Plasmonic nanoparticles interact strongly with light through localized surface plasmon resonance (LSPR). By controlling particle composition, size, shape, and surrounding environment, researchers can tune absorption and scattering across the ultraviolet, visible, and near-infrared spectrum.
These properties make plasmonic nanoparticles useful for sensing, spectroscopy, imaging, diagnostics, optical coatings, and other technologies that require precise control of light.
Applying plasmonic nanoparticles in an optical system?
Explore how nanoparticle composition, size, shape, and optical response can support sensing, spectroscopy, coatings, imaging, and other optical technologies.
What Are Plasmonic Nanoparticles?
Plasmonic nanoparticles are strong absorbers and scatterers of light. Gold and silver nanoparticles are common examples. Their optical response depends on material composition, particle size and shape, and the surrounding environment.
Changing these properties can shift the optical response across the UV, visible, and near-infrared spectrum and alter the relative contributions of absorption and scattering. Small gold nanospheres, for example, typically appear red because their LSPR produces strong extinction in the green portion of the visible spectrum. Many silver nanospheres appear yellow because their dominant plasmon resonance occurs at shorter visible wavelengths.
Particle size, shape, aggregation state, and surrounding refractive index can all produce substantial changes in color and spectral response.
What Is Localized Surface Plasmon Resonance?
When light interacts with a plasmonic nanoparticle, conduction electrons can oscillate collectively in response to the incident electromagnetic field. For discrete nanoparticles, this phenomenon is known as localized surface plasmon resonance (LSPR).
LSPR produces strong wavelength-dependent absorption and scattering as well as enhanced electromagnetic fields near the particle surface. The wavelength and intensity of the resonance depend on both the nanoparticle itself and its local environment.
What Controls the Plasmon Resonance?
For a small spherical nanoparticle, the quasi-static polarizability describes how readily the incident electromagnetic field induces charge separation within the particle:
In this relationship, ε1 represents the wavelength-dependent dielectric function of the nanoparticle and ε2 represents the dielectric function of the surrounding medium. For a small spherical particle, resonance occurs near the condition Re{ε1} = -2ε2.
This relationship helps explain why several variables strongly influence plasmonic optical behavior:
- Composition: Determines the underlying dielectric response and accessible spectral range.
- Particle size: Influences resonance position and the relative contributions of absorption and scattering.
- Particle shape: Introduces different resonance modes and enables broad spectral tuning.
- Surrounding refractive index: Changes the local dielectric environment and can shift the resonance wavelength.
Particle shape provides particularly strong control over optical response. Increasing the aspect ratio of a gold nanorod, for example, shifts its longitudinal plasmon resonance toward longer wavelengths, allowing optical properties to extend from the visible into the near-infrared.
Plasmon Coupling & Interparticle Spacing
Nanoparticle optical properties also depend on the proximity of other plasmonic particles. When particles move close together, their electromagnetic fields can interact. This plasmon coupling can shift and broaden the optical resonance and create highly localized electromagnetic fields between neighboring particles.
The effect becomes stronger as particle separation decreases and depends on particle size, shape, orientation, and surrounding medium. Plasmon coupling underlies the dramatic color changes that can occur during nanoparticle aggregation and supports many plasmonic sensing and surface-enhanced spectroscopy techniques.
Modeling the Optical Response of Plasmonic Nanoparticles
Optical modeling can connect nanoparticle structure with spectral performance and help guide particle selection and design. For spherical and concentric core-shell particles, Mie theory provides an analytical solution to Maxwell's equations for calculating extinction, absorption, and scattering.
The nanoComposix Mie Theory Calculator lets you explore how particle size, composition, shell structure, wavelength, and surrounding refractive index influence calculated optical response. More complex particle shapes, such as nanorods and nanocubes, require other numerical modeling approaches.
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Selected Literature
- Eustis, S. & El-Sayed, M. A. “Why gold nanoparticles are more precious than pretty gold: noble metal surface plasmon resonance and its enhancement of the radiative and nonradiative properties of nanocrystals of different shapes.” Chemical Society Reviews, 35(3), 209–217 (2006).
- Willets, K. A. & Van Duyne, R. P. “Localized surface plasmon resonance spectroscopy and sensing.” Annual Review of Physical Chemistry, 58, 267–297 (2007).
- Barnes, W. L., Dereux, A. & Ebbesen, T. W. “Surface plasmon subwavelength optics.” Nature, 424, 824–830 (2003).