Cavity Optomechanics Radiation pressure cooling of a micromechanical resonator

At the beginning of last century Einstein derived the statistics of the radiation pressure force fluctuactions acting on a moveable mirror. Later, pioneering experiments about the role of radiation pressure and its ability to provide cooling for large objects were carried on by Braginsky in the context of interferometers during the 60s. The dynamical influence of radiation pressure on a harmonically suspended end-mirror of a cavity was investigated, revealing that the retarded nature of the force provides either damping or anti-damping of mechanical motion. The first cavity optomechanical experiment in the optical domain demonstrated bistability of the radiation pressure force acting on a macroscopic end-mirror. The fundamental consquences of the quantum fluctuations of radiation pressure impose a limit on how accurately the position of a test mass can be measured and the standard quantum limit for continuous position detection was then established. The original task concerning my PhD Thesis is the observation of radiation pressure acting on a mechanical object due to light circulating inside a Fabry-Perot cavity. This evidence would trace the path to observe the standard quantum limit through optical methods allowing to perform quantum measurements, and the realization of coherent superposition of the states of macroscopic objects. A plethora of different systems was introduced starting from the last decades of the 20th century in order to explore both theoretically and experimentally the behaviour of optomechanical systems. On the experimental side the most part of the approaches were to miniaturize the investigated structure. Optomechanical effects of retarded radiation forces were observed in microscale setups. However, the task of producing high quality optical cavities below mm-scale remains quite challeging. The route that was followed is the membrane-in-the-middle setup. Previous theoretical proposals and experimental implementations have analysed the possibility of setting up this kind of research using a movable mirror as end mirror of the FP cavity. Later the research focused on a new interpretation of the problem, placing a SiN membrane (a few tens of nanometers of thickness) between the two mirrors. We performed a detailed study of dynamical back-action cooling and of radiation pressure induced modifications of the mechanical properties of the membrane. In particular we have studied how the mechanical susceptibility is modified by such interaction between radiation pressure and a driven cavity mode. We finally observed that the increase of mechanical damping is equivalent to cooling of the vibrational mode and we measured its effective temperature in three different ways, obtaining consistent results, in agreement with theoretical expectations.