Metasurface Device Research Articles

Photoswitchable Anapole Metasurfaces

AbstractNonradiating charge‐current configurations have attracted attention in photonics for the efficient localization of the electromagnetic field. Anapole mode is a unique nonradiating state of light induced by the interference of electric and toroidal dipole that possesses rich physics with potential applications in micro‐nanophotonics. Active control of an anapole is essential for the design and realization of tunable low‐energy photonic devices. Here, an active anapole metasurface device is experimentally demonstrated as a switch for the terahertz waves. The metadevice consists of planar resonators with photoactive inclusions of silicon patches in a hybrid metal–semiconductor configuration. The active element enables dynamic control over the contributions of the multipoles that eventually determine the formation of the exotic anapoles that host extreme nonradiative confinement and its active switching into sub‐radiative Fano resonance and highly radiative electric dipoles. Two orders of magnitude change in the near‐field intensity of the anapole that leads to 201% extinction modulation is further demonstrated. The anapole metadevice provides a platform to efficiently control both the far‐field radiation and near‐field enhancement in metaoptics, promoting active micro‐nanophotonic devices for potential applications in terahertz modulators, lasers, filters, and dynamic near‐field imaging.

High performance reflective microwave split-square-ring metasurface vortex beam generator

The creation of vortex beams becomes increasingly important in microwave region for their potential applications in high-capacity multiplexed wireless communication systems. In this paper, we propose a kind of split-square-ring metasurface for generating microwave vortex beams at 16 GHz. We first analyze the S-parameter of the meta-atoms and then optimize and pick up 8 units which can cover 2π phase shift for the whole array design. Four metasurfaces carrying topological charges from 1 to 4 are, respectively, designed, and full-wave simulation is utilized for evaluating the near-field performance. The orbital angular momentum purity for each topological charge is calculated. Meanwhile, the far-field property has also been simulated and measured, and the simulation results agree well with those in experiments. Our metasurface device provides a flexible way for generating high-quality vortex beams in the microwave region.

Design and Numerical Analysis of an Infrared Cassegrain Telescope Based on Reflective Metasurfaces.

Reflective imaging systems such as Cassegrain-type telescopes are widely utilized in astronomical observations. However, curved mirrors in traditional Cassegrain telescopes unavoidably make the imaging system bulky and costly. Recent developments in the field of metasurfaces provide an alternative way to construct optical systems, possessing the potential to make the whole system flat, compact and lightweight. In this work, we propose a design for a miniaturized Cassegrain telescope by replacing the curved primary and secondary mirrors with flat and ultrathin metasurfaces. The meta-atoms, consisting of SiO2 stripes on an Al film, provide high reflectance (>95%) and a complete phase coverage of 0~2π at the operational wavelength of 4 μm. The optical functionality of the metasurface Cassegrain telescope built with these meta-atoms was confirmed and studied with numerical simulations. Moreover, fabrication errors were mimicked by introducing random width errors to each meta-atom; their influence on the optical performance of the metasurface device was studied numerically. The concept of the metasurface Cassegrain telescope operating in the infrared wavelength range can be extended to terahertz (THz), microwave and even radio frequencies for real-world applications, where metasurfaces with a large aperture size are more easily obtained.

Independent Multichannel Wavefront Modulation for Angle Multiplexed Meta‐Holograms

AbstractMeta‐holograms have been investigated widely because of their capabilities that are not feasible in conventional holograms, such as full‐color, broadband, and bifacial image generation performance. In particular, a high degree of freedom in designing meta‐atoms provides opportunities to multiplex holograms by using light properties, such as angle, polarization, and wavelength. However, further development of techniques for meta‐holograms recording multiple wavefronts is still necessary to expand the amount of optical information and improve the security level of holographic encryption. In this study, detour phase holograms are combined with the spatial multiplexing method to record four phase profiles in a single metasurface device. Independent wavefront modulation, which can process four channels as a bijective function, is realized by engineering the scattering properties of the meta‐atom. Based on the concept, meta‐holograms are demonstrated experimentally, independently generating four different images depending on the incident angles with high fidelity. Because of the expanded capacity and independence, the proposed device provides a potential platform for optical devices in various fields, such as holography, multifunctional flat optics, and off‐axis optical devices.

