2π Phase Control Research Articles

Excitonic van der Waals Metasurfaces for Resonant Wavefront Shaping at Deep Subwavelength Thickness Scale.

Dielectric phase gradient metasurfaces have emerged as promising candidates to shrink bulky optical elements to subwavelength thickness scale based on dielectric meta-atoms. These meta-atoms strongly interact with light, thus offering excellent phase manipulation of incident light. However, to fulfill 2π phase control using meta-atoms, the metasurface thickness, to date, is limited to the order of 102 nm. Here, we present the thickness scaling down of phase gradient metasurfaces to <λ/20 by using excitonic van der Waals metasurfaces. High-refractive-index enabled by exciton resonances and symmetry-breaking nanostructures in the patterned layered tungsten disulfide (WS2) corporately enable quasibound states in the continuum in WS2 metasurfaces, which consequently yield complete phase regulation of 2π with the thickness down to 35 nm. To illustrate the concept, we have experimentally demonstrated beam steering, focusing, and holographic display using WS2 metasurfaces. We envision our results unveiling new venues for ultimate thin phase gradient metasurfaces.

Electrically Tunable Reflective Metasurfaces with Continuous and Full-Phase Modulation for High-Efficiency Wavefront Control at Visible Frequencies.

All-dielectric optical metasurfaces can locally control the amplitude and phase of light at the nanoscale, enabling arbitrary wavefront shaping. However, lack of postfabrication tunability has limited the true potential of metasurfaces for many applications. Here, we utilize a thin liquid crystal (LC) layer as a tunable medium surrounding the metasurface to achieve a phase-only spatial light modulator (SLM) with high reflection in the visible frequency, exhibiting active and continuous resonance tuning with associated 2π phase control and uncoupled amplitude. Dynamic wavefront shaping is demonstrated by programming 96 individually addressable electrodes with a small pixel pitch of ∼1 μm. The small pixel size is facilitated by the reduced LC thickness, strongly suppressing cross-talk among pixels. This device is used to demonstrate dynamic beam steering with a wide field-of-view and high absolute diffraction efficiencies. We believe that our demonstration may help realize next-generation, high-resolution SLMs, with wide applications in dynamic holography, tunable optics, and light detection and ranging (LiDAR), to mention a few.

Thickness-designable acoustic metamaterial for passive phased arrays

Passive phase-controlled acoustic arrays based on metamaterial surfaces artificially manipulate acoustic waves, which can be used in biomedical imaging and nondestructive testing. One of the most representative applications is ultrasonic therapy using a passive phased array within a complete 2π phase. In contrast, the surface thickness is 1/10 the wavelength of the operating frequency. However, conventional design methods feature a fixed array thickness for a specific frequency. This becomes necessary for different application scenarios where different thicknesses are needed. This study proposed a labyrinth resonant cavity metascreen phased array based on Helmholtz resonators and convoluted labyrinth acoustic metamaterials. The thickness of this array can be optimized to accommodate a specific operating frequency to achieve full 2π phase control by adjusting only one parameter. Theoretical simulations and experimental results verified the acoustic wave manipulation effect. The proposed method may find utility in applications for noninvasive ultrasonic therapy, including nondestructive testing, acoustic communication, and biomedical imaging.

Dielectric Polarization‐Filtering Metasurface Doublet for Trifunctional Control of Full‐Space Visible Light

AbstractMultilayered plasmonic metasurfaces have been previously shown to enable multifunctional control of full‐space electromagnetic waves, which are of great importance to the development of compact optical systems. While this structural configuration is practical for acquiring metasurfaces working in microwave frequency, it will inevitably become lossy and highly challenging to fabricate when entering the visible band. Here, an efficient yet facile approach to address this issue by resorting to a dielectric metasurface doublet (DMD) based on two vertically integrated polarization‐filtering meta‐atoms (PFMs) is presented. The PFMs exhibit polarization‐dependent high transmission and reflection, as well as independent and full 2π phase control characteristics, empowering the DMD to realize three distinct incidence‐direction and polarization‐triggered wavefront‐shaping functionalities, including anomalous beam deflection, light focusing, vortex beam generation, and holographic image projection as it is investigated either numerically or experimentally. The presented DMD undoubtedly holds several salient features compared with the multilayered metallic metasurfaces in aspects of design complexity, efficiency, and fabrication. Furthermore, as dielectric meta‐atoms with distinct polarization responses can be deployed to construct the DMD, it is anticipated that diverse full‐space metasurfaces equipped with versatile functionalities can be demonstrated in the future, which will greatly advance the development of multifunctional meta‐optics.

