# Upconversion Luminescence of NaYF4:Ln3+ Nanoparticles on Gold Nanorod Array with Dual-Wavelength Excitation

## TL;DR

This study explores how gold nanorods affect upconversion luminescence in nanoparticles under dual-wavelength excitation, revealing unexpected competition between emission and dissipation processes.

## Contribution

The paper reveals how plasmonic coupling under dual-wavelength excitation enhances dissipative pathways over upconversion in lanthanide-doped systems.

## Key findings

- Plasmonic coupling under dual-wavelength excitation enhances dissipative pathways competing with upconversion.
- Er3+-doped systems show limited upconversion gain despite higher excited state population under 808 nm excitation.
- Tm3+-doped systems experience suppressed luminescence due to amplified stimulated emission and cross-relaxation.

## Abstract

Plasmonic nanostructures have been widely employed to improve upconversion luminescence performance; however, their impact on excitation pathways under multi-wavelength excitation is not yet fully understood. In this work, we constructed hybrid systems composed of gold nanorod arrays and NaYF4:Yb3+/Ln3+ (Ln = Er3+, Tm3+) upconversion nanoparticles to systematically investigate upconversion behavior under dual-wavelength excitation at 808 and 976 nm. Contrary to the expected synergistic enhancement, our experimental results demonstrate that dual-wavelength excitation in the plasmonic hybrid structures produces different responses of upconversion emission. Measurements dependent on excitation power, along with the analysis of emission intensity ratio, indicate that plasmonic coupling under dual-wavelength excitation significantly enhances dissipative pathways that compete with upconversion processes. Notably, these effects strongly depend on the intrinsic energy-level structure of the lanthanide ions. In the Er3+-doped system, excitation at 808 nm facilitates population of higher-lying excited states, but the overall upconversion gain remains limited. In contrast, in the Tm3+-doped system, plasmonic coupling markedly amplifies stimulated emission and cross-relaxation processes, causing rapid depletion of high-energy state populations and substantial suppression of luminescence. These findings elucidate the competition between upconversion and dissipation processes governing plasmon-assisted upconversion under dual-wavelength excitation and provide a physical foundation for manipulating upconversion luminescence using multiple wavelengths.