Metasurface Modulation Research Articles

Flexible switchable mid-infrared metalens optical tweezer based on VO2

Metasurface has developed rapidly since its advent because of its powerful control over electromagnetic waves, but most traditional metasurface can only passively realize a single fixed function, which limits its application and development in integrated systems. To modulate electromagnetic waves more flexibly and efficiently, here, we first propose what we believe to be a novel scheme to design a switchable metalens by utilizing the phase change materials VO2 and double-layer metasurface modulation. The metalens designed by the proposed scheme can achieve flexible conversion between the transmitted focusing and reflected focusing through changing the phase state of VO2. Then, we investigate the optical force phenomenon of these metalenses, the simulation results indicating that the proposed switchable metalens can achieve stable particle manipulation under both the transmission and reflection modes. This makes it a promising device in flexible optical manipulation, and this reversible tuning will also show significant application potentials in biology, medicine, optical communication and other fields.

Multifunctional metasurface for ultrafast all-optical efficient modulation of terahertz wave

Metasurfaces have been widely used to terahertz (THz) modulators due to their unique electromagnetic (EM) modulation properties. However, to realize ultrafast and efficient control of the active metasurfaces remains a critical challenge. Herein, we first demonstrate a multifunctional metasurface modulator based on metal square rings, which have central rotational symmetry with local asymmetry. The former captures more EM energy to achieve strong enhancement of the local EM responses, and the latter generates high quality factor (Q-factor) Fano resonance. By scaling the unit structure, the resonant frequency shifts in an ultra-wideband range of ∼0.29-10 THz, which shows a linear relation between the Q-factor and the scaling factor, and an exponential variation between the full width at half maximum (FWHM) and the scaling factor. The results provide an effective approach to predict the frequency selection characteristics of the modulators in the THz regime. More importantly, by embedding the silicon into the specific position of the metasurface, external optical control results in the unit structure is constructed from meta-atom to molecularization model, the shifting of resonant frequency and the multi-level modulation of amplitude are realized, and the maximum modulation depth (MD) reaches ∼100 %. By systematically studying the dynamics of frequency and amplitude regulation, benefiting from the ultra-short relaxation time of the photo-carriers in silicon bridge, an ultrafast full recovery time of 2000 ps is achieved. Combined with the local field characteristics, the scaled unit structure achieves efficient modulation of the resonant frequency and amplitude under optical pumping, which provides a promising approach for the development of active metasurface modulator suitable for the whole THz band. This work demonstrates the viability of optical control molecularization of specific meta-atoms is applicable to the entire THz band, which has great potential use for future state-of-the-art THz modulators.

High-speed metasurface modulator using perfectly absorptive bimodal plasmonic resonance

Free-space electro-optic (EO) modulators operating at gigahertz and beyond are attractive for a wide range of emerging applications, including high-speed imaging, free-space optical communication, microwave photonics, and diffractive computing. Here, we experimentally demonstrate a high-speed plasmonic metasurface EO modulator operating in a near-infrared wavelength range with a gigahertz modulation bandwidth. To achieve efficient intensity modulation of reflected light from an ultrathin metasurface layer, we utilize the bimodal plasmonic resonance inside a subwavelength metal–insulator–metal grating, which is precisely tuned to satisfy the critical coupling condition. As a result, perfect absorption of −27 dB (99.8%) and a high quality (Q) factor of 113 are obtained at a resonant wavelength of 1650 nm. By incorporating an EO polymer inside the grating, we achieve a modulation depth of up to 9.5 dB under an applied voltage of ±30 V. The 3-dB modulation bandwidth is confirmed to be 1.25 GHz, which is primarily limited by the undesired contact resistance and the output impedance of the driver. Owing to the high electrical conductivity of metallic gratings and a compact device structure with a minimal parasitic capacitance, the demonstrated device can potentially operate at several tens of gigahertz, which opens up exciting opportunities for ultrahigh-speed active metasurface devices in various applications.

