Cryogenic Optomechanics with a Si3N4 Membrane and Classical Laser Noise

TL;DR

This paper presents a cryogenic optomechanical system with a Si3N4 membrane demonstrating high mechanical quality and resolved sideband operation, analyzing classical laser noise effects and discussing future quantum ground state cooling.

Contribution

It introduces a cryogenic optomechanical setup with a high-Q membrane, develops a theory for heterodyne detection with laser noise, and explores laser noise mitigation strategies.

Findings

Mechanical Q > 4 x 10^6 at 261 kHz

Resolved sideband operation achieved

Classical laser noise affects heterodyne measurements

Abstract

We demonstrate a cryogenic optomechanical system comprising a flexible Si3N4 membrane placed at the center of a free-space optical cavity in a 400 mK cryogenic environment. We observe a mechanical quality factor Q > 4 x 10^6 for the 261-kHz fundamental drum-head mode of the membrane, and a cavity resonance halfwidth of 60 kHz. The optomechanical system therefore operates in the resolved sideband limit. We monitor the membrane's thermal motion using a heterodyne optical circuit capable of simultaneously measuring both of the mechanical sidebands, and find that the observed optical spring and damping quantitatively agree with theory. The mechanical sidebands exhibit a Fano lineshape, and to explain this we develop a theory describing heterodyne measurements in the presence of correlated classical laser noise. Finally, we discuss the use of a passive filter cavity to remove classical laser noise, and consider the future requirements for laser cooling this relatively large and low-frequency mechanical element to very near its quantum mechanical ground state.