On-demand terahertz surface wave generation with microelectromechanical-system-based metasurface

During the past decade, metasurfaces have shown great potential to complement standard optics, providing novel pathways to control the phase, amplitude, and polarization of electromagnetic waves utilizing arrays of subwavelength resonators. We present dynamic surface wave (SW) switching at terahertz frequencies utilizing a mechanically reconfigurable metasurface. Our metasurface is based on a microelectromechanical system (MEMS) consisting of an array of micro-cantilever structures, enabling dynamic tuning between a plane wave (PW) and a SW for normal incidence terahertz radiation. This is realized using line-by-line voltage control of the cantilever displacements to achieve full-span ( 2 π ) phase control. Full-wave electromagnetic simulations and terahertz time-domain spectroscopy agree with coupled mode theory, which was employed to design the metasurface device. A conversion efficiency of nearly 60% has been achieved upon switching between the PW and SW configurations. Moreover, a nearly 100 GHz working bandwidth is demonstrated. The MEMS-based control modality we demonstrate can be used for numerous applications, including but not limited to terahertz multifunctional metasurface devices for spatial light modulation, dynamic beam steering, focusing, and beam combining, which are crucial for future “beyond 5G” communication systems.

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.

Complete 2π phase control by photonic crystal slabs

Photonic crystal slabs are the state of the art in the studies of the light confinement, optical wave modulating and guiding, as well as nonlinear optical response. Previous studies have shown abundant real-world implementations of photonic crystals in planar optics, metamaterials, sensors, and lasers. Here, we report a novel full 2π phase control method in the reflected light beam over the interaction with a photonic crystal resonant mode, verified by the temporal coupled-mode analysis and S-parameter simulations. Enhanced by the asymmetric coupling with the output ports, the 2π phase shift can be achieved with the silicon photonics platforms such as Silicon-on-Silica and Silicon-on-Insulator heterostructures. Such photonic crystal phase control method provides a general guide in the design of phase-shift metamaterials, suggesting a wide range of applications in the field of sensing, spatial light modulation, and beam steering.

High-efficiency ultra-broadband orbital angular momentum beam generators enabled by arrow-based fractal metasurface

Vortex beams carrying orbital angular momentum (OAM) are of great importance owing to the capacity to expand channel capacity of communication systems from microwave to optical regimes. In this work, a single-layer ultra-broadband meta-atom is designed by using the method of fractal. The reflected co-polarization amplitude of the arrow-based fractal meta-atom can exceed 0.9 in a frequency range from 6 to 19.7 GHz and the complete 2π phase control can be achieved by rotating the structure. Based on the proposed meta-atom, metasurface-based vortex beam generators are designed. As an illustrative example, a metasurface-based OAM beam generator with a mode of +1 is designed, fabricated, and measured. Both the experimental and simulated results match well with each other, proving the practicability of the proposed high-efficiency ultra-broadband metasurface.

High-efficiency all-silicon metasurfaces with 2π phase control based on multiple resonators

Flat all-silicon metasurfaces have significant potential as ultrathin optical devices and systems that offer the advantages of mature fabrication technology and complementary metal–oxidesemiconductor compatibility. However, previously reported all-silicon metasurfaces suffered from the drawback of low transmission owing to the large intrinsic reflection loss induced by high-index silicon substrates. To achieve high transmission, silicon-based metasurfaces are usually composed of low-index dielectric substrates and silicon pillars. It is still a substantial challenge for an all-silicon metasurface to realize high transmission. In this paper, we present a design to overcome this drawback by integrating two-dimensional periodic arrays of subwavelength silicon cylinders as perfect antireflection resonators on the bottom and top surfaces of the silicon substrate. Using this design, a uniform array of all-silicon unit cells comprising a metasurface can exhibit high transmission, up to 99.5%, at the target wavelength of 10.6 µm, which indicates a significant improvement over the design without the antireflection scheme. Furthermore, as an example of a metasurface application, an all-silicon polarization-insensitive metalens is built based on a series of all-silicon unit cells with high transmission and 2π phase control, which indicates subwavelength-focusing capability. The transmission of the entire metalens reached 81.9%, and the focusing efficiency was 71%. This study not only recommends a means to develop ultrathin all-silicon optical devices with high efficiency but also proposes a general approach to achieve high-quality metasurfaces based on a single high-index dielectric.

