Optomechanical, vibrational and thermal properties of graphene nanoresonators
Graphene is an atomically thin material which is very light, displays an extremely high in-plane stiffness and interacts efficiently with light and electrical fields. Therefore graphene-based mechanical oscillators are very interesting for studying coupling mechanisms, in particular in the context of ultra-sensitive force sensing. However, nanoscale membranes are highly sensitive to any intrinsic heterogeneity because of their high surface to volume ratio. The former can strongly modify their vibrational properties.
I investigate the dynamics of suspended graphene mechanical resonators with an optomechanical detection scheme at the shot noise limit. An optical cavity-free sample design is employed which allows for optical access to the graphene membrane from opposite sides and for spatially resolved pump-probe measurements. I find that spatially inhomogeneous dissipation strongly modifies the thermal noise of a coupled graphene nanoresonator system, while the fluctuation dissipation theorem remains valid. By coupling graphene to a higher Q resonator, the signal-to-noise ratio can be improved in a certain frequency range.
Subsequently, I investigate the thermal properties of suspended graphene with a built-in microstructure by measuring the membrane's static and dynamical response to a thermal wave generated by a tuning laser. This paves the way for a novel method to access the sample's thermal conductivity without physically contacting it, using direct optical imaging of the thermal wave propagating inside the graphene membrane. The spatial dependence of the thermal conductivity is found to be strongly correlated with intrinsic heterogeneities of the graphene.