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Soutenance de thèse

Vendredi 15 Janvier à 14h00,
salle Nevill Mott, D420

Oratrice : Cornelia SCHWARZ
"Optomechanical, vibrational and thermal properties of suspended graphene membranes"

Abstract :

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.

In this work we investigate the vibrational properties of graphene grown by chemical vapour deposition and suspended over up to 20 µm in a drum geometry. The large dimension of the membranes allows us to characterize their spatial properties by various methods. In particular, we were able to map the built-in mechanical strain and charge doping distribution deduced from Raman measurements.

Furthermore, we investigated the graphene membrane dynamics with a new optomechanical approach employing a balanced homodyne detection at the shot noise limit. To this end we developed a sample design, where optical cavity effects are eliminated and the graphene can be accessed optically from opposite sides allowing for spatially resolved pump-probe measurements. We 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 we investigated the thermal properties of intrinsically microstructured suspended graphene by measuring the membrane’s static and dynamical response to a thermal wave generated by a tuning laser. We propose the latter as a novel method to access the sample’s thermal conductivity without physically contacting it, using a direct optical imaging of the thermal wave propagating inside the graphene sample. The spatial dependence of the thermal conductivity is found to be strongly correlated with intrinsic heterogeneities of the membrane.

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