Nanotechnology Now - Press Release: One string to rule them all

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Abstract:
Extreme stresses can be produced in nanoscale structures, a feature which has been used to realize enhanced materials properties, such as the high mobility of silicon in modern transistors. Here we show how nanoscale stress can be used to realize exceptionally low mechanical dissipation, when combined with “soft-clamping” — a form of phononic engineering. Specifically, using a non-uniform phononic crystal pattern, we colocalize the strain and flexural motion of a free-standing Si3N4 nanobeam. Ringdown measurements at room temperature reveal string-like modes with quality (Q) factors as high as 800 million and Q × frequency exceeding 1015 Hz. These results illustrate a promising route for engineering ultra-coherent nanomechanical devices.

One string to rule them all

Lausanne, Switzerland | Posted on April 17th, 2018

Elastic strain engineering is a technique that exploits the change in materials’ properties due to stress, and can lead to remarkable and unusual device functionalities, relevant to fundamental science and engineering alike. Stress can change electronic, optical and thermal transport properties and is in fact already of major commercial and technological importance (most of which we are unaware): For instance, strain engineering enables increased mobility in the silicon transistors used in virtually all chips today. By applying large stress to the silicon channel, electron mobility can be increased beyond the naturally occurring values.

In our manuscript we show for the first time that elastic strain engineering, combined with recently emerged soft-clamping designs, can enable unprecedentedly low dissipation in nanomechanical oscillators. In our work we are able to attain quality factor-frequency) products approaching 1015 Hz at room temperature. This implies that the mechanical oscillator is undergoing hundreds of oscillations in its quantum decoherence time. We achieve this unparalleled performance by introducing a tapered, phononic nanostring—a simple and universally applicable design that achieves an optimized mode shape and simultaneously increases the local stress to near the yield strength of the material. This approach produces Q factors as high as 800 million at room temperature, for a MHz oscillator. This is the highest quality factor attained for a room temperature mechanical oscillator. Importantly, strain engineering allows these values to be achieved in the technologically relevant MHz frequenc
y range. These results signal a paradigm change in the ability to engineer and suppress dissipation in nanomechanical systems, and have far-ranging impact ranging from precision sensing technology to fundamental quantum science.

The attainment of such ultra high Q factors has fascinating consequences. For instance, ground state cooling from room temperature using feedback—a longstanding challenge once perceived to be impossible—becomes a highly realistic endeavor; so does the observation of ponderomotive (i.e. optomechanical) squeezing in a room temperature mechanical oscillator.

Even more remarkably, although we achieve unprecedented products and quality factors, the full potential of strain engineering as outlined here is not yet fully explored. When combining our approach with crystalline strained membranes, produced by molecular beam epitaxy of quaternary compound semiconductors, it should be possible to substantially increase the internal Q factor and even attain higher levels of strain. Realistic estimates, presented in our manuscript, predict that this approach will lead to decoherence rates outperforming trapped ions at UHV, and approaching the coherence times of nuclear spins in NV centers.products of 1017 Hz should, remarkably, be conservative estimates at moderate cryogenic temperatures.

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