Metasurface modulator of frequency diverse multi-mode OAM beams with equal divergence angle for coincidence imaging

Traditional multi-mode orbital angular momentum (OAM) beams suffer from low energy efficiency in detection since the divergence angle tends to increase with increasing order of the OAM mode. Thus, OAM beams with equal divergence angle are highly desirable. In this paper, a frequency-diverse multi-mode (FDMM) OAM modulator with approximately equal divergence angle is proposed. The modulator is composed of an OAM modulation metasurface and a spatial filter (SF) with narrow passband. Firstly, the OAM modulation metasurface is designed based on multiple ring-like regions with different radii in the metasurface aperture. Each mode of OAM beam is generated at a corresponding frequency point in one region. Thus, the metasurface aperture can generate different modes of OAM beams at different frequencies (i.e., FDMM OAM beams) simultaneously. Then, the SF with narrow passband is proposed through synthesizing multi-layer frequency selective metasurfaces, aiming to reduce interferences between different modes of OAM beams. Finally, the FDMM OAM modulator is realized by superposing the SF on the OAM modulation metasurface. The FDMM OAM modulator can generate six modes of OAM beams (i.e., , and modes) at 12.5 GHz, 13.0 GHz and 13.6 GHz, respectively. A multi-mode OAM coincidence imaging model based on the FDMM OAM modulator is established. The simulation and experiment results show that the proposed FDMM OAM modulator can be used for target reconstruction and the coincidence imaging performance can be improved assisted by an optimization algorithm.

Terahertz single pixel imaging with frequency-multiplexed metasurface modulation

Single-pixel imaging is a very attractive technique at the terahertz band due to the lack of mature array detectors, over which the spatial light modulator is the most crucial component. Metasurfaces are usually introduced into the modulator to enhance the modulation depth of active materials. Here we take advantage of the flexible spectral response of pixelated metasurface resonators to encode different spatial masks into different frequency windows, without active materials and external stimuli. The measurements are obtained in parallel as opposed to in sequence in the time-domain spectroscopy system through a single terahertz signal. Images with 2 × 2 pixels as proof of the concept are reconstructed with high fidelity and plenty of choices of the frequency windows. As the measurement time does not increase with the number of pixels, this scheme can be generalized for imaging with higher resolution and increased number of pixels for practical applications in noninvasive inspection and detection.

Tunable metasurface for independent controlling radar stealth properties via geometric and propagation phase modulation.

Metasurfaces have been verified as an ideal way to control electromagnetic waves within an optically thin interface. In this paper, a design method of a tunable metasurface integrated with vanadium dioxide (VO2) is proposed to realize independent control of geometric and propagation phase modulation. The reversible conversion of VO2 between insulator phase and metal phase can be realized by controlling the ambient temperature, which enables the metasurface to be switched quickly between split-ring and double-ring structures. The phase characteristics of 2-bit coding units and the electromagnetic scattering characteristics of arrays composed of different arrangements are analyzed in detail, which confirms the independence of geometric and propagation phase modulation in the tunable metasurface. The experimental results demonstrate that the fabricated regular array and random array samples have different broadband low reflection frequency bands before and after the phase transition of VO2, and the 10 dB reflectivity reduction bands can be switched quickly between C/X and Ku bands, which are in good agreement with the numerical simulation. This method realizes the switching function of metasurface modulation mode by controlling the ambient temperature, which provides a flexible and feasible idea for the design and fabrication of stealth metasurfaces.

Binary THz modulator based on silicon Schottky-metasurface

We propose a metasurface THz modulator based on split-ring resonators (SRRs) formed by four interconnected horizontal Si–Au Schottky diodes. The equivalent junction capacitance of each SRR in the proposed modulator is much smaller than that of the previously reported metasurface counterparts with vertical Schottky junctions, leading to a higher modulation speed. To modulate a THz incident signal by the proposed metasurface, we vary the bias voltage externally applied to the Schottky junctions. Applying a reverse bias of VA = − 5 V to the Au gate, two LC resonances at 0.48 THz, and 0.95 THz are excited in the metasurface. Switching the applied voltage to VA = + 0.49 V, we diminish the oscillator strengths of the LC resonances, creating one dipole resonance at 0.73 THz in the transmission spectrum of the metasurface modulator. The modulation depths at these resonances are more than 45%, reaching 87% at 0.95 THz. The phase modulation for this THz modulator is about 1.12 rad at 0.86 THz. Furthermore, due to the particular design of the meta-atoms, the modulation speed of this device is estimated up to approximately several hundred GHz, which makes this device an appropriate candidate for high-speed applications in wireless communications systems based on external modulators.

