  High-speed tracking of single particles is a gateway to understanding
physical, chemical, and biological processes at the nanoscale. It is also a
major experimental challenge, particularly for small, nanometer-scale
particles. Although methods such as confocal or fluorescence microscopy offer
both high spatial resolution and high signal-to-background ratios, the
fluorescence emission lifetime limits the measurement speed, while
photobleaching and thermal diffusion limit the duration of measurements. Here
we present a tracking method based on elastic light scattering that enables
long-duration measurements of nanoparticle dynamics at rates of thousands of
frames per second. We contain the particles within a single-mode silica fiber
containing a sub-wavelength, nano-fluidic channel and illuminate them using the
fiber's strongly confined optical mode. The diffusing particles in this
cylinderical geometry are continuously illuminated inside the collection focal
plane. We show that the method can track unlabeled dielectric particles as
small as 20 nm as well as individual cowpea chlorotic mottle virus (CCMV)
virions - 4.6 megadaltons in size - at rates of over 2 kHz for durations of
tens of seconds. Our setup is easily incorporated into common optical
microscopes and extends their detection range to nanometer-scale particles and
macromolecules. The ease-of-use and performance of this technique support its
potential for widespread applications in medical diagnostics and micro total
analysis systems.
