Session A38: Focus Session: Acoustic and Optical Instrumentation

Vibration Potential Imaging: Results for the Forward Problem

A colloid is a suspension of charged particles in a liquid, with each particle surrounded by a counter charge. When ultrasound propagates through a colloid where the particles have either a higher or lower density than that of the surrounding fluid, the amplitude and phase of the particle motion, owing to the difference in inertia between the particle and the volume of fluid it displaces, differs from that of the fluid so that the particle and the fluid execute different motions. Since the counter charge is carried by the fluid, the oscillatory motion of the fluid relative to the particle distorts the normally spherical counter charge distribution in the fluid creating an oscillating dipole at the site of each particle. The addition of the polarization created at the sites of the particles over a macroscopic length results in a voltage that can be recorded by a pair of electrodes placed in the solution. The ultrasonic vibration potential can be used as a method of imaging where contrast in the image is governed by the presence of colloidal or ionic regions within the body under consideration. We describe the use of a frequency domain method of imaging where the current in a pair of electrodes is recorded as a function of the frequency of a plane ultrasonic wave. The method is applied to imaging a variety of objects including a thin layer, a thick layer, pairs of layers, layers with differing colloidal concentrations and spheres. The experimental results show agreement with the theory of vibration potential imaging that gives the recorded signal as proportional to the integral of the concentration of colloid over the pressure gradient in the ultrasonic wave.   

Picosecond Ultrasonic Measurement of Liquids

We study acoustic waves launched using a 400 fs blue pulse (407.5 nm) from a Ti:sapphire laser, focused on a SiO$_{2}$/GaAs interface (r $\sim $ 25 $\mu $m, fluence \underline {$<$}15 $\mu $J/cm$^{2})$. Because of the asymmetry of the (114) GaAs, thermoelastic and piezoelastic processes generate quasishear and quasilongitudinal acoustic pulses that propagate in both materials. Pulse echoes in the thin (0.2 $\mu $m) SiO$_{2}$ layer cause a variation in optical reflectivity at the interface. In thicker layers, including the GaAs substrate and liquid layers on top of the SiO$_{2}$ surface, variations in the bulk optical reflectivity caused by the pulses can also be detected if the layer is transparent to the probe wavelength (407 or 815 nm). We look at the pulse propagation in water, ethylene glycol and glycerine. We measure a longitudinal sound velocity for glycerine of 2800 m/s, 32{\%} larger than low frequency values, giving evidence of considerable elastic stiffening. Our pulses have central frequencies $\sim $50 GHz. A more modest 4.6{\%} increase in the longitudinal sound velocity for water (1552 m/s) is also observed. In spite of this notable stiffening, no shear waves were observed in any of the liquids studied, indicating that propagation is still within the hydrodynamic regime.   

Electromagnetic stimulation of the ultrasonic signal for nondestructive detection of the ferromagnetic inclusions and flaws

It was recently shown that thermal or optical stimulation can be used to increase sensitivity of the conventional nondestructive ultrasonic detection of the small crack, flaws and inclusions in a ferromagnetic thin-walled parts. We proposed another method based on electromagnetic modulation of the ultrasonic scattered signal from the inclusions or defects. The electromagnetically induced high density current pulse produces stresses which alter the ultrasonic waves scanning the part with the defect and modulate ultrasonic signal. The excited electromagnetic field can produces crack-opening due to Lorentz forces that increase the ultrasonic reflection. The Joule heating associated with the high density current, and consequent thermal stresses may cause both crack-closure, as well as crack-opening, depending on various factors. Experimental data is presented here for the case of a small cracks near small holes in thin-walled structures. The measurements were taken at 2-10 MHz with a Lamb wave wedge transducer. It is shown that electromagnetic transient modulation of the ultrasonic echo pulse tone-burst suggest that this method could be used to enhance detection of small cracks and ferromagnetic inclusions in thin walled metallic structures.   

Resonant Ultrasound Spectroscopy Characterization of Annealing and Grain Growth in Copper

Resonant Ultrasound Spectroscopy (RUS) is used for determining the bulk elastic properties of a solid material with known dimensions, density and shape from its characteristic vibration frequencies. RUS characterization of polycrystalline materials is based on the assumptions of material uniformity, and the existence of isotropic polycrystal-averaged moduli. The elastic properties of a polycrystalline material depend on the material's microstructure, which can be changed by heat treatment. In this present work, RUS has been applied to heat treated polycrystalline copper specimens; measurements of the resonance frequencies as a function of heat treatment were obtained and used to derive elastic constants. The elastic constants are correlated to the average grain size in the sample, determined by a visual measurement. We find that when the grain size reaches 10{\%} of the sample dimension, elastic constant fit errors suggest that the sample is losing uniformity. We also discuss a number of measurement results that depend on details of the sample mounting and transducer placement.   

Vibrational Modes of MEMS based Directional Sound Sensor

A directional sound sensor was fabricated using micro-electromechanical system (MEMS) technology based on the operational principle of Ormia ochracea fly's hearing organism [1]. The fly uses coupled bars hinged at the center to achieve the directional sound sensing by monitoring the difference in vibration amplitude between them. The MEMS design employed in this work consisted of a 1x2 square millimeter polysilicon membrane hinged at the center. The membrane was positioned about 2 micrometers above the substrate by using a sacrificial silicon dioxide layer. The membrane has two primary vibrational modes (rocking and bending) which were analysis by finite element analysis and found to be at 2.5 kHz and 8 kHz. The incident sound wave causes the two sides of the membrane to oscillates with slightly different amplitudes due to the arrival time difference. In this abstract, the vibrational modes of the system measured using electrical and sound sources will be presented. The experimental data were found to be in good agreement with the modeling. [1] R.N. Miles, et. al.: ``Mechanically coupled ears for directional hearing in the parasitoid fly Ormia ochracea,'' J. Acoust. Soc. Am., \textbf{98}, 3059, (1995).   

