Strain engineering for ultra-coherent nanomechanical oscillators

TL;DR

This paper demonstrates that combining geometric strain engineering with soft-clamping in nanomechanical resonators significantly enhances their quality factors and coherence, reaching unprecedented Q×frequency products at room temperature.

Contribution

It introduces a novel approach of using geometric strain combined with soft-clamping to achieve ultra-high Q nanomechanical resonators near the material yield strength.

Findings

Achieved Q×frequency products approaching 10^{15} Hz at room temperature.

Resonators can perform hundreds of quantum coherent oscillations at room temperature.

Devices attain Q > 400 million at radio frequencies.

Abstract

Elastic strain engineering utilizes stress to realize unusual material properties. For instance, strain can be used to enhance the electron mobility of a semiconductor, enabling more efficient solar cells and smaller, faster transistors. In the context of nanomechanics, the pursuit of resonators with ultra-high coherence has led to intense study of a complementary strain engineering technique, "dissipation dilution", whereby the stiffness of a material is effectively increased without added loss. Dissipation dilution is known to underlie the anomalously high Q factor of Si$_3$N$_4$ nanomechanical resonators, including recently-developed "soft-clamped" resonators; however, the paradigm has to date relied on weak strain produced during material synthesis. By contrast, the use of geometric strain engineering techniques -- capable of producing local stresses near the material yield strength -- remains largely unexplored. Here we show that geometric strain combined with soft-clamping can produce unprecedentedly high Q nanomechanical resonators. Specifically, using a spatially non-uniform phononic crystal pattern, we colocalize the strain and flexural motion of a Si$_3$N$_4$ nanobeam, while increasing the former to near the yield strength. This combined strategy produces string-like modes with room-temperature Q$\times$frequency products approaching $10^{15}$ Hz, an unprecedented value for a mechanical oscillator of any size. The devices we study can have force sensitivities of aN/rtHz, perform hundreds of quantum coherent oscillations at room temperature, and attain Q > 400 million at radio frequencies. These results signal a paradigm shift in the control of nanomechanical dissipation, with impact ranging from precision force microscopy to tests of quantum gravity. Combining the reported approach with crystalline or 2D materials may lead to further improvement, of as yet unknown limitation.