Joseph F. Vignola

Thermoelastic loss in microscale oscillators

A simple model of thermoelastic dissipation is proposed for general, free standing microelectromechanical (MEMS) and nanoelectromechanical (NEMS) oscillators. The theory defines a flexural modal participation factor, the fraction of potential energy stored in flexure, and approximates the internal friction by assuming the energy loss to occur solely via classical thermoelastic dissipation of this component of the motion. The theory is compared to the measured internal friction of a high Q mode of a single-crystal silicon double paddle oscillator. The loss at high temperature (above 150 K) is found to be in good agreement with the theoretical prediction. The importance of this dissipation mechanism as a function of scale is briefly discussed. We find that the relative importance of this mechanism scales with the size of the structure, and that for nanoscale structures it is less important than intrinsic phonon–phonon scattering.

Effect of viscous loss on mechanical resonators designed for mass detection

Simple models are presented for estimating viscous damping of fluid (gas or liquid) loaded mechanical resonators. The models apply to beams in flexural modes of vibration, and to thin beams and plates in longitudinal modes of vibration. Predictions of the associated quality factor are compared with measured values for several macroscale and microscale resonators. The scaling of viscous loss with oscillator size is discussed. The minimum detectable mass is estimated for several oscillator designs and it is shown that, for comparably sized devices, longitudinal resonators have the lowest threshold of detection. This minimum detectable mass is proportional to scale to the power 1.75 for all resonator architectures limited by viscous damping, and it is shown that the viscous loss is 220 times larger in water than in air.

Attachment loss of micromechanical and nanomechanical resonators in the limits of thick and thin support structures

Analytical expressions are provided for the energy loss from vibrating mechanical resonators into their support structures for two limiting cases: supports that can be treated as plates, and supports that act as semi-infinite elastic media, with effectively infinite thickness. The former case is applicable to many microscale resonators, while the latter is appropriate for nanoscale devices. General formulations are given, applicable to a wide range of resonator geometries. These formulations are then applied to two geometries commonly used in microelectromechanical systems and nanelectromechanical systems applications: cantilevered beams and doubly fixed beams. Experimental data are presented to validate the finite-thickness support theory, and the predictions of the theory are also compared to data from existing literature for a microscale rectangular paddle oscillator.

A loss mechanism study of a very high Q silicon micromechanical oscillator

The room-temperature quality factors of silicon micromechanical oscillators have been investigated by scanning laser vibrometry. One of the flexural modes has very little attachment loss to its environment, which enables us to study internal loss mechanisms. After several consecutive annealing steps up to 800°C, the quality factor Q has increased from 8×104 to 6.0×105. However, the Q decays to 1.4×105 over six months in air. We conclude that near-surface lattice defects caused by reactive-ion etching and surface adsorbates are the main source of internal loss while surface adsorbates are responsible for the time dependence. We also discuss the thermoelastic limit in terms of Zener’s theory and flexural modal components of thin plates with vibratory volume change, and compare it with our results.

Laser detection of sound

A differential laser Doppler velocimeter (LDV) has been assembled and tested to provide noninvasive absolute measurements of acoustic particle displacements of standing waves generated in a water‐filled tube. The principle of the technique [see K. J. Taylor, J. Acoust. Soc. Am. 59, 691–694 (1976)] is to measure the Doppler shift of laser light scattered from colloidal microparticles oscillating under the action of an acoustic field. The system tested is capable of detecting particle displacements of the order of a few nanometers with a bandwidth of several kilohertz. The performances and limitations of the system are discussed. In particular, the effect of Brownian motion is shown to produce only negligible broadening of the spectral density of the signal of interest. The sensitivity of the present LDV system is estimated to be very close to the shot noise limit of the photomultiplier tube used to detect the Doppler shift of the scattered light. Experimental results are obtained under controlled laborator...