Cooperative optical wavefront engineering with atomic arrays

AbstractNatural materials typically interact weakly with the magnetic component of light which greatly limits their applications. This has led to the development of artificial metamaterials and metasurfaces. However, natural atoms, where only electric dipole transitions are relevant at optical frequencies, can cooperatively respond to light to form collective excitations with strong magnetic, as well as electric, interactions together with corresponding electric and magnetic mirror reflection properties. By combining the electric and magnetic collective degrees of freedom, we show that ultrathin planar arrays of atoms can be utilized as atomic lenses to focus light to subwavelength spots at the diffraction limit, to steer light at different angles allowing for optical sorting, and as converters between different angular momentum states. The method is based on coherently superposing induced electric and magnetic dipoles to engineer a quantum nanophotonic Huygens’ surface of atoms, giving full 2π phase control over the transmission, with close to zero reflection.

Efficient All-Dielectric Diatomic Metasurface for Linear Polarization Generation and 1-Bit Phase Control.

Optical metasurface has exhibited unprecedented capabilities in the regulation of light properties at a subwavelength scale. In particular, a multifunctional polarization metasurface making use of light polarization to integrate distinct functionalities on a single platform can be greatly helpful in the miniaturization of photonic systems and has become a hot research topic in recent years. Here, we propose and demonstrate an efficient all-dielectric diatomic metasurface, the unit cell of which is composed of a pair of a-Si:H-based nanodisks and nanopillars that play the roles as polarization-maintaining and polarization-converting meta-atoms, respectively. Through rigorous theoretical analyses and numerical simulations, we show that a properly designed diatomic metasurface can work as a nanoscale linear polarizer for generating linearly polarized light with a controllable polarization angle and superior performances including a maximum transmission efficiency of 96.2% and an extinction ratio of 32.8 dB at an operation wavelength of 690 nm. Three metasurface samples are fabricated and experimentally characterized to verify our claims and their potential applications. Furthermore, unlike previously reported dielectric diatomic metasurfaces which merely manipulate the polarization state, the proposed diatomic metasurface can be easily modified to empower 1-bit phase modulation without altering the polarization angle and sacrificing the transmission efficiency. This salient feature further leads to the demonstration of a metasurface beam splitter that can be equivalently seen as the integration of a nonpolarizing beam splitter and a linear polarizer, which has never been reported before. We envision that various metadevices equipping with distinct wavefront shaping functionalities can be realized by further optimizing the diatomic metasurface to achieve an entire 2π phase control.

Direct Optical Lithography of Colloidal Metal Oxide Nanomaterials for Diffractive Optical Elements with 2π Phase Control.

Spatially patterned dielectric materials are ubiquitous in electronic, photonic, and optoelectronic devices. These patterns are typically made by subtractive or additive approaches utilizing vapor-phase reagents. On the other hand, recent advances in solution-phase synthesis of oxide nanomaterials have unlocked a materials library with greater compositional, microstructural, and interfacial tunability. However, methods to pattern and integrate these nanomaterials in real-world devices are less established. In this work, we directly optically pattern oxide nanoparticles (NPs) by mixing them with photosensitive diazo-2-naphthol-4-sulfonic acid and irradiating with widely available 405 nm light. We demonstrate the direct optical lithography of ZrO2, TiO2, HfO2, and ITO NPs and investigate the chemical and physical changes responsible for this photoinduced decrease in solubility. Micron-thick layers of amorphous ZrO2 NPs were patterned with micron resolution and shown to allow 2π phase control of visible light. We also show multilayer patterning and use it to fabricate features with different thicknesses and distinct structural colors. Upon annealing at 400 °C, the deposited ZrO2 structures have excellent optical transparency across a wide wavelength range (0.3-10 μm), a high refractive index (n = 1.84 at 633 nm), and are optically smooth. We then fabricate diffractive optical elements, such as binary phase diffraction gratings, that show efficient diffractive behavior and good thermal stability. Different oxide NPs can also be mixed prior to patterning, providing a high level of material tunability. This work demonstrates a general patterning approach that harnesses the processability and diversity of colloidal oxide nanomaterials for use in photonic applications.

