Tunable plasmons in ultrathin metal films (1809.01449v2)

Abstract: The physics of electrons, photons, and their plasmonic interactions changes greatly when one or more dimensions are reduced down to the nanometer scale. For example, graphene shows unique electrical, optical, and plasmonic properties, which are tunable through gating or chemical doping. Similarly, ultrathin metal films (UTMFs) down to atomic thickness can possess new quantum optical effects, peculiar dielectric properties, and predicted strong plasmons. However, truly two-dimensional plasmonics in metals has so far elusive because of the difficulty in producing large areas of sufficiently thin continuous films. Thanks to a deposition technique that allows percolation even at 1 nm thickness, we demonstrate plasmons in few-nanometer gold UTMFs, with clear evidence of new dispersion regimes and large electrical tunability. Resonance peaks at 1.5-5 micrometer wavelengths are shifted by hundreds of nanometers and amplitude-modulated by tens of per cent through gating using relatively low voltages. The results suggest ways to use metals in plasmonic applications, such as electro-optic modulation, bio-sensing, and smart windows.

Tunable Plasmons in Ultrathin Metal Films

The paper titled "Tunable plasmons in ultrathin metal films" investigates the unique plasmonic properties exhibited by ultrathin metal films (UTMFs), specifically focusing on gold thin films. The paper addresses the challenge of producing continuous UTMFs over large areas, a task that has hindered the exploration of two-dimensional plasmons in metals. A novel deposition technique using a copper seed layer facilitates the fabrication of UTMFs down to 1 nm, overcoming the limitations of island-like growth traditionally observed in unseeded films.

The authors demonstrate that gold films with a nominal thickness of 3 nm exhibit tunable plasmonic resonances, facilitated by electrical gating. The results show that gold UTMFs display significant modulation in plasmonic resonance peaks, shifted by hundreds of nanometers, across near- and mid-infrared wavelengths when subjected to low-voltage gating. For instance, the electrical gating achieved a plasmonic resonance wavelength shift of approximately 200 nm for films with a thickness of 3 nm.

These findings have substantial implications for the practical application of UTMFs in various domains. The ability to tune plasmonic properties electrically could lead to advanced developments in electro-optic modulation, optical biosensing, and the creation of smart windows with dynamically adjustable transparency. The ultrathin nature of the films permits a wide range of optical responses, further enriching their utility in nanophotonic applications and integrated optical devices.

From a theoretical perspective, the paper confirms that UTMFs can sustain plasmons with a strong optical tunability when their effective surface carrier density is modulated. The low surface carrier density in extremely thin films allows for plasmonic behavior akin to that observed in two-dimensional materials like graphene, which inspires new directions in research on metallic films at the nanoscale.

Additionally, the industrial scalability of the deposition technique signifies its potential for widespread application, facilitating the transition of these findings from a research environment into commercial products. This scalability extends the feasibility of incorporating UTMFs into large-area devices, making their application in consumer technologies a tangible goal.

Future developments in the field may focus on optimizing the electrical gating mechanisms and exploring the integration of UTMFs with existing semiconductor technologies. Further research could also investigate the effects of varying the seed layer material or exploring alternative metal films to broaden the spectrum of achievable plasmonic properties.

The paper offers compelling evidence for the deployment of UTMFs in advanced photonic and optoelectronic systems, underscoring their potential to redefine the capabilities of metal-based plasmonic devices in modern technology landscapes.