COOLING AND DISSIPATION IN OPTOMECHANICAL AND NANOMECHANICAL SYSTEMS

Thursday, October 14, 12h00 IFAE: Dr. Ignacio Wilson-Rae, Technische Universität München, Germany.

State of the art optomechanical and nanomechanical setups are close to allowing for the observation of quantum effects in a "macroscopic" mechanical system. Major challenges that remain to be addressed are: (i) understanding and controlling mechanical dissipation at low
temperatures, and (ii) finding practical ways to demonstrate quantum signatures. An important prerequisite for the latter is cooling the relevant mechanical resonator to a sufficiently low effective
temperature. Here we address such issues focusing on laser cooling of mechanical resonators and mechanical dissipation in the quantum regime. Thus, we discuss cavity-assisted backaction cooling and show how in the resolved sideband regime, where the mechanical frequency exceeds the cavity linewidth, this technique allows to prepare the mechanical resonator close to its quantum ground state. We also analyze analogous optomechanical effects in a suspended semiconducting
carbon nanotube induced by exciton-phonon interactions.

In the context of mechanical dissipation [(i)] we analyze the contribution from the unavoidable coupling of the resonator to the vibrations of its substrate (known as clamping losses). Our "phonon
tunneling" approach leads to a "master formula" for the design-limited dissipation 1/Q that is applicable to a very wide range of high-Q resonators including planar structures (e.g. bridges), pedestal geometries (e.g. microdisks), and single-walled carbon nanotubes. Based on this master formula, we have developed an efficient FEM-enabled solver that can be used as an aid to the design of complex resonators. We apply this concept to free-free micromirror structures relevant for Fabry-Perot based optomechanics. In addition, this design allows to isolate support-induced losses from other dissipation channels. Thus, we perform a rigorous test of the theory developed and demonstrate the strong geometric dependence of this loss mechanism. Furthermore, we analyze the case of high-stress nanomechanical resonators and test the theory on silicon nitride membranes with circular and square geometries. For the latter the measured Q-values of different
harmonics present a striking non-monotonic behavior which is successfully explained in terms of interference between the radiated waves.