57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

This work describes a new class of materials—liquid metal metamaterials—whose optical properties can be dynamically tuned to trap light at wavelengths from the ultra-violet to near-infrared. Because of their ability to absorb light strongly (a far-field effect) and to trap light as surface plasmons (a near-field effect), these metamaterials provide unique opportunities to store and convert energy at the nanoscale. Applications include being strong absorbers and concentrators of light to increase the absorption cross-section of active materials in photovoltaic devices and as high surface-area materials to enhance energy conversion in fuel cells.

To realize liquid metal metamaterials for applications, we needed to be able to manipulate, on-demand, plasmonic properties with fast switching rates, high on/off contrast ratios, and materials stability. We pursued these goals by dynamically controlling the surface plasmon polaritons (SPPs)—electromagnetic surface waves coupled to light—in one-dimensional gratings of liquid gallium (Ga). Tuning temperature, we achieved reversible solid-to-liquid phase transitions. The relatively low melting temperature of bulk Ga (29.8 °C) allowed for near-room temperature cycling between the liquid and solid Ga.

The different physical states of matter had very different optical (plasmonic) responses, where the liquid phase exhibited higher SPP coupling efficiencies and narrower resonance widths compared to the solid phase. Hence, we found the SPP lifetime was longer in the liquid phase, which means there is less loss from scattering compared to solid materials. Also, during liquid-to-solid phase transitions, we observed that the freezing point (T_f) decreased as the initial holding temperature (T_H) of the liquid increased. This supercooling effect allowed for dynamic tuning of T_f as well as access to plasmonic properties of liquid Ga at temperatures well below (>30 °C) the bulk melting point.