Surface-normal electro-optic spatial light modulator using graphene integrated on a high-contrast grating resonator

Equivalent circuit model for a graphene-based high efficiency tunable broadband terahertz polarizer

An equivalent circuit model for a graphene-based high-efficiency tunable broadband THz polarizer is presented. The conditions for linear-to-circular polarization conversion in the transmission mode are utilized to derive a set of closed-form design formulas. Given a set of target specifications, the key structural parameters of the polarizer are directly calculated using this model. The proposed model is rigorously validated by comparing the circuit model and full-wave electromagnetic simulation results, from which it is found that the model is accurate and effective, accelerating the analysis and design processes. This offers a further step in developing a high-performance and controllable polarization converter with potential applications in imaging, sensing, and communications.

Equivalent circuit model for the analysis and design of graphene-based tunable terahertz polarizing metasurfaces

In this work, we present an equivalent circuit model that facilitates the analysis and design of graphene-based transmission- and reflection-mode tunable terahertz polarizers. The conditions for polarization conversion are analytically derived, and a set of closed-form design formulas is presented. Given the target specifications, the key structural parameters are directly calculated. The proposed method is rigorously validated for two linear-to-circular polarizers operating in transmission and reflection modes. The results from the circuit model and full-wave electromagnetic simulation are compared, and excellent agreement is observed. The proposed circuit model is accurate and effective, and speeds up the analysis and design processes. The polarizers studied in the present work feature simple geometries and competitive performance with respect to other metasurface polarizers. The tunable fractional bandwidths, over which linear-to-circular polarization conversion is achieved, by varying the graphene chemical potential, are 65% and 36%, respectively, for the two transmission- and reflection-mode polarizers.

Multi-functional high-efficiency reflective polarization converter based on an ultra-thin graphene metasurface in the THz band

In this study, an ultra-thin reflective metasurface is proposed for polarization conversion in the terahertz band. Each unit cell of metasurface is composed of graphene ribbons lying diagonally on silicon substrate. A reflective metal is also placed at the bottom of the structure. Our polarization converter works as a linear polarization converter (LPC) and linear to circular polarization converter (LTC-PC) by variation of the chemical potential of graphene, which can actively be changed by chemical doping or electrical bias of the graphene. The working bandwidth of LPC changes by adjusting the chemical potential of the graphene. The LPC structure has more than 99% polarization conversion ratio in the frequency range of 0.83-0.92 THz, even by changing the angle of incident wave up to 45°, the results are still acceptable. The LTC-PC has less than 3dB axial ratio (AR) in the frequency range of 0.6-0.67 THz for left-handed circularly polarized (LHCP) waves and 0.72-0.97 THz for right-handed circularly polarized (RHCP) waves. To verify the simulation results, an equivalent circuit model based on the structure performance is proposed. Equivalent circuit model results agree very well with the simulation results. Due to the fabrication feasibility, ultra-thin thickness, incident angle insensitive, and high efficiency, our structure has great potential in state-of-the-art technologies such as imaging, sensing, communication, and other optical applications.

High-speed programmable lithium niobate thin film spatial light modulator

High-speed spatial modulation of light is the key technology in various applications, such as optical communications, imaging through scattering media, video projection, pulse shaping, and beam steering, in which spatial light modulators (SLMs) are the underpinning devices. Conventional SLMs, such as liquid crystal (LC), digital micromirror device (DMD), and micro-electro-mechanical system (MEMS) ones, operate at a typical speed on the order of several kilohertz as limited by the slow response of the pixels. Achieving high-speed spatial modulation is still challenging and highly desired. Here, we demonstrate a one-dimensional (1D) high-speed programmable spatial light modulator based on the electro-optic effect in lithium niobate thin film, which achieves a low driving voltage of 10 V and an overall high-speed modulation speed of 5 MHz. Furthermore, we transfer an image by using parallel data transmission based on the proposed lithium niobate SLM as a proof-of-principle demonstration. Our device exhibits improved performance over traditional SLMs and opens new avenues for future high-speed and real-time applications, such as light detection and ranging (LiDAR), pulse shaping, and beam steering.

Low-voltage, broadband graphene-coated Bragg mirror electro-optic modulator at telecom wavelengths

We demonstrate a graphene based electro-optic free-space modulator yielding a reflectance contrast of 20% over a strikingly large 250nm wavelength range, centered in the near-infrared telecom band. Our device is based on the original association of a planar Bragg reflector, topped with an electrically contacted double-layer graphene capacitor structure employing a high work-function oxide shown to confer a static doping to the graphene in the absence of an external bias, thereby reducing the switching voltage range to +/-1V. The device design, fabrication and opto-electric characterization is presented, and its behavior modeled using a coupled optical-electronic framework.

Feasibility of Using High-Contrast Grating as a Point-of-Care Sensor for Therapeutic Drug Monitoring of Immunosuppressants

Point-of-care (POC) testing has demonstrated great transformative potential in personalized medicine. In particular, patients undergoing transplantation require POC testing to ensure appropriate serum immunosuppressant levels so as to maintain adequate graft function and survival. However, no suitable POC device for monitoring immunosuppressant levels is currently available. Exploiting the latest advances in metamaterials can lead to a breakthrough in POC testing. A high-contrast grating (HCG) biosensor is a low-cost, compact, simple-to-fabricate, and easy-to-operate structure. It is highly sensitive and robust in surface-based biomarker detection, which is favorable for the efficiency of a POC device. In this study, the feasibility of using an HCG as a POC sensor for therapeutic drug monitoring of immunosuppressants was evaluated. The detection efficiency of the most commonly prescribed immunosuppressive medication cyclosporine A by using this sensor was demonstrated to be comparable to those of conventional commercial kits, suggesting that the sensor has the potential to be used as a rapid detection and feedback platform for increasing drug compliance and improving new organ transplant survival.