Phase Manipulation‐Based Polarization Profile Realization and Hybrid Holograms Using Geometric Metasurface

The outstanding ability of optical metasurfaces to tailor light's amplitude, phase, and polarization with a subwavelength resolution has attracted big interest. A multifunctional metasurface device is reported and experimentally demonstrated that integrates an arbitrary polarization profile for image concealment and a computer‐generated hybrid hologram. Herein, a high‐resolution image is hidden in the polarization profile of the light beam, while the holographic images are encoded in its phase profile for the reconstruction of two different types of holographic images. The designed polarization profile is realized based on the superposition of two different phase profiles for circular polarization states with opposite handedness. This method can further extend the design degree of freedom in optical metasurfaces, which may open an alternative avenue for anticounterfeiting, optical image encoding, and optical information security.

Near-infrared plasma cavity metasurface with independently tunable double Fano resonances

We report a metasurface device based on nano metal-dielectric-metal plasmon cavity arrays. Investigation of the proposed device indicates the sharp absorption peak of double Fano resonance and multiple periods of time-domain oscillations caused by the three-dimensional double resonance. By adjusting the structure parameters, different absorption patterns can be regulated independently with high degree of freedom. The metasurface exhibits sensitive behavior to the environmental refractive index and possesses the sensitivity of 667 nm RIU−1 as a refractive index sensor in the mid-near infrared band, which can be further considered for glucose or temperature sensing.

Active control of terahertz waves based on p-Si hybrid PIT metasurface device under avalanche breakdown.

Active control of terahertz waves is a critical application for terahertz devices. Silicon is widely used in large-scale integrated circuit and optoelectronic devices, and also shows great potential in the terahertz field. In this paper, a p-Si hybrid metasurface device is proposed and its terahertz characteristics under avalanche breakdown effect is investigated. In the study, a plasmon-induced transparency (PIT) effect caused by the near-field coupling of the bright mode and the dark mode is observed in the transmission spectrum. Due to avalanche breakdown effect, the resonance of the PIT metamaterial disappears as the current increased. Carriers existed in the interface between the metasurface and substrate result to a dipole resonance suppression. When the current continues increasing, the maximal modulation depth can reach up to 99.9%, caused by the avalanche effect of p-Si. Experimental results demonstrate that the avalanche breakdown p-Si can achieve a performance modulation depth, bringing much more possibilities for terahertz devices.

Light Spin Angular Momentum Spatial Mode Converter Based on Dielectric Metasurface

In recent years, the concept of metasurface has brought impact on the way of manipulating light using conventional wavefront shaping components by both reducing the device size and increasing the design flexibility. In this work, a spatial mode converter based on dielectric metasurfaces for spin angular momentum is proposed. The structure parameters of dielectric nanobricks are designed by spatial phase distribution derivation based on the geometric phase and the propagation phase. It is shown that the mode converter can transfer the fundamental spatial mode of spin angular momentum including circular or elliptical polarizations into higher order spatial modes. The generated higher order modes of spin angular momentum contribute a new degree of freedom for light modulation besides the handedness of polarization. The metasurface device can reduce the complexity of the existing mode-division multiplexing systems for spin angular momentum of light, and meanwhile exhibit a high efficiency, which may find applications in optical communication, complex structured beam, and quantum optics.

Large-scale, power-efficient Au/VO2 active metasurfaces for ultrafast optical modulation

Abstract Active metasurfaces, in which the optical property of a metasurface device can be controlled by external stimuli, have attracted great research interest recently. For optical switching and modulation applications, high-performance active metasurfaces need to show high transparency, high power efficiency, as well as ultrafast switching and large-scale fabrication capability. This paper reports Au/VO2-based active metasurfaces meeting the requirements above. Centimeter-scale Au/VO2 metasurfaces are fabricated by polystyrene sphere colloidal crystal self-assembly. The devices show optical modulation on-off ratio up to 12.7 dB and insertion loss down to 3.3 dB at 2200 nm wavelength in the static heating experiment, and ΔT/T of 10% in ultrafast pump-probe experiments. In particular, by judiciously aligning the surface plasmon resonance wavelength to the pump wavelength of the femtosecond laser, the enhanced electric field at 800 nm is capable to switch off the extraordinary optical transmission effect at 2200 nm in 100 fs time scale. Compared to VO2 thin-film samples, the devices also show 50% power reduction for all-optical modulation. Our work provides a practical way to fabricate large-scale and power-efficient active metasurfaces for ultrafast optical modulation.

Design and verify an electrically modulated metasurface

In recent years, tunable terahertz-band plasmon-induced transparency (PIT) metasurface devices have become a research hotspot in the field of nanophotonics. This kind of metasurface device shows extremely high application value in the fields of slow-light devices or optical sensors. Here, a tunable metasurface is designed and experimentally verified in the terahertz band. Two transmission peaks (P1, P2) are achieved at resonance frequencies 2.74 THz and 3.65 THz, which are excited by the coupling of lattice modes and the PIT effect, respectively. Since the periodic mode is an inherent property of all periodic structures, in the experiment, the P1 transmission peak can be obviously strengthened by changing the period of the structure. Moreover, the dielectric properties of KTaO3 (KTO) layer shows voltage tunability. In the experiment, the transmission performance of the metasurface can be modulated by changing the external voltage conditions.

