Optical Lock-in Vibration Detection Using Photorefractive Four-Wave Mixing

Many important applications for photorefractive crystals (PRCs) have been found recently by various investigators [1–3]. These applications range from volumetric information storage in optical computing to adaptive, remote detection of ultrasonic vibration in optical nondestructive evaluation [4]. In this paper, we propose the use of PRCs for lock-in detection of continuously vibrating structures

Laser Acoustic Imaging of Film Bulk Acoustic Resonator (FBAR) Lateral Mode Dispersion

A laser acoustic imaging microscope has been developed that measures acoustic motion with high spatial resolution without scanning. Images are recorded at normal video frame rates and heterodyne principles are used to allow operation at any frequency from Hz to GHz. Fourier transformation of the acoustic amplitude and phase displacement images provides a direct quantitative determination of excited mode wavenumbers at any frequency. Results are presented at frequencies near the first longitudinal thickness mode (~ 900 MHz) demonstrating simultaneous excitation of lateral modes with nonzero wavenumbers in an electrically driven AlN thin film acoustic resonator. Images combined at several frequencies form a direct visualization of lateral mode dispersion relations for the device under test allowing mode identification and a direct measure of specific lateral mode properties. Discussion and analysis of the results are presented in comparison with plate wave modeling of these devices taking account for material anisotropy and multilayer films

Material Property Measurement in Hostile Environments using Laser Acoustics

Acoustic methods are well known and have been used to measure various intrinsic material properties, such as, elastic coefficients, density, crystal axis orientation, microstructural texture, and residual stress. Extrinsic properties, such as, dimensions, motion variables or temperature are also readily determined from acoustic methods. Laser acoustics, employing optical generation and detection of elastic waves, has a unique advantage over other acoustic methods—it is noncontacting, uses the sample surface itself for transduction, requires no couplant or invasive sample surface preparation and can be utilized in any hostile environment allowing optical access to the sample surface. In addition, optical generation and detection probe beams can be focused to the micron scale and/or shaped to alter the transduction process with a degree of control not possible using contact transduction methods. Laser methods are amenable to both continuous wave and pulse-echo measurements and have been used from Hz to 100’s of GHz (time scales from sec to psec) and with amplitudes sufficient to fracture materials. This paper shall review recent applications of laser acoustic methods to determining material properties in hostile environments that preclude the use of contacting transduction techniques. Example environments include high temperature (>1000C) sintering and molten metal processing, thin film deposition by plasma techniques, materials moving at high velocity during the fabrication process and nuclear high radiation regions. Recent technological advances in solid-state lasers and telecommunications have greatly aided the development and implementation of laser acoustic methods, particularly at ultra high frequencies. Consequently, laser acoustic material property measurements exhibit high precision and reproducibility today. In addition, optical techniques provide methods of imaging acoustic motion that is both quantitative and rapid. Possible future directions for laser acoustics shall be discussed drawing from examples in materials science, microelectronic and nuclear fields

Simultaneous Microscopic Imaging of Elastic and Thermal Anisotropy

Simultaneous imaging of elastic and thermal properties of anisotropic materials with micron (lateral) and nanometer (depth) resolution is presented. This approach employs an ultrafast laser for the generation and detection of thermal and acoustic waves. Demonstrations involving the visualization of thermal waves and surface acoustic waves are presented for single crystal quartz and fused silica substrates supper coated with chromium films. These images dramatically reveal and contrast the symmetry of thermal and elastic properties and compare favorably with theoretical prediction. This hybrid approach shows great promise to investigate fundamental properties of materials and interfacts on both a low-frequency (elastic wave) and a high-frequency (phonon diffusion) scale

Quantitative Determination of Lateral Mode Dispersion in Film Bulk Acoustic Resonators through Laser Acoustic Imaging

Film Bulk Acoustic Resonators are useful for many signal processing applications. Detailed knowledge of their operation properties are needed to optimize their design for specific applications. The finite size of these resonators precludes their use in single acoustic modes; rather, multiple wave modes, such as, lateral wave modes are always excited concurrently. In order to determine the contributions of these modes, we have been using a newly developed full-field laser acoustic imaging approach to directly measure their amplitude and phase throughout the resonator. This paper describes new results comparing modeling of both elastic and piezoelectric effects in the active material with imaging measurement of all excited modes. Fourier transformation of the acoustic amplitude and phase displacement images provides a quantitative determination of excited mode amplitude and wavenumber at any frequency. Images combined at several frequencies form a direct visualization of lateral mode excitation and dispersion for the device under test allowing mode identification and comparison with predicted operational properties. Discussion and analysis are presented for modes near the first longitudinal thickness resonance (~900 MHz) in an AlN thin film resonator. Plate wave modeling, taking account of material crystalline orientation, elastic and piezoelectric properties and overlayer metallic films, will be discussed in relation to direct image measurements

