Integrated tuning fork nanocavity optomechanical transducers with high fMQM product and stress-engineered frequency tuning

Abstract

Cavity optomechanical systems are being widely developed for precision force and displacement measurements. For nanomechanical transducers, there is usually a trade-off between the frequency (f M ) and quality factor (Q M ), which limits temporal resolution and sensitivity. Here, we present a monolithic cavity optomechanical transducer supporting both high f M and high Q M . By replacing the common doubly-clamped, Si 3 N 4 nanobeam with a tuning fork geometry, we demonstrate devices with the fundamental f M ≈ 29 MHz and Q M ≈ 2.2×105 , corresponding to an f M Q M product of 6.35×10 12 Hz, comparable to the highest values previously demonstrated for room temperature operation. This high f M Q M product is partly achieved by engineering the stress of the tuning fork to be 3 times the residual film stress through clamp design, which results in an increase of f M up to 1.5 times. Simulations reveal that the tuning fork design simultaneously reduces the clamping, thermoelastic dissipation, and intrinsic material damping contributions to mechanical loss. This work may find application when both high temporal and force resolution are important, such as in compact sensors for atomic force microscopy.Cavity optomechanical systems are being developed for many applications in precision force and displacement measurements 1,2 . Monolithic systems in which nanomechanical transducers are combined with integrated optical readout have been developed in geometries where optical resonances and mechanical modes are co-located within the same physical structure 3-7 , and in systems for which optical and mechanical modes are supported by different physical structures and are near-fieldcoupled 8,9 . Such near-field coupling enables the mechanical resonator size to be scaled down to the nanoscale while maintaining high displacement sensitivity 8,9 in contrast to far-field optical readout, where diffraction effects limit the mechanical resonator size that can be sensitively detected 10 . For a given desired mechanical stiffness (determined by the force sensing application), a nanoscale cantilever can have much higher resonant frequency f M (and therefore transduction bandwidth/temporal resolution) than a microscale counterpart, due to its smaller effective motional mass (m). Such a high frequency mechanical resonator would ideally exhibit a high mechanical quality factor (Q M ), as the force sensitivity scales as 1/(fHowever, there is usually a tradeoff between f M and Q M because shrinking down the resonator size comes at the expense of a reduction in Q M , due to increased clamping losses 11,12 . Here, we demonstrate an integrated silicon nitride cavity optomechanical system where high f M and Q M are simultaneously achieved. Using microdisk optical resonators with intrinsic optical quality factor Q o > 6×10 5 for readout, we develop a doubly-clamped tuning fork geometry as a) Electronic mail: yliu11@wpi.edu the mechanical resonator, where we take advantage of elastic wave interference to limit the mechanical loss, while retaining th...

Responsivity

“…3.8. The former configuration is often used in optomechanics [23][24][25]. Here fluctuations in the laser power could result in frequency noise of the mechanical resonator.…”

Mechanical Resonators for Quantum Optomechanics Experiments at Room Temperature

“…In order to achieve such remarkably low dissipation rates of Γ m =2π ¼ f=Q m ≈ 1.4 mHz with a tethered system, we design ultrathin high-stress Si 3 N 4 membranes which enhance the intrinsic stress in crucial tether regions-significantly reducing clamping and bending losses [29]. A key observation is that high-stress membranes have mechanical frequencies which are stress dominated, meaning that one can minimize the thickness of the resonator in order to reduce bending losses without significantly reducing the mechanical mode frequencies.…”

Efficient anchor loss suppression in coupled near-field optomechanical resonators

“…Therefore, although very high Q mechanical resonators have been reported, they can not always be easily integrated with optical cavities using large-scale photonic integration technology. On the other hand, exploring the destructive interference of mechanical waves is a simple method to obtain both low [24] and high [25] frequency high-Q mechanical resonators, without impacting the device's design or footprint. Yet, the constraints of simultaneously supporting optical and mechanical modes still challenge optomechanical device's design.…”