Introduction:

At the invitation of the founding editor-in-chief of Photonics Insights, Prof. Sergey I. Bozhevolnyi and Associate Prof. Fei Ding from University of Southern Denmark authored a review paper titled "Electrically tunable optical metasurfaces" which was published in the Issue 3 of Photonics Insights, 2024. (Fei Ding, Chao Meng, Sergey I. Bozhevolnyi. Electrically tunable metasurfaces[J]. Photonics Insights, 2024, 3(3): R07)

This review provides a comprehensive summary of recent research achievements in electrically tunable metasurfaces. It systematically introduces various materials and configurations of tunable optical metasurfaces, along with their tuning mechanisms and key design parameters. The review highlights the metasurfaces' ability to control the amplitude, phase and polarization of light fields, as well as related applications, such as compact intensity/phase modulators, dynamic wavefront shaper, tunable polarization optics, display and hologram, spatial light modulators and more. This serves as a highly valuable reference for both researchers and newcomers to the field. At the end of the review, future research directions and potential application scenarios that could further advance the development of tunable optical metasurfaces are proposed.

Fig. 1 Overview of electrically tunable optical metasurfaces: materials, configurations, and applications.

1. Electrically Tunable Optical Metasurfaces Based on Liquid Crystals

Liquid crystals represent a unique state of matter, exhibiting characteristics that fall between conventional liquids and solid crystals. The orientation of liquid crystal molecules can be precisely controlled through external stimuli, such as electric fields, temperature, light, or pressure. This tunability allows liquid crystals to dynamically adjust their refractive index (Δn ≈ 0.1−0.3), making them suitable for fabricating reconfigurable optical devices. By integrating liquid crystal with metasurfaces, the advantages of both can be effectively utilized: metasurfaces offer high spatial resolution for precise light field manipulation, while liquid crystals enable dynamic tuning of the optical response.

The Current state-of-the-art liquid crystal-based tunable metasurface configurations can be classified into two main types: (1) Hybrid liquid crystal metasurfaces, which integrate liquid crystal molecules with metasurface elements in a heterogeneous manner. (2) Cascaded configurations of liquid crystal modules and metasurfaces. Liquid crystal-based tunable metasurfaces have already been applied to create a series of compact, lightweight tunable optics, including dynamic display devices, tunable wavefront shaper, tunable color filters, varifocal lenses, and spectropolarimeters. From a manufacturing perspective, liquid crystals offer distinct advantages: The material itself has been studied for years, with well-established processes that allow for large-scale production. However, there are still several challenges in the integration of tunable metasurfaces with liquid crystals: (1) Strong temperature and polarization sensitivity of liquid crystals; (2) Modulation frequency is generally in the kHz range, limited by the mechanical rotation of liquid crystal molecules. This limits their suitability for high-speed optoelectronic systems.

Figure 2. Electrically Tunable Optical Metasurface Integrated with Liquid Crystals

2. Tunable Optical Metasurfaces Based on Phase-Change Materials

The optoelectronic properties of phase-change materials can be controlled through electrical, thermal, or optical stimuli, making them promising for applications in thermal energy storage and data storage. As a result, these materials have garnered widespread attention from both academia and industry. Phase-change materials can exhibit significant changes in refractive index, making them ideal for constructing highly efficient tunable metasurfaces. Some typical phase-change materials and corresponding tuning mechanisms include: (1) Vanadium dioxide (VO₂) and its insulator-to-metal phase transition. (2) Phase-change chalcogenides (GeSbTe, GeSbSeTe, Sb₂Se₃) and their amorphous-to-crystalline phase transitions. By applying an electric current, these materials can undergo reversible phase changes at specific temperatures, resulting in substantial changes in refractive index (Δn ≈ 1). Metasurfaces based on phase-change materials have been explored for creating devices such as intensity/phase modulators, dynamic deflectors, varifocal lenses, and fast spectral imaging systems. However, tunable metasurfaces based on phase-change materials face several challenges: (1) Slow modulation speed: Although the theoretical modulation frequency of phase-change materials can reach MHz levels, achieving high-speed modulation in practice requires optimizing the device's thermal management, i.e., efficient heating and cooling processes. (2) Material durability and stability: Repeated phase transitions can lead to material fatigue, affecting the long-term reliability of the devices. (3) Heterogeneous integration and process compatibility: When integrating phase-change materials with other materials, differences in thermal expansion, chemical, and mechanical properties may lead to the degradation of device performance and lifetime. These issues must be addressed to further improve the performance and scalability of phase-change material-based tunable metasurfaces.

Figure 3. Electrically Tunable Optical Metasurface Based on Phase-Change Materials

3. Tunable Optical Metasurfaces Based on Reversible Electrochemical Reactions

Electrochemical reactions are fundamental processes in energy storage, such as in batteries. In recent years, the combination of reversible electrochemical reaction mechanisms with metasurfaces has led to the development of novel tunable metasurfaces. Electrochemical redox reactions can effectively modulate the complex refractive index of the involved materials, particularly the imaginary part (i.e., the extinction coefficient). The key materials used in these systems include: (1) Inorganic transition metal oxides (e.g., tungsten trioxide (WO₃), titanium dioxide (TiO₂)); (2) Conductive polymers (e.g., poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANI)). Tunable metasurfaces based on electrochemical reactions allow for the control of transparency and/or color through electrical input. The frequency response of these devices typically ranges from Hz to kHz, making them suitable for applications such as displays. Current research in this area mainly focuses on electrochromic devices, dynamic displays, and smart windows.

