Ultralow loss torsion micropendula for chipscale gravimetry

Abstract: We explore a new class of chipscale torsion pendula formed by Si$_3$N$_4$ nanoribbon suspensions. Owing to their unique hierarchy of gravitational, tensile, and elastic stiffness, the devices exhibit damping rates of $\sim 10\;\mu$Hz and parametric gravity sensitivities near that of an ideal pendulum. The suspension nonlinearity can also be used to cancel the pendulum nonlinearity, paving the way towards fully isochronous, high $Q$ pendulum gravimeters. As a demonstration, we study a 0.1 mg, 32 Hz micropendulum with a damping rate of $16\;\mu$Hz, a thermal acceleration sensitivity of $2\;\text{n}g/\sqrt{\text{Hz}}$, and a parametric gravity sensitivity of $5$ Hz/$g_0$. We record Allan deviations as low as 2.5 $\mu$Hz at 100 seconds, corresponding to a bias stability of $5\times 10^{-7}g_0$. We also demonstrate a 100-fold cancellation of the pendulum nonlinearity. In addition to inertial sensing, our devices are well suited to proposed searches for new physics exploiting low-loss micro- to milligram-scale mechanical oscillators.