Steve D. Sharples

Spatially resolved acoustic spectroscopy for fast noncontact imaging of material microstructure

We have developed a noncontact and nondestructive technique that uses laser-generated and detected surface acoustic waves to rapidly determine the local acoustic velocity, in order to map the microstructure of multi-grained materials. Optical fringes excite surface waves at a fixed frequency, and the generation efficiency is determined by how closely the fringe spacing matches the acoustic wavelength in the excitation region. Images of titanium alloys are presented, acquired using the technique. Methods to improve the current lateral resolution of 0.8mm are discussed, and the ability to measure velocity change to an accuracy of one part in 3300 is experimentally demonstrated.

All-optical adaptive scanning acoustic microscope☆

We have constructed a fast laser-based surface acoustic wave (SAW) microscope, which may be thought of as a non-perturbing scanning acoustic microscope. The instrument is capable of rapid high resolution vector contrast imaging at several discrete frequencies, without any damage to the sample. Tailoring the generating optical distribution using computer-generated holograms allows us to both focus the acoustic waves (increasing their amplitude) and to spread the optical power over the sample surface (preventing damage). Accurate quantitative amplitude and phase (velocity) measurements and unique acoustic contrast mechanisms are possible with our instrument based on this technology due to the non-perturbing nature and the instrument geometries.However, the complexity of the optical generation profile leads to a strong dependence on material properties such as the SAW velocity and material anisotropy. We address these issues in this paper, and demonstrate how a spatial light modulator may be used to adapt the generating optical distribution to compensate for the material properties. This facilitates simpler alignment and velocity matching, and, combined with an acoustic wavefront sensor, will allow real-time adjustment of the generating source to enable imaging on anisotropic materials.

Determination of crystallographic orientation of large grain metals with surface acoustic waves

A previously described laser ultrasonic technique known as spatially resolved acoustic spectroscopy (SRAS) can be used to image surface microstructure, using the local surface acoustic wave (SAW) velocity as a contrast mechanism. It is shown here that measuring the SAW velocity in multiple directions can be used to determine the crystallographic orientation of grains. The orientations are determined by fitting experimentally measured velocities to theoretical velocities. Using this technique the orientations of 12 nickel and 3 aluminum single crystal samples have been measured, and these are compared with x-ray Laue backreflection (LBR) measurements with good agreement. The root mean square difference between SRAS and LBR measurements in terms of an R-value is less than 4.1°. The influence of systematic errors in the SAW velocity determination due to instrument miscalibration, which affects the accurate determination of the planes, is discussed. SRAS has great potential for complementary measurements or even for replacing established orientation determination and imaging techniques.

Measurement of material nonlinearity using surface acoustic wave parametric interaction and laser ultrasonics

A dual frequency mixing technique has been developed for measuring velocity changes caused by material nonlinearity. The technique is based on the parametric interaction between two surface acoustic waves (SAWs): The low frequency pump SAW generated by a transducer and the high frequency probe SAW generated and detected using laser ultrasonics. The pump SAW stresses the material under the probe SAW. The stress (typically <5 MPa) is controlled by varying the timing between the pump and probe waves. The nonlinear interaction is measured as a phase modulation of the probe SAW and equated to a velocity change. The velocity-stress relationship is used as a measure of material nonlinearity. Experiments were conducted to observe the pump-probe interaction by changing the pump frequency and compare the nonlinear response of aluminum and fused silica. Experiments showed these two materials had opposite nonlinear responses, consistent with previously published data. The technique could be applied to life-time predictions of engineered components by measuring changes in nonlinear response caused by fatigue.

Fast, all-optical Rayleigh wave microscope: Imaging on isotropic and anisotropic materials

A fast, non-contact Rayleigh wave scanning microscope is demonstrated, which is capable of scan rates of up to a maximum of 1000 measurements/s with typical speeds of up to 250 measurements/s on real samples. The system uses a mode-locked, Q-switched Nd:YAG laser operating at a mode-locked frequency of 82 MHz and a Q-switch frequency of 1 kHz. The Q-switch frequency determines the upper limit of the scanning rate. The generating laser illumination is delivered and controlled by a computer-generated hologram (CGH). The generating laser produces around 30 pulses at 82 MHz and additional harmonics at 164 and 246 MHz and above. The microscope can operate at these harmonics provided the spatial bandwidth of the optics and the temporal bandwidth of the electronics are suitable. The ultrasound is detected with a specialized knife-edge detector. The microscope has been developed for imaging on isotropic materials. Despite this, the system can be used on anisotropic materials, but imaging and interpreting images can be difficult. The anisotropy and grain structure of the material can distort the Rayleigh wavefront, leading to signal loss. A model has been developed to simulate polycrystalline-anisotropic materials; this is discussed along with possible solutions that would overcome the problems associated with anisotropy. Rayleigh wave amplitude images are demonstrated on silicon nitride at 82 and 164 MHz and on polycrystalline aluminium at 82 MHz.