Design of beam shaping and focusing metasurface device based on G-S algorithm

We demonstrate a sub-diffraction flat-top beam shaping device based on sub-wavelength square silicon pillar array at operating wavelength of 10.6 μm, followed with improved Gerchberg-Saxton (G-S) algorithm to enable the advanced shaping processing capability. The theoretical simulation results show that the device generates a sub-diffraction flat-top optical needle with length of 10.6 cm along the optic axis, and its diffraction efficiency is higher than 90%. Moreover, the focal spot, with a transverse size of 3.5 λ, is smaller than the diffraction limit of 6.8 λ. This sub-diffraction beam shaping device with large focal depth has practical applications in laser machining technologies.

Electrically-controlled digital metasurface device for light projection displays

Light projection displays play an increasingly important role in our modern life. Core projection systems including liquid crystal displays and digital micromirror devices can impose spatial light modulation and actively shape light waves. Recently, the advent of metasurfaces has revolutionized design concepts in nanophotonics, enabling a new family of optical elements with exceptional degrees of freedom. Here, we demonstrate a light projection display technology based on optical metasurfaces, called digital metasurface device (DMSD). Each metasurface pixel in a DMSD is electrically reconfigurable with well-controlled programmability and addressability. The DMSD can not only continuously modulate the intensity of light with high contrast, but also shape the wavefront of light generated by each metasurface pixel and dynamically switch between arbitrary holographic patterns. Our approach will pave an avenue towards the development of customized light projection devices. It will also drive the field of dynamic optical metasurfaces with fresh momentum and inspiring concepts.

Free-Standing Complementary Asymmetric Metasurface for Terahertz Sensing Applications.

We designed and tested a highly sensitive metasurface device based on free-standing complementary asymmetric split-ring resonators at terahertz frequencies. It is utilized for sensing a galactose film. We characterized the device using the induced red shift of a Fano resonance observed in the THz transmission. The sensor has a high sensitivity of 91.7 GHz/RIU due to a significant interaction between the galactose overlayer and the metasurface.

Magnesium-Based Metasurfaces for Dual-Function Switching between Dynamic Holography and Dynamic Color Display.

Metasurface-based color display and holography have greatly advanced the state of the art display technologies. To further enrich the metasurface functionalities, recently a lot of research endeavors have been made to combine these two display functions within a single device. However, so far such metasurfaces have remained static and lack tunability once the devices are fabricated. In this work, we demonstrate a dynamic dual-function metasurface device at visible frequencies. It allows for switching between dynamic holography and dynamic color display, taking advantage of the reversible phase transition of magnesium through hydrogenation and dehydrogenation. Spatially arranged stepwise nanocavity pixels are employed to accurately control the amplitude and phase of light, enabling the generation of high-quality color prints and holograms. Our work represents a paradigm toward compact and multifunctional optical elements for future display technologies.

Mid- to long-wave infrared computational spectroscopy with a graphene metasurface modulator

In recent years there has been much interest concerning the development of modulators in the mid- to long-wave infrared, based on emerging materials such as graphene. These have been frequently pursued for optical communications, though also for other specialized applications such as infrared scene projectors. Here we investigate a new application for graphene modulators in the mid- to long-wave infrared. We demonstrate, for the first time, computational spectroscopy in the mid- to long-wave infrared using a graphene-based metasurface modulator. Furthermore, our metasurface device operates at low gate voltage. To demonstrate computational spectroscopy, we provide our algorithm with the measured reflection spectra of the modulator at different gate voltages. We also provide it with the measured reflected light power as a function of the gate voltage. The algorithm then estimates the input spectrum. We show that the reconstructed spectrum is in good agreement with that measured directly by a Fourier transform infrared spectrometer, with a normalized mean-absolute-error (NMAE) of 0.021.

Switching the Optical Chirality in Magnetoplasmonic Metasurfaces Using Applied Magnetic Fields.

