Light emission from gold nanoparticles under ultrafast near-infrared excitation: thermal emission, inelastic light scattering or multiphoton luminescence?

TL;DR

This study investigates the mechanisms behind broad-band upconverted luminescence in gold nanoparticles under ultrafast near-infrared excitation, distinguishing between multiphoton processes and thermal effects through power-law analysis.

Contribution

It provides a spectrally resolved power-law analysis revealing two emission regimes and clarifies that high-intensity luminescence is due to intraband transitions linked to heated electron gas, not interband transitions or inelastic scattering.

Findings

Low irradiance emission follows a multiphoton power law.

High irradiance emission shows photon-energy-dependent nonlinearity.

Luminescence at high irradiance is consistent with intraband electronic transitions.

Abstract

Gold nanoparticles emit broad-band upconverted luminescence upon irradiation with pulsed infrared laser radiation. Although the phenomenon is widely observed, considerable disagreement still exists concerning the underlying physics - most notably over the applicability of concepts such as multiphoton absorption, inelastic scattering, and interband and intraband electronic transitions. Here, we study single particles and small clusters of particles by employing a spectrally resolved power-law analysis of the irradiation-dependent emission as a sensitive probe of these physical models. Two regimes of emission are identified: at low irradiance levels of kW/cm2, the emission follows a well-defined integer-exponent power law suggestive of a multiphoton process. However, at higher irradiance levels of several kW/cm2, the nonlinearity exponent itself depends on the photon energy detected, a tell-tale signature of a radiating heated electron gas. We show that in this regime, the experiments are incompatible with both interband transitions and inelastic light scattering as the cause the luminescence, while they are compatible with the notion of luminescence linked to intraband transitions.