Microbubble-enhanced ultrasound-modulated fluorescence in a turbid medium

The feasibility of using ultrasound to modulate fluorescence in a turbid medium is still in debate due to the difficulty of detecting the modulated signal. We have demonstrated a system that could detect the weak signals of ultrasound-modulated fluorescence (UMF) by using a broadband lock-in amplifier and microbubbles as enhancement agents. By detecting the microbubble-enhanced UMF signal, a sub-millimeter fluorescent tube submerged in a turbid medium with a depth of 2 cm has been clearly observed with an ultrasonic spatial resolution. The modulation efficiency was significantly improved by using microbubbles, and was found to linearly increase with the drive voltage applied to the ultrasound transducer and the fluorophore concentration within the range adopted in this study. Possible modulation mechanisms are discussed.

Dissipation from microscale and nanoscale beam resonators into a surrounding fluid

Simple models for estimating viscous damping, acoustic radiation loss, and loss due to collisions with individual molecules of rarefied gas are presented and examined to show how quality factor of microscale and nanoscale resonators varies with ambient fluid pressure p and resonator geometry. For sufficiently low length to thickness aspect ratios, acoustic radiation loss is the dominant loss mechanism at pressures great enough that the fluid acts as a continuum, and the acoustic loss transitions seamlessly at low pressure to loss due to direct transfer of momentum to individual fluid molecules (both models have loss proportional to fluid pressure). For beams with greater aspect ratio, viscous loss, with loss proportional to p, dominates over a certain range of pressures, and the width of this region depends on the acoustic radiation efficiency of the beam. The transition between rarefied gas behavior and viscous (continuum) behavior occurs when the mean free path lmfp, a function of pressure, becomes shor...

Architectural considerations of micro- and nanoresonators for mass detection in the presence of a fluid

The sensitivity of various microscale and nanoscale resonator platforms, for use as mass sensors for detection of chemical or biological agents in air or water, is examined in terms of architectural considerations, including shape, scale, vibration mode, and fluid environment. Simple models for estimating damping due to various sources are used to calculate Q for several resonator designs: cantilevers and doubly fixed beams in flexure and extensional bar and disk resonators. The scaling of various contributions to Q is discussed, and the effects of support loss and fluid loss are compared as a function of aspect ratio for beam resonators. The minimum detectable mass is estimated for each of the four resonator designs, both for the case in which additional mass adsorbs uniformly over the resonator surface and the case in which functionalization of the surface is limited in order to maximize sensitivity and minimize added dissipation. The mass sensitivity is best for resonators undergoing extensional motion...

Effects of annealing and temperature on acoustic dissipation in a micromechanical silicon oscillator

The temperature dependence (15–320K) of the acoustic dissipation was studied for some lower vibrational modes of a suspended silicon plate 1.5μm thick. Our oscillator was exposed to the laboratory environment prior to measurement, laser annealed while in a cryogenic vacuum, and remeasured. We find a dissipation peak at 160K, similar to results by others, and a second dissipation peak near 30K. Annealing reduced the dissipation at 160K by as much as a factor of 10, and gave quality factors as high as 1.4×106 at 470kHz and our lowest temperature. Our data support the idea that the 160K peak is related to adsorbates, and show this mechanism is important at room temperature. Post-anneal room-temperature dissipation appears to be limited by thermoelastic loss for certain modes.

Airgun inter-pulse noise field during a seismic survey in an Arctic ultra shallow marine environment

Offshore oil and gas exploration using seismic airguns generates intense underwater pulses that could cause marine mammal hearing impairment and/or behavioral disturbances. However, few studies have investigated the resulting multipath propagation and reverberation from airgun pulses. This research uses continuous acoustic recordings collected in the Arctic during a low-level open-water shallow marine seismic survey, to measure noise levels between airgun pulses. Two methods were used to quantify noise levels during these inter-pulse intervals. The first, based on calculating the root-mean-square sound pressure level in various sub-intervals, is referred to as the increment computation method, and the second, which employs the Hilbert transform to calculate instantaneous acoustic amplitudes, is referred to as the Hilbert transform method. Analyses using both methods yield similar results, showing that the inter-pulse sound field exceeds ambient noise levels by as much as 9 dB during relatively quiet conditions. Inter-pulse noise levels are also related to the source distance, probably due to the higher reverberant conditions of the very shallow water environment. These methods can be used to quantify acoustic environment impacts from anthropogenic transient noises (e.g., seismic pulses, impact pile driving, and sonar pings) and to address potential acoustic masking affecting marine mammals.