Multispectral higher-order Fano resonant metasurface based on periodic twisted DNA-like split ring arrays with three modulation methods.

The active modulation of the Fano resonance is rare but desirable. However, recent studies mostly focused on a single modulation method and few reported the use of three photoelectric control methods. A tunable graphene DNA-like metamaterial modulator with multispectral Fano resonance is demonstrated. In experimentally fabricated metamaterials with six photoelectric joint modulation patterns, each joint shows different optoelectrical response characteristics. Ultrahigh modulation depth (MD) up to 982% was achieved at 1.5734 THz with a 1.040 A external laser pump by involving combined optoelectrical methods. These results show that the metasurface modulator is a promising platform for higher-order Fano resonance modulation and communication fields.

Physical insight into the terahertz polarization modulator by using Smith chart

Polarization modulation (PoM) provides an additional degree of freedom for the modulation of carrier waves and may be an alternative to conventional wireless communication techniques that use amplitude, frequency, and phase modulations. In this study, we proposed a metasurface-based modulator that transforms a linearly polarized wave into a circularly polarized wave in the terahertz range of 1.53 THz–1.57 THz. The polarization manipulation of the metasurface is analyzed using the Smith chart method, which builds a bridge between the metasurface pattern and its equivalent circuit impedance. We show the reflection/transmission coefficient and impedance of the metasurface in the Smith chart, providing clear physical images for the designers. According to the Smith chart analysis, we obtained a two-layer metasurface modulator with 90% efficiency for the transmitted field. All the results are validated from the simulation. The proposed Smith chart method opens a new design methodology of metasurface-based devices.

Design and theoretical characterization of high speed metasurface modulators based on electro-optic polymer.

Metasurfaces have attracted extraordinary interest in achieving novel, ultrathin and compact photonic devices. To date, however, the realization of electrically tunable high-speed metasurfaces remains a great challenge. In this work, we present an electro-optic (EO) polymer/silicon hybrid metasurface modulator with an estimated 3dB modulation bandwidth up to 118 GHz. The specially designed metasurface utilizes a broken in-plane inversion symmetry structure to generate a high-Q resonance. The high-Q property enhances the EO modulation effect, so that a 16 dB extinction ratio is theoretically verified under a driving voltage Vp-p of 4V. The pulse modulation results in an ultra-fast single-lane data rates up to 300 Gbps driven by a low RF power. The presented modulator should be applicable for high-speed and low-energy intelligent tunable metasurface, space optical communication and so on.

Dual-polarization programmable metasurface modulator for near-field information encoding and transmission

Controlling the polarization state of electromagnetic waves is an important topic in microwaves due to the enormous application potential in radar technology and mobile communications. Here, we propose a programmable metasurface based on single-pole double-throw switches to realize multifunctional polarization conversions. A structure of the double-sided metallic pattern is adopted in the metasurface, in which a novel double-pole double-throw hub is achieved to guide the energy direction. Such a mechanism successfully induces multiple transmission channels into the metasurface structure for functional design. By controlling the states of the switches with a field programmable gate array, the x- and y-polarizations of the incident waves can be efficiently modulated into linear co- and cross-polarizations of transmitted waves, suggesting a higher degree of freedom on wave manipulations. The proposed metasurface can be developed as a near-field information encoder to transmit binary coding sequence according to the energy distribution. Character transmissions are realized by programming binary ASCII codes on the transmitted fields. Nine supercells on the metasurface can encode 9-bit binary information in one frame of near-field imaging, which can be switched in real time with high speed. We envision that this work will develop digital coding applications to control the polarization information.

Spread-Spectrum Selective Camouflaging Based on Time-Modulated Metasurface

This article presents the concept of spread-spectrum selective camouflaging based on time-modulated metasurface. The spectrum spreading is realized by switching the metasurface between the reflective states of a perfect electric conductor (PEC) mirror and a perfect magnetic conductor (PMC) mirror, using an array of microstrip patches connected to the ground via diode switches, according to a periodic pseudorandom noise sequence. As the spectrum spreading induces a drastic reduction of the power spectral density of the signal, the level of the scattered wave falls below the noise floor of the interrogating radar, and the object covered by the metasurface is hence perfectly camouflaged to a foe radar. Moreover, the object can be detected by a friend radar possessing the spread-spectrum demodulation key corresponding to the metasurface modulation, and this detection is robust to interfering signals. The proposed system is analyzed theoretically and demonstrated by both simulation and experimental results.