Figure 4. Tunable Optical Metasurface Based on Electrochemical Reactions

4. Ultrafast Tunable Optical Metasurfaces Based on 2D Materials, Transparent Conductive Oxides, and Electro-Optic Nonlinear Materials

Electrically tunable optical metasurfaces based on liquid crystals, phase-change materials, and electrochemical reactions enable effective modulation of material refractive indices, achieving efficient dynamic control of light fields. However, due to the limitations of the materials and their respective tuning mechanisms, these approaches inherently cannot achieve high-speed dynamic light field control (above GHz levels). To realize ultrafast dynamic light field control, materials such as 2D materials, transparent conductive oxides, or electro-optic nonlinear media must be employed for the development of ultrafast tunable optical metasurfaces:

(1) 2D Materials: These materials consist of single or few atomic layers, including graphene and its derivatives, transition metal dichalcogenides, black phosphorus and more. Due to their atomically thin structure and electrically tunable bandgaps, 2D materials can be heterogeneously integrated with ultrathin planar metasurfaces, forming hybrid electrically tunable metasurfaces. By designing metasurfaces to generate various resonance modes, the interaction between light and 2D materials can be significantly enhanced, enabling efficient dynamic light field control. In addition to hybrid metasurfaces, 2D materials themselves possess a wealth of electrically tunable resonance mechanisms. For instance, graphene exhibits electrically tunable plasmonic resonances, while transition metal dichalcogenides have electrically tunable exciton resonances. Thus, electrically tunable metasurfaces can also be developed based solely on the inherent properties of 2D materials. These materials open new pathways for achieving ultrafast, high-efficiency control of optical fields, which is essential for applications in high-speed photonics and optoelectronics.

Figure 5. Tunable Optical Metasurface Based on 2D Materials

(2) Transparent Conductive Oxides (TCO): Transparent conductive oxides (TCOs) include materials such as indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), and cadmium oxide. By injecting carriers, the refractive index of TCO materials can be modulated. When combined with epsilon-near-zero (ENZ) modes, these materials can enable highly efficient tunable metasurfaces. Typically, these metasurface units are designed in a metal-oxide-semiconductor (MOS) configuration to ensure efficient carrier injection while simultaneously optimizing the resonant design of the metasurface units. Remarkably, only a few nanometers thick of TCO films are sufficient to create efficient tunable metasurfaces. In nanoscale TCO films, carrier modulation occurs at extremely fast speeds, allowing modulation frequencies to reach up to the GHz range. This makes TCO-based metasurfaces highly suitable for ultrafast dynamic optical control applications, such as in high-speed communication and photonics.

Figure 6. Tunable Optical Metasurface Based on Transparent Conductive Oxides

(3) Electro-Optic Nonlinear Materials: Electro-optic nonlinear materials, such as lithium niobate and electro-optic polymers, are key components for high-speed tunable metasurfaces. The electro-optic effects include the linear electro-optic Pockels effect and the second-order nonlinear Kerr effect. For instance, in the linear electro-optic Pockels effect, the refractive index of the material changes linearly with the applied electric field, enabling ultrafast modulation frequencies (>100 GHz). However, the range of refractive index modulation through the Pockels effect is typically small (on the order of 10⁻³), and since metasurfaces are subwavelength in thickness, the tunable capabilities of such metasurfaces can be limited. By designing high-quality resonances, the modulation efficiency can be significantly improved, but this also tends to narrow the operational bandwidth of the device. Currently, tunable metasurfaces based on electro-optic nonlinear materials are primarily focused on uniform amplitude and phase modulation. More complex functionalities, such as dynamic polarization control and wavefront shaping, still require further exploration and development to enhance their capabilities in dynamic optical control.

Figure 7. Tunable Optical Metasurface Based on Electro-Optic Nonlinear Effects

5. MEMS/NEMS-Based Tunable Optical Metasurfaces

Tunable metasurfaces based on active materials are typically limited by the tuning range of the material's refractive index and the operational wavelength. Recently, the development of micro/nano-electromechanical systems (MEMS/NEMS) has provided new strategies for achieving highly efficient tunable metasurfaces. MEMS/NEMS offer nanometer-precision movement and a range of motion from 100 nanometers to 100 micrometers, making them well-suited for fine, continuous, and wide-range adjustments in tunable optics. MEMS/NEMS-based tunable metasurfaces can generally be categorized into three configurations: (1) Modulating the resonance characteristics of individual meta-atoms by altering their geometric structures. (2) Stretching or moving the entire metasurface to dynamically control the overall response. (3) Adjusting the relative positions of multiple cascaded metasurfaces to modulate the overall optical response. These configurations have led to the development of various highly efficient tunable metasurface devices, including intensity/phase modulators, dynamic wavefront/polarization control devices, varifocal lenses, and dynamic displays. The response speed of MEMS/NEMS-integrated tunable metasurfaces is determined by the inherent resonant frequency of the MEMS, typically in the kHz to MHz range.

Figure 8. MEMS/NEMS Tunable Optical Metasurface

6. Summary and Outlook

Optical metasurfaces represent a revolutionary technique in the field of optics, offering a new platform technology for multidimensional light field manipulation at the subwavelength scale. In this review, we have systematically examined, analyzed, and summarized the current state of research on electrically tunable metasurfaces, including material systems, basic configurations, and related devices and applications. Looking ahead, the future of electrically tunable optical metasurfaces is filled with vast potential, with several promising research directions, including: (1) Innovating designs, and structures to achieve tunable metasurfaces with optimal performance. (2) Improving material stability and durability. (3) Developing large-scale, low-cost micro/nanofabrication techniques. (4) Exploring new materials and novel tuning mechanisms. (5) Promoting the integration of tunable metasurface devices into compact intelligent optoelectronic systems, such as augmented reality/virtual reality (AR/VR) and portable biomedical imaging systems.