Picosecond acoustics for mass characterization of nanoparticles on thin membranes

In this paper, we demonstrate the potential of laser picosecond acoustics for characterizing the averaged mass of nanoparticles distributed on the surface of submicron-thin membranes. Specifically, we investigate guided-wave (GW) propagation in thin membranes covered with silicon oxide nanospheres. By employing picosecond acoustics, we excite and detect GWs, generating spatiotemporal maps of acoustic behavior that visualize the GW dispersion curves. Such dispersion curves are also calculated using a computer model, which implements a semianalytical Green's-matrix-based solution to the corresponding boundary-value problem. Based on these data, inverse problems are solved to determine the effective elastic parameters of the membrane and the average mass of particles per unit surface. The method exhibits high sensitivity in detecting masses as small as 200 femtograms per square micron. The wide field of view provided by the picosecond technique enables efficient scanning of millimetric surfaces with the potential to detect mass variations at the hundred-femtogram level over large areas.