Spatially resolved acoustic spectroscopy for rapid imaging of material microstructure and grain orientation

Measuring the grain structure of aerospace materials is very important to understand their mechanical properties and in-service performance. Spatially resolved acoustic spectroscopy is an acoustic technique utilizing surface acoustic waves to map the grain structure of a material. When combined with measurements in multiple acoustic propagation directions, the grain orientation can be obtained by fitting the velocity surface to a model. The new instrument presented here can take thousands of acoustic velocity measurements per second. The spatial and velocity resolution can be adjusted by simple modification to the system; this is discussed in detail by comparison of theoretical expectations with experimental data.

Non-contact acoustic microscopy

We demonstrate a fast all-optical surface acoustic wave (SAW) microscope. This acoustic microscope may be thought of as a non-contact (hence non-perturbing) surface acoustic microscope. The key to producing a sufficiently high SAW amplitude for imaging without producing surface damage is to tailor the generating optical distribution. This can be used to spread the optical power on the sample surface (preventing damage) and to focus the acoustic waves (increasing the amplitude). This paper discusses the general and specific design features of our microscope and important new developments in the general design of such instruments. Our microscope based on this technology is capable of producing high quality, quantitative images of SAW amplitude and phase (velocity) on many materials; this and new, unique, forms of acoustic contrast are demonstrated and discussed.

Multichannel, time-resolved picosecond laser ultrasound imaging and spectroscopy with custom complementary metal-oxide-semiconductor detector

This paper presents a multichannel, time-resolved picosecond laser ultrasound system that uses a custom complementary metal-oxide-semiconductor linear array detector. This novel sensor allows parallel phase-sensitive detection of very low contrast modulated signals with performance in each channel comparable to that of a discrete photodiode and a lock-in amplifier. Application of the instrument is demonstrated by parallelizing spatial measurements to produce two-dimensional thickness maps on a layered sample, and spectroscopic parallelization is demonstrated by presenting the measured Brillouin oscillations from a gallium arsenide wafer. This paper demonstrates the significant advantages of our approach to pump probe systems, especially picosecond ultrasonics.

Rapid photoreflectance spectroscopy for strained silicon metrology

We present an improved photoreflectance (PR) spectroscopy technique upon the prior art in providing a rapid acquisition method of the PR spectrum in a simultaneous and multiplexed manner. Rapid PR (RPR) application is the on-line monitoring of strained silicon. Shrinkage in the silicon bandgap is measured and converted to strain, using theoretical models. Experimental RPR results are in good correlation with Raman spectroscopy.

Fast noncontact imaging of material microstructure using local surface acoustic wave velocity mapping

The make-up of the material microstructure of multi-grained materials such as titanium alloys and aluminum is of great interest to many in industries such as aerospace. The ability to map the material microstructure - in effect to image the grains - quickly and in a nondestructive manner would be useful from both a process control perspective and in the area of nondestructive evaluation. There are several techniques in the field that are capable of imaging grain structure, including simple etching, orientation imaging microscopy and scanning acoustic microscopy; all have their strengths and weaknesses. We present a new ultrasonic technique that can directly and quantitatively image the local surface acoustic wave (SAW) velocity over the surface of a material. Material microstructure can be determined if the phase velocity of the grains varies with grain orientation. The acoustic waves are generated and detected by lasers and as well as being noncontact, the technique is relatively fast, can cope with large samples, and is totally nondestructive. The new technique involves varying the spatial parameters of the excitation pattern in real time to maximize the generation efficiency of the surface acoustic waves at the phase velocity of the material, in the region of excitation. By repeating this over the sample surface, a surface wave phase velocity map can be produced. As well as describing the velocity mapping technique in detail, several example results of industrially-relevant materials acquired using our new instrument are presented. The limit to the quantitative lateral resolution of the instrument is discussed, and how this relates to the qualitative lateral resolution. Results indicating the practical limit of the accuracy of velocity measure- ments are also presented. We demonstrate imaging on several samples (40mm 2 )a nd different materials. This shows that the the instrument can reveal the underlying microstructure and image areas of anomalous grain structure that may be significant for the performance of the material. The instrument currently works on smooth samples but can be extended to work on rough surfaces and components with unprepared surfaces and consequently this has high industrial relevance.