# Interferometric near-field characterization of plasmonic slot waveguides in single- and poly-crystalline gold films

## TL;DR

This study uses interferometric near-field measurements to analyze how material quality and geometry affect plasmonic slot waveguides' performance in gold films, revealing better propagation in single-crystalline gold for narrow slots.

## Contribution

It provides detailed experimental and numerical analysis of plasmonic slot waveguides in single- and poly-crystalline gold films, highlighting the impact of material quality and geometry on mode propagation.

## Key findings

- Single-crystalline gold improves performance for narrow slots below 100 nm.
- Propagation lengths are similar in both gold types for larger slots.
- Loss mechanisms shift from Ohmic to leakage radiation with increasing gap size.

## Abstract

Plasmonic waveguides are a promising platform for integrated nanophotonic circuits and nanoscale quantum optics. Their use is however often hampered by the limited propagation length of the guided surface plasmon modes. A detailed understanding of the influence of the material quality and the waveguide geometry on the complex mode index is therefore crucial. In this letter, we present interferometric near-field measurements at telecommunication wavelength on plasmonic slot waveguides fabricated by focused ion beam milling in single- and poly-crystalline gold films. We observe a significantly better performance of the slot waveguides in the single-crystalline gold film for slot widths below $100\,\mathrm{nm}$. In contrast for larger slot widths, both gold films give rise to comparable mode propagation lengths. Our experimental observations indicate that the nature of the dominant loss channel changes with increasing gap size from Ohmic to leakage radiation. Our experimental findings are reproduced by three dimensional numerical calculations.