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.

Gate tunable graphene-integrated metasurface modulator for mid-infrared beam steering.

The ability to integrate graphene into metasurface devices has attracted enormous interest as a means of achieving dynamic electrical control of their electromagnetic response. In this manuscript, we experimentally demonstrate a graphene-integrated metasurface modulator that establishes the potential to actively control the amplitude and phase of mid-infrared light with high modulation depth and speed, in good agreement with simulation results. Our simulations also show it is possible to construct a reconfigurable surface with tunable phase profile by incorporating graphene-integrated metasurface modulators with specific geometric parameters. This reconfigurable surface is able to manipulate the orientation of the wave reflected from it, achieving a high-speed, switchable beam steering reflective interface. The results here could inspire research on dynamic reflective display and holograms.

Tensorial metasurface antennas radiating polarized beams based on aperture field implementation

This paper presents a procedure for the design of tensorial metasurface antennas radiating polarized beams through aperture field synthesis. The aperture field is generated using the surface wave to leaky wave conversion resulting from metasurface modulation. The design procedure of the feeder is presented with a description of the geometrical parameters affecting the antenna matching. Numerical solutions for single beam and multi-beam metasurface antennas working respectively, at 12.25 and 20 GHz are presented. In addition, a metasurface prototype working at 12.25 GHz is manufactured and measured.

Active metasurface modulator with electro-optic polymer using bimodal plasmonic resonance

Electrically tunable metasurfaces have gained special interest as they can realize ultrathin surface-normal modulators in planar geometries. In this paper, we demonstrate a novel metasurface modulator based on electro-optic (EO) polymer that utilizes bimodal resonance inside a metallic subwavelength grating to increase the modulation efficiency. When two metal-insulator-metal (MIM) resonant modes are excited simultaneously inside the grating, they couple strongly to generate a sharp dip in the reflected spectrum. As a result, efficient intensity modulation with 15-dB extinction ratio can be obtained at the resonant wavelength under a small refractive index change of 8.5 × 10-3, corresponding to modulation voltage of less than 10 V. Due to the low parasitic capacitance of EO polymer and high conductivity of metallic gratings which is also used as the electrodes, the RC bandwidth of the device should easily exceed 100 GHz, potentially applicable to high-speed surface-normal modulators.

Hybrid metasurface for ultra-broadband terahertz modulation

We demonstrate an ultra-broadband free-space terahertz modulator based on a semiconductor-integrated metasurface. The modulator is made of a planar array of metal cut-wires on a silicon-on-sapphire substrate, where the silicon layer functions as photoconductive switches. Without external excitation, the cut-wire array exhibits a Lorentzian resonant response with a transmission passband spanning dc up to the fundamental dipole resonance above 2 THz. Under photoexcitation with 1.55 eV near-infrared light, the silicon regions in the cut-wire gaps become highly conductive, causing a transition of the resonant metasurface to a wire grating with a Drude response. In effect, the low-frequency passband below 2 THz evolves into a stopband for the incident terahertz waves. Experimental validations confirm a bandwidth of at least 100%, spanning 0.5–1.5 THz with −10 dB modulation depth. This modulation depth is far superior to −5 dB achievable from a plain silicon-on-sapphire substrate with effectively 25 times higher pumping energy. The proposed concept of ultra-broadband metasurface modulator can be readily extended to electrically controlled terahertz wave modulation.

Metasurfing: Addressing Waves on Impenetrable Metasurfaces

Metasurfaces constitute a class of thin metamaterials, which are used from microwave to optical frequencies to create new antennas and microwave devices. Here, we propose the use of variable-impedance metasurfaces for transforming surface or guided waves into different wavefield configurations with desirable properties. We will shortly refer to this metasurface-driven wavefield transformation as “metasurfing.” Metasurfing can be obtained by an appropriate synthesis of inhomogeneous metasurface reactance that allows a local modification of the dispersion equation and, at constant operating frequency, of the local wave vector. The general effects of metasurface modulation are similar to those obtained in solid (volumetric) inhomogeneous metamaterial as predicted by the transformation optics-namely, readdressing the propagation path of an incident wave. However, significant technological simplicity is gained. Several examples are shown as a proof of concept.