# Nonclassical optical properties of mesoscopic gold

## TL;DR

This study reveals that mesoscopic gold exhibits size-dependent band structure changes below 30 nm, significantly affecting its nonlinear optical properties, as demonstrated through experiments and density functional calculations.

## Contribution

It provides the first experimental evidence of size-dependent band structure modifications in mesoscopic gold films below 30 nm thickness.

## Key findings

- 100-fold increase in nonlinear photoluminescence signal for films below 30 nm
- Optical resolution of single-unit-cell steps in ultrathin gold films
- Density functional calculations explain the observed band structure changes

## Abstract

Gold nanostructures have important applications in nanoelectronics, nano-optics as well as in precision metrology due to their intriguing opto-electronic properties. These properties are governed by the bulk band structure but to some extend are tunable via geometrical resonances. Here we show that the band structure of gold itself exhibits significant size-dependent changes already for mesoscopic critical dimensions below 30 nm. To suppress the effects of geometrical resonances and grain boundaries, we prepared atomically flat ultrathin films of various thicknesses by utilizing large chemically grown single-crystalline gold platelets. We experimentally probe thickness-dependent changes of the band structure by means of two-photon photoluminescence and observe a surprising 100-fold increase of the nonlinear signal when the gold film thickness is reduced below 30 nm allowing us to optically resolve single-unit-cell steps. The effect is well explained by density functional calculations of the thickness-dependent 2D band structure of gold.