  We demonstrate multimode optomechanical sensing of individual nanoparticles
with radius of a hundred of nanometers. A semiconductor optomechanical disk
resonator is optically driven and detected under ambient conditions, as
nebulized nanoparticles land on it. Multiple mechanical and optical resonant
signals of the disk are tracked simultaneously, providing access to several
physical informations about the landing analyte in real-time. Thanks to a fast
camera registering the time and position of landing, these signals can be
employed to weigh each nanoparticle with precision. Sources of error and
deviation are discussed and modeled, indicating a path to evaluate the
elasticity of the nanoparticles on top of their mere mass. The device is
optimized for future investigation of biological particles in the high
megadalton range, such as large viruses.
