Ultimate limit for optical losses in gold, revealed by quantitative near-field microscopy

TL;DR

This study measures the propagation lengths of surface plasmon polaritons on atomically flat gold surfaces, revealing the ultimate optical loss limits in gold and emphasizing the importance of accurate dielectric property characterization.

Contribution

It provides the first direct experimental measurement of SPP propagation lengths on nearly perfect gold interfaces, establishing the fundamental loss limits in gold plasmonics.

Findings

Measured SPP propagation lengths are shorter than those predicted by some dielectric models.

Experimental results match predictions based on properly fabricated polycrystalline gold.

Results suggest the measured propagation lengths represent the ultimate optical loss limit in gold.

Abstract

We report thorough measurements of surface plasmon polaritons (SPPs) running along nearly perfect air-gold interfaces formed by atomically flat surfaces of chemically synthesized gold monocrystals. By means of amplitude- and phase-resolved near-field microscopy, we obtain their propagation length and effective mode index at visible wavelengths (532, 594, 632.8, 729, and 800 nm). The measured values are compared with the values obtained from the dielectric functions of gold that are reported in literature. Importantly, a reported dielectric function of monocrystalline gold implies $\sim 1.5$ times shorter propagation lengths than those observed in our experiments, whereas a dielectric function reported for properly fabricated polycrystalline gold leads to SPP propagation lengths matching our results. We argue that the SPP propagation lengths measured in our experiments signify the ultimate limit of optical losses in gold, encouraging further comprehensive characterization of optical material properties of pure gold as well as other plasmonic materials.