Designing tunable optical metamaterials is one of the great challenges in photonics. Strategies for reversible tuning of nanoengineered devices are currently being sought through electromagnetic or piezo effects. For example, bottom-up self-assembly of nanoparticles at solid | liquid or liquid | liquid interfaces can be used to tune optical responses by varying their structure either chemically or through applied voltage. Here, we report on a fully reversible tunable-color mirror based on a TiN-coated Ag substrate immersed in an aqueous solution of negatively charged Au-nanoparticles (NPs). Switching electrode potential can be used to fully control the assembly/disassembly of NPs at the electrode | electrolyte interface within a 0.6 V wide electrochemical window. The plasmon coupling between the electrode and the adsorbed NP array at high positive potentials produces a dip in the optical reflectance spectrum, creating the "absorber" state. Desorption of NPs at low potentials eliminates the dip, returning the system to the reflective "mirror" state. The intensity and wavelength of the dip can be finely tuned through electrode-potential and electrolyte concentration. The excellent match between the experimental data and the theory of optical response for such system allows us to extract valuable information on equilibrium and kinetic properties of NP-assembly/disassembly. Together with modeling of the latter, this study promotes optimization of such meta-surfaces for building electrotunable reflector devices.
Bibliographical noteFunding Information:
The project was supported by an Engineering and Physical Sciences Research Council Grant, “Electrotunable Molecular Alarm”, EP/L02098X/1. J.B.E. also acknowledges receipt of European Research Council Consolidator Grant (NanoPD). Y.M. has been supported in part by a China Scholarship Council, Imperial Scholarship (201506320194). D.J.K. and S.-H.O. acknowledge support from Seagate Technology through the Centre for Micromagnetics and Information Technologies (MINT) at the University of Minnesota. D.S. also acknowledges the support of a H2020-MSCA individual fellowship. Discussions and previous work with Prof. Michael Urbakh (TAU) are gratefully acknowledged.
- electrochemical interface
- nanoparticle self-assembly
- realtime tunability