58th Annual Report on Research 2013 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 ultra-violet to near-infrared wavelengths. Since they can strongly absorb light (a far-field effect) and efficiently trap light as surface plasmons (a near-field effect), these metamaterials offer unique prospects to store and convert energy at the nanoscale. For example, they can function as 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.

We have demonstrated control over surface plasmon polaritons (SPPs)—electromagnetic surface waves coupled to light—in one-dimensional gratings of liquid gallium (Ga). We found that the liquid phase of the metal exhibited higher SPP coupling efficiencies and narrower resonance widths compared to the solid phase. Hence, we found that the SPP lifetime was longer in the liquid phase, which means there is less loss from scattering compared to solid materials. In Ga, these solid-liquid phase transitions occur above room temperature (T_m = 303 K). We were interested in testing light trapping systems at room temperature, and so we focused on a In-Ga eutectic alloy (T_m = 289 K) with 14.2 atom% In. In addition to observing switchable plasmonic properties by changing the phase of the material, we tuned the location of the plasmonic resonances by altering the composition of the alloy. The incorporation of In into the In-Ga alloy shifted the plasmonic resonances further into the ultra-violet.

Besides these experimental results, we worked with collaborators to calculate the plasmonic properties of the liquid metal alloys from first principles, with no a priori knowledge of the system. We used a model based on density functional theory molecular dynamics to calculate the dielectric constants of the liquid metal alloy. We found exceptional agreement between the simulated and measured optical constants of liquid Ga and liquid In−Ga eutectic alloy. Modeling also enabled us to understand why the liquid state produced superior plasmonic properties compared to the solid state: during the solid-to-liquid phase change, the amorphous nature of the liquid reduced the losses from interband transitions, which improved the plasmonic coupling efficiencies as well as the SPP lifetimes.