Nanomechanical resonators are exquisite force sensors and have recently been used to “feel” the vacuum fluctuations of a laser field. I’ll describe a system consisting of a glass nanostring coupled to an optical microcavity and how it has been used to not only sense radiation pressure shot noise, but also squeeze it, to cool a vibration of the string to near its ground state, and to witness its zero-point energy as motional sideband asymmetry—all long-standing goals in the field of optomechanics. Underlying these advances are new insights into dissipation of nanomechanical resonators. Combining strain and mode-shape engineering, we've recently fabricated strings with effective masses of picograms, frequencies of megahertz, and quality factors approaching 1 billion at room temperature. These numbers spark the imagination, inviting speculation about applications ranging from ultrasensitive accelerometry to tests of quantum collapse models.
The pursuit of resonators with ultra-high coherence has led to intense study of “dissipation dilution”, where the stiffness of a material is effectively increased without added loss. Interestingly, the paradigm has to date relied on weak strain produced during material synthesis—the use of geometric strain engineering techniques, capable of producing local stresses near the material yield strength, remains largely unexplored. We show that geometric strain combined with soft-clamping can produce exceptionally high Q mechanical oscillators.
Loss due to curvature at the clamping points has been a major limiting factor in engineering of high Q mechanical oscillators. Soft-clamped resonators recently developed by Tsaturyan et al. bypass this limitation by localizing the mode away from the clamping points. We adapt their technique to 1D nanobeams. By corrugating the width of the nanobeam, we create a bandgap in the mechanical mode spectrum. A defect in the center of the beam allows a localized mode to exist in the bandgap. Then, by tapering the overall width of the nanobeam, we increase the stress in the center of the beam to near the yield strength. This combined strategy produces picogram-mass flexural modes with room-temperature Q factors as high as 800 million and Qf products of 10^15 Hz—both unprecedented for a mechanical oscillator of any size. Additionally, strain-engineering allows us to tune the frequency of the ultra-high Q mode from 1-6 MHz while retaining Qf products above 10^14 Hz. The extremely low dissipation and low effective mass of these devices make them promising for performing cavity quantum optomechanics.