In the nanoscale world, nanoparticles are measured in billionths of a meter, which often make them only a little bit larger than the size of atoms. Because these nanoparticles are typically smaller than the wavelengths of visible light––which varies from 700 nanometers for red light to 400 nanometers for violet light–– they are literally invisible to even the most powerful optical microscopes.
Now, scientists at Los Alamos have constructed a novel device for "seeing" tiny metal nanoscale particles by combining sub-wavelength, near-field imaging with broadband interference spectroscopy that uses the high-intensity illumination produced by an ultrafast laser–– a laser that emits pulse durations lasting only of a few quadrillionths of a second. The technique could help scientists around the world gain a deeper understanding of the largely unseen nanoscale world.
The design of the device, along with details of how it was recently used for studies of collective oscillations of electrons in individual gold nanoparticles and their assemblies, are discussed in last week's issue of the journal Optics Letters.
The technique begins by directing light through a thin optical fiber that has been previously heated and stretched until, like stretched taffy, the middle becomes far thinner than the ends. This tapered fiber is then cut at its thinnest diameter and clad in aluminum to create––in effect, a tiny "nanoscale flashlight" –– with an aperture only 50 to 100 nanometers across.
"Since there are no white-light lasers that would make it possible to "see" nanoparticles in more than one wavelength of the visible light spectrum," says Victor Klimov, leader of the research team, "it was necessary for our team to create a high-intensity illumination source for the optical fiber. We did this by focusing the beam of an ultrafast laser onto a transparent sapphire plate, which converted the single-wavelength laser output into a broadband spectrum of high- intensity light that is somewhat equivalent to white light and, therefore, is referred to as "femtosecond white-light continuum."
The important property of the "femtosecond white-light continuum" is its low, laser-beam-like divergence that allows researchers to efficiently couple it into an optical fiber and to create a high-intensity, multi-color, near-field light source.
For use, the "nanoscale flashlight" was positioned just a few nanometers away from a sample mounted on a near-field scanning optical microscope. As the emitted light is transmitted past and through the sample, a photomultiplier tube, a device that amplifies the effect of a single photon to measurable levels, collects and measures it. This signal is used to reconstruct a nanoscale image while the near-field tip is raster-scanned across the sample. At the same time, the transmitted light is also dispersed by a spectrometer and is detected by a CCD recording device to create a broadband absorption/extinction spectrum for each sample point. The combined "multidimensional" data is giving scientists their first real look into the nanoscale world.
Because of its ability to both image the nanostructure and to interrogate it spectroscopically, the instrument developed by Los Alamos researchers is ideally suited to guide the design of nanophotonic and nanoplasmonic structures and devices. In addition, this new capability may provide a powerful new tool for "real-time" studies of electronic dynamics at the nanoscale level with high resolutions in both time and spatial domains.