2 research outputs found
Thermal intermodulation noise in cavity-based measurements
Thermal frequency fluctuations in optical cavities limit the sensitivity of
precision experiments ranging from gravitational wave observatories to optical
atomic clocks. Conventional modeling of these noises assumes a linear response
of the optical field to the fluctuations of cavity frequency. Fundamentally,
however, this response is nonlinear. Here we show that nonlinearly transduced
thermal fluctuations of cavity frequency can dominate the broadband noise in
photodetection, even when the magnitude of fluctuations is much smaller than
the cavity linewidth. We term this noise "thermal intermodulation noise" and
show that for a resonant laser probe it manifests as intensity fluctuations. We
report and characterize thermal intermodulation noise in an optomechanical
cavity, where the frequency fluctuations are caused by mechanical Brownian
motion, and find excellent agreement with our developed theoretical model. We
demonstrate that the effect is particularly relevant to quantum optomechanics:
using a phononic crystal membrane with a low mass, soft-clamped
mechanical mode we are able to operate in the regime where measurement quantum
backaction contributes as much force noise as the thermal environment does.
However, in the presence of intermodulation noise, quantum signatures of
measurement are not revealed in direct photodetectors. The reported noise
mechanism, while studied for an optomechanical system, can exist in any optical
cavity
Hierarchical tensile structures with ultralow mechanical dissipation
Structural hierarchy is found in myriad biological systems and has improved
man-made structures ranging from the Eiffel tower to optical cavities.
Hierarchical metamaterials utilize structure at multiple size scales to realize
new and highly desirable properties which can be strikingly different from
those of the constituent materials. In mechanical resonators whose rigidity is
provided by static tension, structural hierarchy can reduce the dissipation of
the fundamental mode to ultralow levels due to an unconventional form of soft
clamping. Here, we apply hierarchical design to silicon nitride nanomechanical
resonators and realize binary tree-shaped resonators with quality factors as
high as at 107 kHz frequency, reaching the parameter regime of levitated
particles. The resonators' thermal-noise-limited force sensitivities reach
at room temperature and $\mathrm{90\
zN/\sqrt{Hz}}$ at 6 K, surpassing state-of-the-art cantilevers currently used
for force microscopy. We also find that the self-similar structure of binary
tree resonators results in fractional spectral dimensions, which is
characteristic of fractal geometries. Moreover, we show that the hierarchical
design principles can be extended to 2D trampoline membranes, and we fabricate
ultralow dissipation membranes suitable for interferometric position
measurements in Fabry-P\'erot cavities. Hierarchical nanomechanical resonators
open new avenues in force sensing, signal transduction and quantum
optomechanics, where low dissipation is paramount and operation with the
fundamental mode is often advantageous.Comment: 19 pages, 11 figures. Fixed link to Zenodo repositor