Strained crystalline nanomechanical resonators with ultralow dissipation*

In strained mechanical resonators, the concurrence of tensile stress and geometric nonlinearity dramatically reduces dissipation. This phenomenon, dissipation dilution, is employed in mirror suspensions of gravitational wave interferometers and at the nanoscale, where soft clamping and strain engineering have allowed extremely high quality factors. However, these techniques have so far mostly been applied in amorphous materials, specifically Si₃N₄. Crystalline materials exhibit significantly lower intrinsic damping at cryogenic temperatures, due to the absence of two level systems in the bulk, and engineering dissipation dilution in strained crystalline mechanical resonators could enable exquisite force sensors and optomechanical transducers, due to the combination of reduced internal friction, high intrinsic strain, and high yield strength. Here, we demonstrate that single crystal strained silicon, a material developed for high mobility transistors, can be used to realize ultracoherent mechanical resonators. We fabricate high aspect ratio (> 10⁵ ) nanostrings supporting MHz mechanical modes with Q > 10¹⁰ at 7 K, the highest reported at liquid He temperatures. We characterize thoroughly the dissipation temperature dependence for localized and non-localized modes. Finally, we will discuss our progress in characterizing the nanostrings at He dilution temperatures, where the scaling laws of single-crystal materials hint at outstanding mechanical performance.