Cavity optomechanics has led to advances in quantum sensing, optical manipulation of mechanical systems, and macroscopic quantum physics. However, previous studies have typically focused on cavity optomechanical coupling to translational degrees of freedom, such as the drum mode of a membrane, which modifies the amplitude and phase of the light field. Here, we discuss recent advances in “imaging-based” cavity optomechanics – where information about the mechanical resonator’s motion is imprinted onto the spatial mode of the optical field. Torsion modes are naturally measured with this coupling and are interesting for applications such as precision torque sensing, tests of gravity, and measurements of angular displacement at and beyond the standard quantum limit. In our experiment, the high-Q torsion mode of a Si3N4 nanoribbon modulates the spatial mode of an optical cavity with degenerate transverse modes. We demonstrate an enhancement of angular sensitivity read out with a split photodetector, and differentiate the “spatial” optomechanical coupling found in our system from traditional dispersive coupling. We discuss the potential for imaging-based quantum optomechanics experiments, including pondermotive squeezing and quantum back-action evasion in an angular displacement measurement.
Torsion resonators loom large in the history of precision measurement; however their role in modern nanomechanics experiments is limited. In this presentation I will describe a new class of ultra-high-Q torsion nanoresonators fashioned from strained nanoribbons, and how they might be used for imaging-based quantum optomechanics experiments and chip-scale intertial sensing. Specifically, using an optical lever, we have resolved the rotation of one such nanoribbon with an imprecision 100 times smaller than the zero-point motion of its fundamental torsion mode, paving the way towards observation of radiation pressure shot noise in torque. We have also found that a strained nanoribbon can be mass-loaded without changing its torsional Q. We have used this strategy to engineer a chip-scale torsion pendulum with an ultralow damping rate of 7 micro-hertz, sufficient to resolve micro-g fluctuations of the local gravitational field.
We present a new class of ultra-high-Q nanomechanical resonators based
on torsion modes of high-stress nanoribbons, and explore their
application for quantum optomechanics experiments and precision
optomechanical sensing. Specifically, we show that nanoribbons made of
high stress silicon nitride support torsion modes which are naturally
soft-clamped, yielding dissipation dilution factors as high as 10^4
and Q factors as high as 10^8 for the fundamental mode. We show that
these modes can be read out with optical lever measurements with an
imprecision below that at the standard quantum limit, paving the way
for a new branch of torsional quantum optomechanics. We also show
that nanoribbons can be mass-loaded without changing their torsional Q
factor. We use this strategy to engineer a chip-scale torsion balance
with an damping rate of 10 micro-hertz. We use this torsion balance
as a clock gravimeter to sence micro-g fluctuation in the local
gravitational field strength.
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