Efficient Optical Reflection Modulation by Coupling Interband Transition of Graphene to Magnetic Resonance in Metamaterials

Designing powerful electromagnetic wave modulators is required for the advancement of optical communication technology. In this work, we study how to efficiently modulate the amplitude of electromagnetic waves in near-infrared region, by the interactions between the interband transition of graphene and the magnetic dipole resonance in metamaterials. The reflection spectra of metamaterials could be significantly reduced in the wavelength range below the interband transition, because the enhanced electromagnetic fields from the magnetic dipole resonance greatly increase the light absorption in graphene. The maximum modulation depth of reflection spectra can reach to about 40% near the resonance wavelength of magnetic dipole, for the interband transition to approach the magnetic dipole resonance, when an external voltage is applied to change the Fermi energy of graphene.

Tunable broadband terahertz polarizer using graphene-metal hybrid metasurface

We demonstrate an electrically tunable polarizer for terahertz (THz) frequency electromagnetic waves formed from a hybrid graphene-metal metasurface. Broadband (>3 THz) polarization-dependent modulation of THz transmission is demonstrated as a function of the graphene conductivity for various wire grid geometries, each tuned by gating using an overlaid ion gel. We show a strong enhancement of modulation (up to ∼17 times) compared to graphene wire grids in the frequency range of 0.2-2.5 THz upon introduction of the metallic elements. Theoretical calculations, considering both plasmonic coupling and Drude absorption, are in good agreement with our experimental findings.

Mechanically tunable focusing metamirror in the visible

A compact, flat lens with dynamically tunable focal length will be an essential component in advanced reconfigurable optical systems. One approach to realize a flat tunable lens is by utilizing metasurfaces, which are two-dimensional nanostructures capable of tailoring the wavefront of incident light. When a metasurface with a hyperboloidal phase profile, i.e., a metalens, is fabricated on a substrate that can be actuated, its focal length can be adjusted dynamically. Here, we design and realize the first reflection type, tunable metalens (i.e., metamirror) operating in the visible regime (670 nm). It is shown that the focal length can be continuously adjusted by up to 45% with a 0% to 20% lateral stretching of the substrate, while maintaining diffraction-limited focusing and high focusing efficiency. Our design as a flat optics element has potential in widespread applications, such as wearable mixed reality systems, biomedical instruments and integrated optics devices.

Ultranarrow-band metagrating absorbers for sensing and modulation

Nanostructured plasmonic metamaterials are an excellent platform for narrowband optical absorption, which has wide applications in sensing, filtering, modulation, and emission tailoring. However, achieving a subnanometer absorption bandwidth for optical sensing and dynamical control of light is still challenging. Here, we propose an asymmetric metagrating structure and make use of the propagating surface plasmonic mode that has a small dissipation rate, to achieve perfect optical absorption with a bandwidth of 0.28 nm near the wavelength of 1.55 μm. Our proposed structure can be used in solution environments as a chemical or biological sensor in the visible spectral range just by changing the structural parameters. The sensor possesses a sensitivity of 440 nm/RIU and figure of merit of 1333.33 RIU^-1. In addition, by combining an organic electro-optic material with this metagrating, our device can be reconfigurable with a dynamic range of 15.52 dB. Therefore, our proposed metagrating platform not only works as an ultranarrow-band absorber, but also can be employed for optical sensing and dynamic control of light.

Electro-optic phase modulation with a symmetrical metal-cladding waveguide

A bulk electro-optic (EO) modulator based on the ultrahigh-order guided modes, which are excited in a symmetrical metal-cladding waveguide (SMCW), has been exploited. This kind of mode in a SMCW has high sensitivity to phase shift by changing the refractive index of the guiding layer. Compared with phase modulation via the bulk EO modulator without a waveguide, the applied half-wave voltage is reduced for one magnitude. This work may have practical applications in optical information processes.

Demonstration of a thermo-optic phase shifter by utilizing high-Q resonance in high-index-contrast grating

A thermo-optic phase shifter is proposed and demonstrated by utilizing the high-Q resonance in high-index-contrast grating (HCG). The Q-factor up to ∼12000 is measured in a footprint of 110  μm×300  μm. By heating the HCG with paired metal strip micro-heaters, the optical resonance shifts, which induces phase modulation. A phase shift of ∼1.2π under heating power of ∼32  mW is directly observed and demodulated from the fringes shifting in a Michelson interferometer. The proposed configuration can also be extended to realize high-speed phase shift by adopting electro-optical modulation.

Solid-State Electrolyte-Gated Graphene in Optical Modulators

The gate-tunable wide-band absorption of graphene makes it suitable for light modulation from terahertz to visible light. The realization of graphene-based modulators, however, faces challenges connected with graphene's low absorption and the high electric fields necessary to change graphene's optical conductivity. Here, a solid-state supercapacitor effect with the high-k dielectric hafnium oxide is demonstrated that allows modulation from the near-infrared to shorter wavelengths close to the visible spectrum with remarkably low voltages (≈3 V). The electroabsorption modulators are based on a Fabry-Perot-resonator geometry that allows modulation depths over 30% for free-space beams.