Broadband cross-slots fractal nano-antenna and its extraordinary optical transmission characteristics

To overcome the disadvantages of narrow frequency band and low transmittance for traditional nano-antenna, a nano-antenna structure based on cross-slots fractal was designed. The extraordinary optical transmission characteristics of the cross-slots fractal nano-antenna and the differences between the cross-slots fractal nano-antenna and the uniform cross-slots nano-antenna were analyzed by the finite difference time domain method. Meanwhile, the influence of physical parameters on the extraordinary optical transmission characteristics of the cross-slots fractal nano-antenna and the relationship of transmission spectrum of the nano-antenna between the fractal size and the non-fractal size were discussed. The results show that the fractal cross-slots structure is more miniaturized, and realizes extraordinary optical transmission and full 2π phase control of transmission beam, and the transmittance is higher than the uniform cross-slots structure, the full width at half maximum (FWHM) is wider, and the highest transmittance is up to 99.51%. By adjusting the physical parameters, the transmission spectrum exhibits red-shift or blue-shift characteristics, achieving controllability of the transmission spectrum. When h=50 nm, the full width at half maximum is about 356 nm, and the transmittance is still as high as 95.66%, which is generally higher than traditional structures, and the peak transmittance is still greater than 74% at large incident angles (70 degrees). In short, the cross-slots fractal nano-antenna has the characteristics of wide frequency, controllable and adjustable, and more miniaturized structure compared with other nano-antenna structures, and realizes extraordinary optical transmission.

High-Efficiency Metasurfaces with 2π Phase Control Based on Aperiodic Dielectric Nanoarrays.

In this study, the high-efficiency phase control Si metasurfaces are investigated based on aperiodic nanoarrays unlike widely-used period structures, the aperiodicity of which providing additional freedom to improve metasurfaces’ performance. Firstly, the phase control mechanism of Huygens nanoblocks is demonstrated, particularly the internal electromagnetic resonances and the manipulation of effective electrical/magnetic polarizabilities. Then, a group of high-transmission Si nanoblocks with 2π phase control is sought by sweeping the geometrical parameters. Finally, several metasurfaces, such as grating and parabolic lens, are numerically realized by the nanostructures with high efficiency. The conversion efficiency of the grating reaches 80%, and the focusing conversion efficiency of the metalens is 99.3%. The results show that the high-efficiency phase control metasurfaces can be realized based on aperiodic nanoarrays, i.e., additional design freedom.

Phase engineering with all-dielectric metasurfaces for focused-optical-vortex (FOV) beams with high cross-polarization efficiency

Metasurfaces, the two-dimensional (2D) metamaterials, facilitate the implementation of abrupt phase discontinuities using an array of ultrathin and subwavelength features. These metasurfaces are considered as one of the propitious candidates for realization and development of miniaturized, surface-confined, and flat optical devices. This is because of their unprecedented capabilities to engineer the wavefronts of electromagnetic waves in reflection or transmission mode. The transmission-type metasurfaces are indispensable as the majority of optical devices operate in transmission mode. Along with other innovative applications, previous research has shown that Optical-Vortex (OV) generators based on transmission-type plasmonic metasurfaces overcome the limitations imposed by conventional OV generators. However, significant ohmic losses and the strong dispersion hampered the performance and their integration with state-of-the-art technologies. Therefore, a high contrast all-dielectric metasurface provides a compact and versatile platform to realize the OV generation. The design of this type of metasurfaces relies on the concept of Pancharatnam-Berry (PB) phase aiming to achieve a complete 2π phase control of a spin-inverted transmitted wave. Here, in this paper, we present an ultrathin, highly efficient, all-dielectric metasurface comprising nano-structured silicon on a quartz substrate. With the help of a parameter-sweep optimization, a nanoscale spatial resolution is achieved with a cross-polarized transmission efficiency as high as 95.6% at an operational wavelength of 1.55 µm. Significantly high cross-polarized transmission efficiency has been achieved due to the excitation of electric quadrupole resonances with a very high magnitude. The highly efficient control over the phase has enabled a riveting optical phenomenon. Specifically, the phase profiles of two distinct optical devices, a lens and Spiral-Phase-Plate (SPP), can be merged together, thus producing a highly Focused-Optical-Vortex (FOV) with a maximum focusing efficiency of 75.3%.

