In experiments using a device dubbed the "nanoscale flashlight," a team of Los Alamos National Laboratory scientists have applied a new nanoscale spectroscopic technique to studies of the collective oscillations of electrons in individual gold nanoparticles and their assemblies. A deeper understanding of these oscillations and their interactions will not only provide a foundation for research in the new and emerging field of nanoplasmonics, but may have practical applications in the ultrasensitive detection of chemical and biological molecules.
The details of the gold nanoparticles studies, along with information about the design of the imaging/spectroscopic device, are discussed in last week's issue of the journal Optics Letters.
Ever since the English scientist, Michael Faraday's first experiments on gold colloids, gold nanoparticles have been characterized by very strong absorption in the green region of the visible light spectrum, as measured by a absorption spectrometer. Using nanoscale spectrometry, however, Los Alamos researchers discovered that a single gold nanoparticle was indeed "absorbing" light, but only above the plasmon resonance. Below the plasmon resonance, the particle "transmitted" more light than was sent onto it. The puzzling behavior is an illustration of the fact that physics at the nanoscale level can be quite different from what is seen in the "macroworld."
In their experiments, a team of Los Alamos researchers applied the "nanoscale flashlight" to measure spectrally resolved absorption of individual gold nanoparticles. They illuminated a nanoparticle through a tiny, 50-nanometer aperture using a femtosecond white-light continuum as a source. The transmitted light was spectrally dispersed to produce the nanoscale absorption spectra.
The team's principal investigator, Victor Klimov, explains that in this experiment the use of the new technique allowed the visualization of a so-called "nanoantenna" effect, that is, the re-emission of the secondary radiation by excited plasmon oscillations.
Because the oscillations of electrons result in periodic modulations of surface charges, they are called surface plasmons. Plasmon oscillations lead to the generation of local electric fields and the local fields associated with plasmons become particularly strong if the frequency of the excited light is approaching the frequencies of so-called plasmon resonances. The control of these resonances via the nanoscale materials engineering represents an important area of nanoplasmonics, the field that studies surface plasmons in nanoscale metal systems.
Of the many potential practical applications of nanoplasmonics, perhaps the greatest interest is in methods for the ultrasensitive detection of chemical and biological molecules, which can be characterized by the strong "local" fields associated with surface plasmons. The instrument developed at Los Alamos is ideally suited to advance the field of nanoplasmonics by allowing researchers to not only "see" at the nanoscale, but also to perform nanoscale spectroscopy.