Mechanical dissipation and phonon-induced decoherence in nanomechanical and optomechanical systems

• Applicant: Dr. Ignacio Wilson-Rae
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2012 to 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 204526181

• Final Report Year: 2014

Final Report Abstract

State of the art opto- and nano-mechanical setups are allowing for the first observations in macroscopic mechanical resonators of effects that reveal their fundamental quantum nature. Typical optomechanical setups are equivalent to a Fabry-Perot optical cavity where one of the end mirrors is mechanically compliant so that its motion is driven by the forces exerted by the stored light. Analogously nano-electromechanical setups involve a capacitor with a compliant electrode. These unprecedented developments will both, further the investigation of the foundations of quantum mechanics and provide promising alternatives for key technological applications, such as sensing at the single particle scale and tunable inter-conversion between optical and microwave signals. Important prerequisites for these advances are to (i) understand and control mechanical dissipation in these systems, and (ii) identify ways of inducing the sufficiently strong controllable interactions necessary to manipulate them at the level of single quanta. The latter applies, in particular, to ‘atomicsized’ nanomechanical structures such as carbon nanotubes, which are emerging as ideal candidates for these pursuits. In the context of (i), experiments have established across many different nanomechanical systems that subjecting the resonator material to high-stress dramatically reduces the dissipation. However the relation between stress and high mechanical quality is not fully settled. This project has made substantial progress in elucidating this connection by establishing precise relations between the degrees of mechanical quality that result with and without stress for a restricted but relevant class of models, in which dissipation is only induced by extensions and compressions inside the resonator. Our analysis suggests that current modelling of these effects of stress is, in general, only consistent when the mechanical dissipation is dominated by surface losses. In turn, we have proposed and analysed two promising optomechanical approaches to the second challenge (ii) for nanotube mechanical resonators. In these resonators a carbon nanotube is suspended across a gap forming a bridge that can oscillate sideways. The first approach is based on manipulating electronic excitations of a semiconducting nanotube known as excitons, which can be excited by light and whose energy can be modulated by stresses inside the nanotube. This modulation effectively couples the excitons to the nanotube’s motion, which in the quantum regime involves vibrational quanta known as phonons. Due to the small transverse dimensions, that involve around ten atoms, these exciton-phonon interactions are strong enough that a single exciton can substantially affect the motion of a nanotube mechanical resonator. We have shown that by applying suitable electric fields and shining a laser on the nanotube one can sequentially generate single excitons that upon recombining extract energy from the resonator. This generates a net cooling that can be exploited to prepare the resonator close to its lowest-energy state predicted by quantum mechanics: its quantum ground state. Furthermore, we have analysed how the same setup could be operated in regimes where the resonator will hybridise with the exciton leading to striking quantum signatures such as states where the nanotube resonator presents a coexistence of two opposite deflections. The second optomechanical approach that we have analysed in the context of (ii) involves exploiting an intrinsic nonlinearity of the nanotube resonator’s response which originates from the fact that when a bridge is deflected sideways its length suffers an extension. The relative importance of this effect can be enhanced by again applying electric fields that modify the energy of the nanotube. We have analysed the feasibility of a scheme where this strongly nonlinear mechanical system is placed in the vicinity of an optical microcavity excited by a laser, so as to drive the nanotube resonator towards a motional state revealing the inherent quantisation of energy predicted by quantum mechanics.

Publications

• Approaching intrinsic performance in ultra-thin silicon nitride drum resonators, J. Appl. Phys. 112, 064323 (2012)
  V.P. Adiga, B. Ilic, R.A. Barton, I. Wilson-Rae, H.G. Craighead, and J.M. Parpia
  
• Exciton-assisted optomechanics with suspended carbon nanotubes, New J. Phys. 14, 115003 (2012)
  I. Wilson-Rae, C. Galland, W. Zwerger, and A. Imamoglu
  
• Nonlinear nanomechanical resonators for quantum optoelectromechanics, Phys. Rev. A 89, 013854 (2014)
  S. Rips, I. Wilson-Rae, and M. J. Hartmann