Nonlinear optical studies of single gold nanoparticles

Gold nanoparticles are spherical clusters of gold atoms, with diameters typically between 1 and 100 nanometers. The applications of these particles are rather diverse, from optical labels for biological experiments to data carrier for optical data storage. The goal of my project was to develop new methods to study the physical properties of single gold nanoparticles on ultra-short timescales. Exciation with a short laser pulse brings a nanoparticle out of equilibrium, which makes it vibrate with a period that depends on the particle diameter and the speed of sound in gold. The vibrational period of a gold nanoparticle with a diameter of 60 nanometer is 20 picoseconds. This acoustic vibration has been detected by us for the first time for single particles. The main advantage of single-particle studies over bulk detection of these particles lies in the fact that all particles in an ensemble vibrate at slightly different frequencies, which causes increased damping due to dephasing. The damping of the vibrations of single particles only depends on the elastic coupling between the particle and its environment, which offers the possibility of using these particles as mechanical nanosensors.