Tunable fiber-tip lens based on thermo-optic effect of amorphous silicon

We theoretically demonstrate a compact all-dielectric metasurface fiber-tip lens composed of sub-wavelength amorphous silicon on the end face of a multimode fiber. The full 2π phase control was realized by varying the widths of resonant units. The tunable focal length is achieved by using the thermal-optic effect of amorphous silicon. The focal length increases from 309 μm to 407 μm when the temperature changes by 300 K. The temperature controlled all-fiber integrated lens is compact and with high efficiency and provides an excellent platform of a fiber-tip lab. Meanwhile, the proposed fiber lens does not have any structural changes during dynamic tuning, which improves the durability and repeatability of the devices.

Analysis of polarization-dependent continuous 2π phase control mechanism for trapezoidal nano-antennas through multipole expansion method

In order to explore the polarization-dependent continuous 2π phase control mechanism of nano-antennas and to provide theoretical bases for the subsequent design of polarization beam splitters, unidirectional radiation antennas, and other devices, we design a trapezoidal nano-antenna structure with high transmission efficiency and polarization-dependent continuous 2π phase variation under the wavelengths of 600–1000 nm. Then we analyze the relationship between phase control and the excited electric/magnetic dipoles and quadrupoles under different linearly polarized incident light beams. The results show that, in the y -direction, the antenna mainly realizes polarization-dependent continuous 2π phase control at the wavelengths where the first Kerker’s condition is satisfied. As the antenna’s unidirectional angular radiation is mainly achieved by compensating the wavefront phase, based on the above analysis, the relationship between the maximum radiation angle of unidirectional angular radiation and the excited electric/magnetic dipoles and quadrupoles in the antenna is further analyzed. Finally, we also briefly consider the influence of the structural sizes of the designed antenna on the resonances and coupling of the excited electric/magnetic dipoles and quadrupoles under different linearly polarized incident light beams for future applications.

Mechanisms of 2π phase control in dielectric metasurface and transmission enhancement effect

Metasurfaces, two-dimensional structures composed of nanoantennas in an array configuration, can be used to fully control electromagnetic waves, which requires a 2π phase shift. Herein, we apply the silicon metasurface as an example to interpret the mechanisms of full 2π phase coverage. It is found that the mechanism varies from Fabry-Pérot resonance to Mie resonance as the period increases for a metasurface with certain height. Particularly, there is a transition region between these two types of resonance. We present the corresponding periods and wavelength regions of the different mechanisms when considering the phase-gradient metasurface with at most three diffraction orders. Moreover, the transmission enhancement of metasurface is investigated. The transmission efficiency can be effectively improved when the nanoantenna is changed from a uniform structure to a gradient-index one. We expect that the results can simplify the design process and provide a reference for the future design of all-dielectric metasurface with 2π phase control.

Temporal color mixing and dynamic beam shaping with silicon metasurfaces.

Metasurfaces offer the possibility to shape optical wavefronts with an ultracompact, planar form factor. However, most metasurfaces are static, and their optical functions are fixed after the fabrication process. Many modern optical systems require dynamic manipulation of light, and this is now driving the development of electrically reconfigurable metasurfaces. We can realize metasurfaces with fast (>105 hertz), electrically tunable pixels that offer complete (0- to 2π) phase control and large amplitude modulation of scattered waves through the microelectromechanical movement of silicon antenna arrays created in standard silicon-on-insulator technology. Our approach can be used to realize a platform technology that enables low-voltage operation of pixels for temporal color mixing and continuous, dynamic beam steering and light focusing.

Lead Halide Perovskite‐Based Dynamic Metasurfaces

AbstractLead halide perovskites CH3NH3PbX3 (MAPbX3) are known to have high refractive index and controllable bandgap, making them attractive for all‐dielectric and tunable metasurfaces. Till now, perovskite metasurfaces have only been used in structural colors. More interesting meta‐devices with 2π phase control are still absent. Here, MAPbX3‐perovskite‐based metasurfaces are experimentally demonstrated with a complete control of phase shift in a reflection mode. By utilizing MAPbBr3 cut‐wires as meta‐atoms on a ground metal film, it is found that the MAPbBr3 perovskite metasurface can produce full phase control from 0 to 2π and high reflection efficiency simultaneously. Consequently, high‐efficiency polarization conversion, anomalous reflection, and meta‐holograms are successfully produced. Most interestingly, the bandgap of the MAPbX3 perovskite can be post‐synthetically and reversibly tuned via anion exchange, providing a new approach to dynamically control of the all‐dielectric meta‐devices with novel function such as anomalous reflection and hologram.