In quest of improved nanomechanical sensitivity at larger damping and applications in ambient conditions

• Author / Creator

  Roy, Swapan

• In the current state-of-the-art, a wide variety of devices, from computer clocks to smartphone accelerometers and from pressure gauges to atomic force microscope sensors, rely on mechanical resonators, either in microscale or nanoscale. Nanoscale mechanical resonators have even more potential than microscale by offering unprecedented sensitivity through weighing a single proton (10^(-24) g) or by measuring aN (10^(-18) N) force or µK temperature. The smaller size of nanometric mechanical resonators allows high sensitivity to their environment; ensuring excellent frequency stability, through high resonance frequency with a higher quality factor, Q, takes the best advantage of this sensitivity. At the same time, their smaller dimension makes these more susceptible to environmental fluctuations, such as thermomechanical (TM) noise, and to energy dissipation, which degrades the Q at atmospheric pressure. The amplitude of TM noise sets a limit to the frequency stability via limiting the signal to noise ratio (SNR). This limit on SNR happens to improve (decrease) as Q is lowered at a rate of Q^(1/2). Meanwhile, nonlinearity practically limits the maximum signal and also causes SNR to improve as Q is lowered with signal proportional to Q^(-1/2). By definition, SNR is the ratio of amplitude at the onset of nonlinearity to the TM noise peak. As a consequence, in widely accepted Robins' picture, frequency stability improves inversely with Q×SNR and motivates nanomechanical sensor operations mostly at high vacuum to get better performance via high-Q by resolving TM noise.

  The negative effect on the stability of early appearance of nonlinearity in high-Q mechanical resonators is underappreciated by researchers and the mantra that the better the Q, the better the frequency stability is well known. Interestingly, if the SNR can be improved at the same rate that Q is degraded, then mass sensitivity can be maintained despite lower Q conditions. Amongst excellent demonstrations of high-Q NEMS sensors to date, such interplay between Q and SNR is not well studied, despite importance not only for fundamental studies but also for practical applications. This thesis shows that the high displacement sensitivity of a nano-optomechanical (NOMS) transduction scheme ensures that mass sensing is generally occurring with only fundamental limitations to the stability. Put another way, NOMS transduction tends to resolve thermomechanical noise at orders of magnitude above the instrumentation noise background. Frequency stability measurements by phase-locked loop and open loop method validate Robins' picture by attaining the same level of stability at different Q. We test experimental results, both in the analytical and numerical frameworks, and provide a full model to unfold different fundamental noise sources existing in the system and noise suppression effects in phase-locked loop Allan-deviation experiments. Phase-locked loop experiments show increasing noise signal suppression with damping due to loop bandwidth artifacts. Surprisingly, open-loop experiments also show some improvement of mechanical sensor performance with increasing damping, which is attributed to reduced frequency-fluctuation noise in resonance through improved temperature fluctuation noise level via heat conduction by air molecules at atmospheric pressure. We confirm these findings by demonstrating better temperature resolution in atmosphere than in vacuum.

  For temperature sensing experiments, we study the temperature dependent properties of the optical ring and NEMS to develop NOMS thermometry. Our thermometry results reveal the existence of nanoscale heat transfer issue for the NEMS that results in very high 0.7 MWm-2K-1 heat transfer coefficient in atmosphere for the doubly clamped beam resonator and agrees well with COMSOL multiphysics simulations.

  We further apply the obtained frequency stability at atmospheric pressure by integrating the NOMS system with a commercial gas-chromatography. The designed, integrated GC-NOMS gas sensor demonstrates 1 ag (10^(-18) g) mass resolution, even at truncated SNR of the mechanical resonator, by detecting GC separated toluene and xylene in a mixture. The retention time of toluene and xylene are identical both in NEMS and FID and asserts the success of GC-NOMS integration in ambient condition. The obtained ambient ag mass resolution is comparable to that predicted by Robins' formula at 100 µTorr even though the Q of 29 at 760 Torr is ≈300× lower than that at 100 µTorr. Such an intriguing mass resolving capacity by the GC-NOMS in atmosphere is a baby step for future generations of portable GC-MS in ambient air.

• Subjects / Keywords

  • Gas sensing
  • NOMS
  • Nanoscale heat transfer
  • Frequency stability
  • Damping
  • Frequency fluctuation noise
  • Phase locked loop
  • GC-MS
  • Quality factor
  • Nanomechanical sensitivity
  • Mass sensitivity
  • Thermometry
  • Allan deviation
  • NEMS

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-4ayq-bn15

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.