Gate-Tunable Conducting Oxide Metasurfaces

Yao-Wei Huang Department of Physics, National Taiwan University, Taipei, Taiwan Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, USA Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California, USA Ho Wai Lee Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, USA Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California, USA Ruzan Sokhoyan Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, USA Krishnan Thyagarajan Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, USA Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California, USA Georgia Papadakis Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, USA Seunghoon Han Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, USA Samsung Advanced Institute of Technology, Samsung, Suwon, Gyeonggi-do, South Korea Din Ping Tsai Department of Physics, National Taiwan University, Taipei, Taiwan Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan Harry A. Atwater Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California, USA Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California, USA

In the past decade, metasurfaces composed of sub-wavelength artificial ultrathin structures show the promises to exhibit extraordinary light-manipulation and to overcome the limitations of conventional optical components such as lens, wave plates, orbital angular detection, and holograms in any electromagnetic wavelength region. Recent developments in metasurface employs the flexible and reconfigurable optical responses in light manipulation that promises to achieve tunable metasurface devices. Electrical tuning methods based on carrier-induced index changes in transparent conducting oxide provide a capability for potential technology in dynamic response and compatibility with silicon technology to mass-produce.

We report an original approach to create electrically tunable metasurfaces with amplitude and phase modulation based on the voltage applied on conducting oxide active region. The electrically tunable metasurface consists of connected gold nanoantenna array patterned on a 5 nm thin Al₂O₃ and indium tin oxide (ITO) layers on a gold mirror. By increasing the bias voltage on the nanoantenna, the carrier concentration of the conducting oxide material increases and forms an accumulation layer at the Al₂O₃-ITO interface, resulting in the higher plasma frequency of active ITO and the modification of its complex permittivity. The real part of permittivity of active ITO approaches near zero region (epsilon-near-zero, ENZ) at wavelength 0.5 – 3 μm corresponding to carrier concentration about 1×10²² – 2×10²⁰ cm^-3, which occurs a large electric field enhancement in this region. Using the electrical modulation on the active ITO layer, we show that coupling ENZ region of ITO with plasmonic resonance of gold nanoantenna can generate two resonances, which can be used in amplitude-modulated metasurface. In contrast, the wavelength region between these two resonances shows capable of obtaining large phase modulation of 2π versus carrier concentration in active ITO, leading to a phase-modulated metasurface. These results have potential applications in performing dynamic beam shaping, reconfigurable imaging, and high capacity data storage based on electrically tunable metasurfaces.

[1] N. Yu et al., Science 334, 333-337 (2011).

[2] S. Sun et al., Nano Lett. 12, 6223-6229 (2012).

[3] H. W. Lee et al., Nano Lett. 14, 6463-6468 (2014).

gpapadak@caltech.edu