NONLINEAR TRANSDUCTION OF BROWNIAN MOTION IN A NANO-OPTOMECHANICAL SYSTEM

The motion of tiny resonators can be probed with high sensitivity, employing systems that are both mechanical and optical resonators at the same time and in which the mechanical mode of vibration is coupled to the optical field. Optomechanical interactions can be especially effective when light is stored for significant time in a small optical cavity. Due to the coupling between the optical and mechanical degrees of freedom, exchange of energy can cause a transfer of information between light and mechanics. Sensitive readout of mechanical motion through an optical signal is therefore possible allowing to realize sensors of tiny forces or small masses. In this thesis, we explore the response of a photonic crystal cavity to thermal fluctuations of the environment, that causes brownian motion of the resonator itself. Ultimately we investigate a regime of strong interaction in which the response of the system is nonlinear, due to nonlinear dynamics involved in the process of transduction. In this work, different readout methods are investigated, together with a numerical model for the prediction of the mechanical spectra. We compare our model to experiments, in which thermal fluctuations (for temperature ranging from 3 to 300 K) and optomechanical coupling are large enough to cause strongly nonlinear transduction. The results obtained are applicable to other systems and especially to those that could approach the regime of quantum nonlinear optomechanical coupling in the near future.