Can Copper Nanostructures Sustain High-Quality Plasmons?

TL;DR

This study demonstrates that copper nanowires can support high-quality vis-IR plasmons with quality factors exceeding 60, challenging the notion that copper is too lossy for plasmonic applications, due to energy distribution outside the metal.

Contribution

The paper provides experimental evidence that copper nanowires can sustain high-quality plasmons, with detailed analysis of loss mechanisms, positioning copper as a viable, cost-effective plasmonic material.

Findings

Copper nanowires exhibit plasmons with quality factors over 60.

Most plasmon energy resides outside the copper metal, reducing losses.

Thermal effects enable observation of electron energy gains at low energies.

Abstract

Silver is considered to be the king among plasmonic materials because it features low inelastic absorption in the visible and infrared (vis-IR) spectral regions compared to other metals. In contrast, copper is commonly regarded as being too lossy for plasmonic applications. Here, we experimentally demonstrate vis-IR plasmons in long copper nanowires (NWs) with quality factors that exceed a value of 60, as determined by spatially resolved, high-resolution electron energy-loss spectroscopy (EELS) measurements. We explain this counterintuitive result by the fact that plasmons in these metal wires have most of their electromagnetic energy outside the metal, and thus, they are less sensitive to inelastic losses in the material. We present an extensive set of data acquired on long silver and copper NWs of varying diameters supporting this conclusion and further allowing us to understand the relative roles played by radiative and nonradiative losses in plasmons that span a wide range of energies down to $<20\,$meV. At such small plasmon energies, thermal population of these modes becomes significant enough to enable the observation of electron energy gains associated with plasmon absorption events. Our results support the use of copper as an attractive cheap and abundant material platform for high quality plasmons in elongated nanostructures.