Ultracoherent Nanomechanical Resonators Achieve >1 Billion Quality Factors, Enabling Force Sensing Beyond Atomic Force Microscopy

Ultracoherent nanomechanical resonators represent a rapidly advancing technology with potential applications ranging from precision measurement to fundamental scientific investigation, but their performance near other materials remains poorly understood. Amirali Arabmoheghi, Alessio Zicoschi, Guillermo Arregui, and colleagues at the Swiss Federal Institute of Technology Lausanne, along with Yi Xia and Nils J. Engelsen of Chalmers University of Technology, now demonstrate a previously unrecognised source of energy loss in these devices. The team discovered that the presence of nearby dielectric materials introduces a novel dissipation mechanism, effectively creating ‘noncontact friction’ that limits the coherence of the resonator. This finding is significant because it explains a long-standing puzzle in atomic force microscopy and establishes fundamental limitations on integrating these highly sensitive resonators into practical devices, particularly highlighting the detrimental effects of charged defects within the surrounding materials.

Nanobeam Friction Limits Quality Factor

Researchers investigated non-contact friction (NCF) in nano-optomechanical devices, a phenomenon limiting their performance by reducing their quality factor. These devices, consisting of silicon nitride nanobeams coupled to photonic crystal cavities, were fabricated using two routes, one starting with a thermal oxide layer on silicon and the other directly using silicon nitride. Both processes involved lithography, etching, and release steps to create free-standing structures. Characterisation revealed that NCF significantly limits the quality factor, with narrower beams and shorter lengths proving more susceptible.

Thermal annealing at 80°C initially showed improvement, but 120°C reduced performance, indicating a complex temperature relationship. UV irradiation and buffered oxide etching had no significant impact. Analysis suggests surface charges on the silicon nitride nanobeams are the primary cause of NCF, strongly influenced by the fabrication process. Detailed examination of fabrication parameters and device geometry provided valuable insights. Mitigating NCF remains a significant challenge, and further research is needed to understand the origin of surface charges and develop effective strategies for reducing friction in these nanoscale devices. This work provides a comprehensive understanding of the challenges associated with fabricating and characterising nano-optomechanical systems.

High Q Nanobeam Ringdown Measurements Reveal Loss Mechanisms

Scientists achieved quality factors exceeding one billion at room temperature in nanomechanical oscillators, demonstrating their potential for precision measurement. They fabricated uniform silicon nitride strings suspended above a silicon dioxide substrate and employed a ringdown measurement technique, using an optical interferometer to precisely monitor the decay of mechanical oscillations. Initial theoretical models predicted quality factor limitations due to bending loss, but experimental results deviated significantly, indicating additional damping sources. Researchers discovered that lower frequency mechanical modes experienced greater damping, suggesting a frequency-dependent dissipation mechanism.

They developed a model incorporating both bending loss and non-contact friction, accurately fitting experimental data and extracting key parameters like effective linear charge densities. Analysis of multiple fabrication runs and various modes revealed consistent results, demonstrating the influence of fabrication-dependent charge accumulation. Finite element method simulations validated the experimental findings and provided insights into charge distribution. The team successfully integrated a silicon nitride photonic crystal microcavity with a binary-tree string resonator, demonstrating the potential for on-chip integration of ultracoherent nanomechanical systems.

Dielectric Loss Limits Nanoresonator Coherence

Researchers have identified a previously unrecognised source of energy loss in ultracoherent nanomechanical resonators, stemming from interactions with nearby dielectric materials. These resonators, capable of detecting extraordinarily weak forces, are hampered by energy dissipation, and this work reveals that dielectric materials introduce a novel damping mechanism, more pronounced at lower mechanical frequencies. The observed phenomenon closely mirrors non-contact friction observed in atomic force microscopy, suggesting a shared underlying physical principle. Through detailed analysis and finite element method simulations, scientists established that this effect is linked to the motion of static charges within the resonator and the surrounding environment.

Analysis of fundamental and higher-order modes yielded consistent results, strengthening the evidence for non-contact friction as the dominant loss mechanism. Experiments on silicon nitride strings revealed a significant deviation from predicted quality factors based on bending loss alone. The team successfully fitted experimental data using a combined model accounting for both bending loss and non-contact friction, confirming their findings. This work provides crucial insights into the limitations of integrating ultracoherent nanomechanical resonators and highlights the importance of controlling charge defects in these systems.

Dielectric Loss Limits Nanomechanical Oscillator Coherence

Scientists have discovered a novel dissipation mechanism affecting ultracoherent nanomechanical oscillators, stemming from interactions with nearby dielectric materials. This research demonstrates that the presence of these materials introduces losses that hinder the performance of these sensitive devices, linked to the motion of static charges within the resonator and the surrounding environment. The magnitude of this effect scales with the frequency of the mechanical modes, being more pronounced at lower frequencies. These findings are consistent with observations of non-contact friction in atomic force microscopy, suggesting a common underlying physical principle.

The team developed methods for accurately modelling this non-contact friction in various geometries, using standard characterisation techniques. This work highlights limitations in integrating ultracoherent nanomechanical resonators, particularly concerning the impact of trapped charges. Looking forward, researchers suggest opportunities to leverage static charges for new functionalities, such as coupling nanomechanical resonators to electric fields and other quantum systems. Furthermore, the observed non-contact friction mechanism itself could serve as a sensitive probe for dissipative processes in materials, offering a new avenue for materials characterisation. The theoretical and numerical methods developed during this study are readily adaptable to these future investigations.