Gate-tunable conducting oxide metasurfaces

Abstract: Metasurfaces composed of planar arrays of sub-wavelength artificial structures show promise for extraordinary light manipulation; they have yielded novel ultrathin optical components such as flat lenses, wave plates, holographic surfaces and orbital angular momentum manipulation and detection over a broad range of electromagnetic spectrum. However the optical properties of metasurfaces developed to date do not allow for versatile tunability of reflected or transmitted wave amplitude and phase after fabrication, thus limiting their use in a wide range of applications. Here, we experimentally demonstrate a gate-tunable metasurface that enables dynamic electrical control of the phase and amplitude of the plane wave reflected from the metasurface. Tunability arises from field-effect modulation of the complex refractive index of conducting oxide layers incorporated into metasurface antenna elements which are configured in a reflectarray geometry. We measure a phase shift of {\pi} and ~ 30% change in the reflectance by applying 2.5 V gate bias. Additionally, we demonstrate modulation at frequencies exceeding 10 MHz, and electrical switching of +/-1 order diffracted beams by electrical control over subgroups of metasurface elements, a basic requirement for electrically tunable beam-steering phased array metasurfaces. The proposed tunable metasurface design with high optical quality and high speed dynamic phase modulation suggests applications in next generation ultrathin optical components for imaging and sensing technologies, such as reconfigurable beam steering devices, dynamic holograms, tunable ultrathin lens, nano-projectors, and nanoscale spatial light modulators. Importantly, our design allows complete integration with electronics and hence electrical addressability of individual metasurface elements.

• The paper demonstrates dynamic electrical control using field-effect modulation in ITO, achieving ~π phase shift and a 30% change in reflectance with a 2.5V bias.
• It employs a metasurface reflectarray design to enable modulation speeds exceeding MHz frequencies and selective beam steering.
• The study paves the way for ultrathin, reconfigurable optical devices with applications in holography, tunable lenses, and on-chip sensing.

Gate-Tunable Conducting Oxide Metasurfaces

The paper under review presents an in-depth exploration of gate-tunable conducting oxide metasurfaces, focusing on achieving dynamic electrical control over the phase and amplitude of reflected plane waves. By integrating conducting oxide layers within a metasurface reflectarray geometry, the authors demonstrate significant advancements in the electrical modulation capabilities of metasurfaces. Specifically, the utilization of field-effect modulation in transparent conducting oxide (TCO) materials, such as indium tin oxide (ITO), provides a mechanism for high-speed, low-power modulation.

The authors document a notable phase shift of approximately π and a 30% change in reflectance achieved through a mere 2.5 V gate bias, highlighting the practical efficiency of their approach. These results are bolstered by the demonstration of modulation speeds exceeding MHz frequencies and selective electrical switching of diffracted beams. Collectively, these findings indicate the potential for electrically tunable beam-steering phased array metasurfaces that maintain high optical quality and exhibit rapid dynamic responses.

Theoretical and Experimental Implications

The research carries significant implications for both theoretical exploration and practical applications. Theoretically, the ability to dynamically control the phase and amplitude of reflected waves represents a critical advancement in the area of metasurface wavefront engineering. The experimental validation underscores the feasibility of utilizing field-effect modulation in TCO-based metasurfaces to achieve epsilon-near-zero (ENZ) optical properties, a concept that has been traditionally challenging to implement in practice.

From an application perspective, the electrically tunable metasurfaces proposed could play a transformative role in developing next-generation ultrathin optical components. Potential applications include dynamic holography, tunable lenses, nano-projectors, and reconfigurable beam-steering devices. Notably, further enhancements could refine the design to support even faster modulation speeds, possibly extending into the THz range by leveraging optimized antenna array designs with reduced footprints and increased capacitance efficiencies.

Future Research Directions

Potential future research directions are manifold. Advancements in gate dielectric materials could provide enhancements in modulation speed and device reliability, particularly by addressing issues related to electrical breakdown. Exploring alternative materials that offer higher capacitance and field-effect efficiencies might enable the realization of metasurfaces with even more rapid and energy-efficient modulation capabilities.

Further exploration into the integration of these metasurfaces with existing electronic circuit architectures could facilitate novel on-chip sensing and imaging applications, particularly in fields such as LIDAR, telecommunications, and real-time spatial light modulation. As the field of nanophotonics continues to evolve, these findings establish a foundational framework for ongoing innovations in adaptable and miniaturized optical devices.

In summary, this paper contributes substantially to the field of metasurfaces by providing both a theoretical framework and practical demonstration of gate-tunable properties in conducting oxide-based metasurfaces. The results open new avenues for research and technological development, promising to impact various sectors reliant on advanced optical manipulation technologies.