Metamaterials are nanocomposite structures which exhibit unusual behaviors that cannot be found in nature. These materials are used to create objects with unique electromagnetic properties for applications in, e.g., antennae, sensors, and superlenses. However, tuning and modulating optical metamaterials in real time remains a challenge.

Anthony R. Kucernak, Joshua B. Edel, and Alexei A. Kornyshev, Imperial College London, UK, and colleagues have developed a reversible electrotuneable window-mirror that emulates either a reflective mirror or a transmissive window depending on the polarity of the applied potential.

The self-assembly/disassembly of functionalized gold nanoparticle anions (ca. 16 nm) at the interface between two immiscible electrolyte solutions is the key to the electrotuneable liquid mirror. A negative polarization of the aqueous phase relative to the organic phase pushes the gold nanoparticles anions towards the interface. A dense monolayer of gold nanoparticles is formed that reflects light due to coupled plasmon resonances. When a positive polarization is applied to the aqueous phase, the nanoparticles disperse from the interface and the material acts as a window.

The team also demonstrated that both the peak reflectance wavelength and reflectance intensity of the liquid mirror can be tuned by changing the applied potential. This is due to a decrease of the interparticle distances of the gold nanoparticles within the formed monolayer with an increased potential. The observed effects were in excellent agreement with theoretical calculations.

• Electrotunable nanoplasmonic liquid mirror,
  Yunuen Montelongo, Debabrata Sikdar, Ye Ma, Alastair J. S. McIntosh, Leonora Velleman, Anthony R. Kucernak, Joshua B. Edel, Alexei A. Kornyshev,
  Nat. Mater. 2017.
  DOI: 10.1038/nmat4969