Plasmonics | Nanostructures

Plasmon Resonant Nanoparticles

Our group specializes in the study of plasmon resonant nanoparticles. Plasmon resonant nanoparticles are nanosized (~40–100 nanometers in diameter) metallic particles (usually silver or gold) that scatter light with remarkable efficiency. The reason that nanoparticles respond to light so strongly is because the conduction electrons in the metal undergo a collective resonance called a surface plasmon resonance. The magnitude, peak wavelength and spectral bandwidth of the plasmon resonance associated with a nanoparticle are dependent on the particle's size, shape, and material composition, as well as its local dielectric environment.

In the movie at the top of this page, a darkfield microscope image of silver nanoparticles is shown. The nanoparticles can be individually identified under the microscope and their spectra acquired. The same set of nanoparticles can also be imaged by an electron microscope to ascertain their physical characteristics (see below).

Nanoparticle Spectroscopy

We utilize a high magnification optical microscope to observe plasmonic nanoparticles. Because the plasmon resonant nanoparticles scatter optical light with such great efficiency, we are able to observe individual nanoparticles and acquire their spectra. Although the nanoparticles are much smaller than the wavelength of visible light, their apparent size in the microscope images is set by the diffraction limit, set by the aperture of the microscope objective used. The nanoparticles thus appear as bright, colorful circles that are about half of one micrometer. Comparison between the electron and the optical microscope images of a group of these silver nanoparticles (as shown below left) demonstrates the diffraction effect and the mapping technique we use to correlate the physical and spectral characterisics of the nanoparticles. The Dependence of the Plasmon Resonance (color) on the Index of Refraction of the surrrouding medium can be observed as a change in scattering color by Darkfield microscopy (as shown below right).

Correlation of optical spectra with transmission electron microscope image. By analyzing individual nanoparticles, we can relate the optical characteristics of plasmonic nanoparticles to their physical properties.

The nanoparticle plasmon resonance is dependent upon the surrounding dielectric environment, as shown here for a group of Ag nanoparticles on a glass slide in air and 1.5 index oil. Note the spectral shift.

Dark-Field Microscopy

Because we are interested in the light that is reflected just from the nanoparticles, we must employ methods by which the incident light is rejected from the final image. These methods are collectively known as darkfield imaging;darkfield illumination is necessary to maximize the scattering from the nanoparticles while minimizing the background scattering from the substrate. There are a number of illumination schemes we utilize which meet this requirement, such as standard darkfield (utilizing commercially available darkfield objectives); total internal reflection (evanescent field illumination); and brightfield oil immersion.

Standard darkfield.
Total internal reflection imaging.
Oil immersion darkfield imaging.

The optical darkfield microscope we employ is a custom design utilizing Nikon BF/DF optics, a Prior mechanical stage, a Z-scope focusing column, custom built image plane rotate-able pinhole aperture, Acton Spectra Pro spectrometer, Photometrics CCD detectors for imaging (CoolSnap ES) and spectroscopy (CoolSnap HQ), and Nikon D70 color digital imaging camera. The scope is also equipped with a Prism sample mount and goniometer controlled T.I.R. white light illumination from a fiber coupled Xe source. Darkfield illumination can thus be achieved through a variety of schemes as illustrated above. Additionally, the microscope is equipped with laser excitation (and notch and laser line filters required for SERS) at 457nm, 488nm, 514nm, 632nm and 785nm. Tightly focused laser excitation of a diffraction limited spot is achieved in epi-illumination mode. Alternatively, we can excite a wide-field area with the laser by directing a focused the beam onto the sample in the space provided by the extra long working distance (ELWD) 50x objective.