Wednesday June 29, 2005
Thursday June 30, 2005

"Optical detection methods for nanoparticles and viruses"
Mr. Filipp Ignatovich
University of Rochester

Currently existing optical single particle sensors are capable of detecting single particles down to 200nm in diameter. The main difficulty to detect smaller particles is due to a strong dependence of the measured signal on particle size. Typically, light scattered by a particle is collected by one or an array of photodetectors that measure power of the scattered light, hence the signal depends on the sixth power of the particle size. Attempts to reduce the lower detection size limit require a dramatic improvement in the noise levels, which often is not possible due to scattering in the media or the optical elements. We developed two methods for single nanoparticle detection, which are based on weaker signal dependence on particle size. In one approach, nanoparticles are recognized by measuring the optical force acting on nanoparticles in a strongly focused laser beam. As particles are moving through the laser focus, their motion is perturbed by the optical gradient force. The magnitude of the perturbation depends on particle size. In another approach, we measure the scattered electric field strength in an interferometric setup. The configuration is background-free and allows us to detect polystyrene beads in water down to 15nm in radius, as well as live viruses.