Nature publishes new findings by Xu Haitan from PKU School of Physics

APR . 18 2019
Peking University, April 18, 2019: Nature has recently published a paper co-authored by Xu Haitan from PKU State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, Professor Jack Harris’ research team from Yale University, and Professor Aashish Clerk from University of Chicago. The paper, titled “Nonreciprocal control and cooling of phonon modes in an optomechanical system,” introduces a scheme that uses the standard cavity-optomechanical interaction to produce robust nonreciprocal coupling between phononic resonators. 

The harmonic oscillator model is very important in physics, because any mass subject to a force in stable equilibrium acts as a harmonic oscillator for small vibrations. Harmonic oscillators occur widely in nature and are exploited in many manmade devices, such as clocks and radio circuits.  They are the source of virtually all sinusoidal vibrations and waves. Reciprocity is a generic feature of linear, time-invariant oscillator systems, requiring the transmission of information or energy between two points in space to be symmetric for opposite propagating directions. While a nonreciprocal harmonic oscillator exhibits different received-transmitted field ratios when its source(s) and detector(s) are changed. It allows full duplex communication between the phonon modes and is of great importance to the isolation of phonics.

Optomechanics is a sub-field of physics involving the study of the interaction of electromagnetic radiation with mechanical systems via radiation pressure or the manufacture and maintenance of optical parts and devices. The phononic resonators they studied are two normal modes of a SiN membrane. The membrane is positioned inside a high-finesse optical cavity. Near-resonant coupling can be induced between these modes by modulating the dynamical backaction. Such modulation arises from the intracavity beat note produced when the cavity is driven by two tones. In this arrangement, a photon can scatter from one drive tone to the other by transferring a phonon between the modes. This process occurs via a virtual state in which the photon is at a mechanical sideband of the drive tones. The participation of the various mechanical sidebands can be enhanced or suppressed by the cavity’s resonance.

The energy-domain illustration of the optomechanical control scheme

Their experiments describe measurements which directly probe nonreciprocal devices’ internal degrees of freedom. This opens up the possibility of controlling the state of the resonators via their nonreciprocal interactions. They use the nonreciprocity described in their paper to modify the thermal fluctuations of the resonators and to realize a form of cooling with no equivalent in reciprocal systems. When the energy transport is reciprocal, thermal phonons are exchanged between the modes, tending to bring the temperatures closer together. By contrast, if given unidirectional energy transport, then the isolated mode emits thermal phonons into the other mode but not vice versa. This leads to cooling of the isolated mode and heating of the other mode, even if the former is initially the colder of the two.

Their work was supported by PKU State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, Forefront Research Center of Nano-Optoelectronics of Ministry of Education, and International Center for Quantum Materials .

Written by: Yang Hongyun
Edited by: Huang Yadan
Source: PKU News (Chinese)