Real-Time Measurement of Material Elastic Properties in a High Gamma Irradiation Environment

This paper describes the first noncontact elastic vibration measurements of an object in a high gamma radiation field. Using a laser-coupled resonant ultrasound technique, the vibration modes of an Inconel hollow capped cylinder were measured as the gamma radiation field was increased to 104 Gy/h. This measurement technique allowed shifts in the resonant frequency of the sample’s vibration modes to be tracked over a 170-h period. The vibration mode frequencies changed in a manner consistent with the temperature dependence of the elastic stiffness coefficients of the material. These results demonstrate the efficacy of the laser approach for real-time resonant ultrasound measurements in this severely hostile nuclear environment

REAL-TIME IN-SITU MEASUREMENT OF MATERIAL ELASTIC PROPERTIES IN A HIGH GAMMA IRRADIATION ENVIRONMENT

The first measurements of elastic vibrations of an object in-situ to a high gamma irradiation field using a laser coupled resonant ultrasound method are described. A vibration mode of an Inconel hollow capped cylinder was measured throughout a period of 170 hours as the gamma radiation field was increased to 104 Gray/hour. The vibration mode frequency was observed to change in a manner consistent with the temperature dependence of the elastic stiffness coefficients of the material. These results illustrate the efficacy of the laser approach for real-time resonant ultrasound measurements in this severely hostile nuclear environment

Laser Measurement of SAM Bulk and Surface Wave Amplitudes for Material Microstructure Analysis

Scanning Acoustic Microscopy (SAM) at ultra high frequencies has proven to be a useful tool for investigating materials on the scale of individual grains. This technique is normally performed in a reflection mode from one side of a sample surface. Information about the generation and transmission of bulk acoustic waves into the material is inferred from the reflection signal amplitude. We present an adaptation to the SAM method whereby the acoustic bulk waves are directly visualized through laser acoustic detection. Ultrasonic waves were emitted from a nominal 200 MHz point focus acoustic lens into a thin silicon plate (thickness 75ìm) coupled with distilled water. A scanned laser beam detected the bulk and surface acoustic waves at the opposite surface of the thin silicon plate. Distinct amplitude patterns exhibiting the expected symmetry for Silicon were observed that alter in predictable ways as the acoustic focal point was moved throughout the plate. Predictions of the acoustic wave fields generated by the acoustic lens within and at the surface of the Silicon are being investigated through the angular spectrum of plane waves approach. Results shall be presented for plates with (100) and (111) orientations followed by discussion of applications of the technique for material microstructure analysis

OSTEOBLAST ADHESION OF BREAST CANCER CELLS WITH SCANNING ACOUSTIC MICROSCOPY

Breast cancer frequently metastasizes to the bone. Upon colonizing bone tissue, the cancer cells stimulate osteoclasts (cells that break bone down), resulting in large lesions in the bone. The breast cancer cells also affect osteoblasts (cells that build new bone). Conditioned medium was collected from a bone-metastatic breast cancer cell line, MDA-MB-231, and cultured with an immature osteoblast cell line, MC3T3-E1. Under these conditions the osteoblasts acquired a changed morphology and appeared to adherer in a different way to the substrate and to each other. To characterize cell adhesion, MC3T3-E1 osteoblasts were cultured with or without MDA-MB-231 conditioned medium for two days, and then assayed with a mechanical scanning acoustic reflection microscope (SAM). The SAM indicated that in normal medium the MC3T3-E1 osteoblasts were firmly attached to their plastic substrate. However, MC3T3-E1 cells cultured with MDA-MB-231 conditioned medium displayed both an abnormal shape and poor adhesion at the substrate interface. The cells were fixed and stained to visualize cytoskeletal components using optical microscopic techniques. We were not able to observe these differences until the cells were quite confluent after 7 days of culture. However, using the SAM, we were able to detect these changes within 2 days of culture with MDA-MB-231 conditioned mediu

Microstructure Characterization with a Pulsed Laser Ultrasonic Source

Localized heating produced by absorption from a pulsed laser provides an efficient noncontacting source of ultrasonic waves in materials. This paper describes the results of experiments conducted to illustrate the feasibility of this type of source fot microstructure characterization in metal and ceramic materials. Piezoelectric and capacitive wide bandwidth detection transducers have been used to record attenuation and scattering in these materials for comparison with the conventional pulse echo technique. The laser source was found to be art efficient, versatile, and wide bandwidth noncontacting source.</p