All-optical in-depth detection of the acoustic wave emitted by a single gold nanorod

Asingle gold nanorod dropped on the surface of a silica substrate is used as a transient optoacoustic source of gigahertzhypersounds.We demonstrate the all-optical detection of the as-generated acousticwave front propagatingin the silica substrate. For this purpose, time-resolved femtosecond pump-probe experiments are performed in areflection configuration. The fundamental breathing mode of the nanorod is detected at 23 GHz by interferometry,and the longitudinal acousticwave radiated in the silica substrate is detected by time-resolved Brillouin scattering.By tuning the optical probe wavelength from 750 to 900 nm, hypersounds with wavelengths of 260–315 nmare detected in the silica substrate, with corresponding acoustic frequencies in the range of 19–23 GHz. To confirmthe origin of these hypersounds, we theoretically analyze the influence of the acoustic excitation spectrum onthe temporal envelope of the transient reflectivity. This analysis proves that the acoustic wave detected in thesilica substrate results from the excitation of the breathing mode of the nanorod. These results pave the way forperforming local in-depth elastic nanoscopy.Vibration GHz d'une nanoparticule unique : sonde d'élasticité locale à une échelle submicroniqu