Chiral nanophotonic devices are promising candidates for chiral molecule sensing, polarization of diverse nanophotonics, and display technologies. Active chiral nanophotonic devices, where the optical chirality can be controlled by an external stimulus has triggered great research interest. However, efficient modulation of the optical chirality has been challenging. Here, we demonstrate switching of the extrinsic chirality by applied magnetic fields in a magnetoplasmonic metasurface device based on a magneto-optical oxide material, Ce1Y2Fe5O12 (Ce:YIG). Due to the low optical loss and strong magneto-optical effect of Ce:YIG, we experimentally demonstrated giant and continuous far-field circular dichroism (CD) modulation by applied magnetic fields from -0.6 ± 0.2° to +1.9 ± 0.1° at 950 nm wavelength under glancing incident conditions. The far-field CD modulation is due to both magneto-optical circular dichroism and near-field modulation of the superchiral fields by applied magnetic fields. Finally, we demonstrate magnetic-field-tunable chiral imaging in millimeter-scale magnetoplasmonic metasurfaces fabricated using self-assembly.

Tying Polarization‐Switchable Optical Vortex Knots and Links via Holographic All‐Dielectric Metasurfaces

AbstractTailoring the spatial structure of light field in multiple degrees of freedom is a research hotspot in recent years. The topology of light field, as one of the most fascinating structures, such as vortex knots and links associated with the phase singularities, is evoking increasing attention both in fundamental research and practical application. Here, an all‐dielectric metasurface device is proposed and experimentally demonstrated that can construct vortex knots and links in light field at the micro scale. These two distinct topological configurations can be switched by changing the incident polarization. Utilizing the digital holographic interference method, these topological configurations of vorticity lines are accurately characterized in three‐dimensions, and the topology‐preserving evolution of such ultra‐small fields is demonstrated. These two classical configurations exemplify the capability of multichannel manipulating topology by compact metadevice. The work may promote the application of structured light filed at the micro scale and even the creation of other physical fields with ultra‐small size.

A Deep Learning Approach for Objective-Driven All-Dielectric Metasurface Design

Metasurfaces have become a promising means for manipulating optical wavefronts in flat and high-performance optical devices. Conventional metasurface device design relies on trial-and-error methods to obtain target electromagnetic (EM) responses, an approach that demands significant efforts to investigate the enormous number of possible meta-atom structures. In this paper, a deep learning modeling approach is introduced that significantly improves on both speed and accuracy compared to techniques currently used to characterize the subwavelength optical structures. Our neural network approach overcomes two key challenges that have limited previous neural-network-based design schemes: input/output vector dimensional mismatch and accurate EM-wave phase prediction. Additionally, this is the first neural network to characterize 3-D dielectric structures. By combining with optimization algorithms or neural networks, this approach can be generically applied to a wide variety of metasurface device designs across the entire electromagnetic spectrum. Using this new methodology, examples of neural networks capable of producing on-demand designs for meta-atoms, metasurface filters, and phase-change reconfigurable metasurfaces are demonstrated.

3D-Integrated metasurfaces for full-colour holography

Metasurfaces enable the design of optical elements by engineering the wavefront of light at the subwavelength scale. Due to their ultrathin and compact characteristics, metasurfaces possess great potential to integrate multiple functions in optoelectronic systems for optical device miniaturisation. However, current research based on multiplexing in the 2D plane has not fully utilised the capabilities of metasurfaces for multi-tasking applications. Here, we demonstrate a 3D-integrated metasurface device by stacking a hologram metasurface on a monolithic Fabry–Pérot cavity-based colour filter microarray to simultaneously achieve low-crosstalk, polarisation-independent, high-efficiency, full-colour holography, and microprint. The dual functions of the device outline a novel scheme for data recording, security encryption, colour displays, and information processing. Our 3D integration concept can be extended to achieve multi-tasking flat optical systems by including a variety of functional metasurface layers, such as polarizers, metalenses, and others.

Ultrasmall Optical Vortex Knots Generated by Spin‐Selective Metasurface Holograms

AbstractTying isolated vortex knots in complex optical fields has been recently realized using spatial light modulators (SLMs). These traditional devices suffer from low efficiency, narrow bandwidth, and they cannot be integrated to nanophotonic systems. Here, the design and realization of metasurface holograms (MHs) is reported to efficiently generate the optical vortex knots in a broad spectral region. In comparison with conventional optical devices, the thicknesses and pixels of MHs are both in subwavelength scale, and the size of created optical vortex knot can be six orders of magnitude smaller than those produced by SLMs. Such extremely little vortex knots may find widespread applications in templating knotted micro/nano‐structures in photosensitive materials and 3D optical trapping of cold atoms in complex topologies at microscale. In particular, it is found that the spatial position of the generated off‐axis optical vortex knot is able to be symmetrically reversed by just changing the helicity of the incident light. This spin‐selective vortex knot generator can be regarded as a new type of metasurface device for manipulating the topological structure of optical vortex lines, which has potential applications in the field of singular optics.