augmented reality, virtual reality technologies, three-dimensional imaging, and so forth.