Development of a new ABS Acoustic Bubble Spectrometer$^{\mbox{{\textregistered}}\copyright }$ system

D\textsc{ynaflow} has developed an acoustic based device, the ABS Acoustic Bubble Spectrometer$^{\mbox{{\textregistered}}\copyright }$, that measures bubble size distributions and void fractions in liquids based on the measurement of sound propagation through the liquid. In the original system, a pair of hydrophones is used to transmit and receive short monochromatic bursts of sound at different frequencies through the liquid. These signals are processed and analyzed to obtain the frequency dependent attenuation and phase velocities of the acoustic waves. Subsequently, the bubble size distribution is obtained following solution of an inverse problem. In the new system, we have utilized multiple hydrophone pairs that have different frequency response ranges to cover a wider range of bubble size measurement. A transmission signal amplifier is integrated into the system to improve the signal noise ratio. We have also implemented an adaptive control scheme that automatically adjusts the transmitting signal strength and acquisition resolution to optimize the measurement process and used a rectangular and a sine acoustic wave pattern to improve accuracy of signal analysis.   

Numerical modeling of the impact of the propagation of a finite wave in a bubbly media on the Acoustic Bubble Spectrometer ABS$^{\copyright \mbox{{\textregistered}}}$.

The propagation of acoustic waves in bubbly media has been extensively studied over the years. Several methods have been developed for the inversion of the propagation characteristics in order to compute the size and number of bubbles present in the field. At the core of these inversion methods are the assumptions that bubbles are homogeneously dispersed and behave in a steady-state monochromatic linear fashion. However, instruments designed for the detection and measurements of bubbles (such as Dynaflow's Acoustic Bubble Spectrometer ABS$^{\copyright \mbox{{\textregistered}}})$ are limited to the use of finite duration and amplitude signals. Consequently, the transient characteristics of the bubble field can provide a significant impact on the received signal. We will present some recent work in the numerical modeling of transient and finite amplitude effects and their impact on the received signals and inversion procedure.   

Resolving dynamics of acoustic phonons by surface plasmons

In this work, we employ surface plasmons as a sensitive probe technique to detect acoustic phonons in metal films following impulsive optical excitation. Surface plasmons are shown to have an enhanced sensitivity in detecting acoustic phonons in metals. Our study shows that the surface plasmon technique is a promising tool to detect small optical or mechanical property changes in metals at a miniature scale, suitable for a variety of applications, such as sensors and MEMS.   

Long Range Surface Plasmon Fluorescence Spectroscopy

Surface plasmon modes, excited at the two sides of a thin metal layer surrounded by two (nearly) identical dielectric media interact via the overlap of their electromagnetic fields. This overlap results in two new-coupled modes, a short and a long-range surface plasmon (LRSP). We demonstrate that combining the LRSP optics with fluorescence spectroscopy can result in a huge enhancement of the fluorescence signal due to the enhanced optical field of the LRSP at the metal dielectric interface, and to its increased evanescent depth into the analyte. This was demonstrated for the detection of the fluorescence intensity of chromophore labeled protein bound to the surface sensor. Beside that, some fundamentals were studied leading to some interesting difference between SPFS and LRSPFS.   

Evanescent field response to patterned features on a planar waveguide measured with a buried detector array.

The evanescent field of an appropriately designed waveguide can be very sensitive to the local refractive indices of the cladding layers surrounding the core. In this study, a planar waveguide has been fabricated on a chip that contains buried p-Si photo-detectors, located about 1 micron from the waveguide core and arrayed down the length of the waveguide. Local changes in the index of the upper cladding, such as the formation of an adlayer, result in signal changes at the detector. The buried detector format provides significant opportunities for localized detection of chemical or biological analytes in complex milieu through monitoring of the evanescent field. To test the responses to refractive index changes in the upper cladding, small photoresist features were fabricated on the surface of the waveguide. This material was selected because it is easily patterned, its thickness can be tightly controlled, and its refractive index is similar to that of biological molecules. The results of the experiments measuring evanescent field intensity as well as detector fabrication details will be presented; these results are compared with parallel numerical modeling studies.   

AlGaN based Tunable Hyperspectral Detector: Growth and Device Structure Optimization

We report on fabrication and growth optimization of an AlGaN/GaN based tunable hyperspectral detector. III-Nitride based detectors possess the potential to detect a large portion, from UV to IR, of the electromagnetic spectrum. Control over the detection wavelength with applied bias across an AlInGaN heterostructure can provide a compact tunable hyperspectral detector eliminating use of filters and gratings which make current tunable detectors bulky. Challenges involved in the development of the device include controlled deposition and characterization of thin layers of Al$_{x}$Ga$_{1-x}$N with Al composition varying from 0{\%} to 100{\%} and back to 0{\%}. Performance of such detector is greatly affected by the thickness and quality of the thin heteroepitaxially grown layers which control the dark current and operating voltage of the device. We will present the effect of growth conditions and heterostructure parameters such as composition and thickness on the device performance.   

Dielectric Effects in Electro-optic Field Sensors

The use of electro-optic (EO) crystals for electromagnetic field detection is an attractive alternative to conventional techniques, due to their compactness, large intrinsic bandwidth, and ability to measure field amplitude and phase with minimum field perturbation. In our LiNbO$_{3}$ sensors, anomalously large detection sensitivities were observed, which were found to be due to dielectric relaxation effects within the crystals. In this presentation, we demonstrate these effects, their impact on the EO sensor responsivity, and discuss implications for improving future EO field sensors.