Probing a century old prediction one plasmonic particle at a time

Area of Science:

• Plasmonics and Nanophotonics
• Optical properties of metallic nanoparticles
• Single-particle spectroscopy

Background:

• Gustav Mie's 1908 solution to Maxwell's equations describes plasmonic particle optics.
• Metallic nanoparticles exhibit size-dependent optical properties.
• Ensemble measurements typically provide only total extinction cross sections.

Purpose of the Study:

• To separately probe size-dependent surface absorption and scattering of gold nanoparticles.
• To investigate the validity of Mie theory for varying nanoparticle sizes.
• To identify factors influencing optical properties beyond Mie theory predictions.

Main Methods:

• Utilized dark-field scattering spectroscopy to measure scattering.
• Employed photothermal imaging to selectively measure absorption.
• Correlated optical measurements with scanning electron microscopy for precise size determination (43-274 nm).

Main Results:

• Experimental data largely followed Mie theory trends across a broad size range.
• Observed significant variations in scattering and absorption intensities for small size changes.
• These variations were not solely explained by Mie theory or deviations from spherical shape.

Conclusions:

• Mie theory provides a good approximation for nanoparticle optical properties over large size ranges.
• Factors beyond simple spherical geometry significantly influence absorption and scattering at smaller size scales.
• Single-particle spectroscopy is crucial for dissecting complex optical behaviors of nanoparticles.