We use ultra-short laser pulses to probe the mechanical properties of individual gold nanoparticles and answer such questions as: Are the elastic properties of a 10 nm object the same as those of a macroscopic object? How is the vibration energy dissipated into the environment? In liquid environments, how do viscosity and acoustic impedance influence dissipation? Gold nanoparticles are strong light absorbers, so that mechanical vibrations are launched by absorption of a laser pulse. Heat deposition occurs on a timescale shorter than the vibration period (10-100 ps), giving rise to sudden thermal expansion and to an increase in electron pressure. The mechanical vibrations are then followed in time by monitoring the plasmon resonance of the particle, whose spectral position accompanies the particle’s size and shape oscillations. The main advantage of single-particle studies is the complete elimination of heterogeneity of size and shape in ensembles of nanoparticles. This gives access to intrinsic damping mechanisms, which are often hidden in ensemble measurements.
The vibrations of single particles live significantly longer than the signals detected on ensembles, enabling a more precise determination of the vibration frequencies. We have investigated gold particles of various shapes (nanorods, pairs of spheres or dumbbells) deposited on glass substrates. The next step will be to repeat these measurements on particles in a liquid (water) by means of an optical trap (see this project).