  We use an all-optical pump-probe method to develop a mass sensing mechanism
in a molecular plasmonic system at room temperature. The system consists of a
double-clamped graphene nanoribbon that parametrically interacts with two types
of isotropic and anisotropic spherical plasmonic cavities in the presence of a
strong pump field and a weak probe pulse. Based on the mode-selective
quantization scheme and analogy with the canonical model of the cavity
optomechanics, we formulate the Hamiltonian of the system in terms of the
electromagnetic Greens tensor. In this manner, we derive an explicit form of
size-dependent optomechanical coupling function and plasmonic damping rate,
which include the modal, geometrical, and material features of the plasmonic
structure. Engineering material features of the plasmonic nanostructure, we
find that the intensity of the probe field transmission spectrum for radially
anisotropic spherical nanocavity enhances significantly compared to the silver
sphere nanocavity due to the mode volume reduction. This scheme can provide to
achieve the minimum measurable mass at room temperature.
