Measurements of Elastic Modulus Using Laser-Induced Surface Waves
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where R(v) =(0.862 + 1.14v)/(l + v) is the ratio of the Rayleigh wave velocity to the shear wave velocity and ranges from 0.86 to 0.95 as v takes on values from 0 to 0.5. Anisotropic materials are discussed further in reference 1. Experimental A Neodymium :YAG pulsed laser was used as the energy source to thermo-elastically excite surface waves on a surface-polished and coated specimens. The front of the surface waves produced by the YAG laser are detected by the knife edge/beam deflection technique. Figure 1 shows a schematic of the optical layout. The YAG laser produces single laser pulses of 15-ns duration with near-Gaussian temporal and spatial energy distribution. The repetition frequency of the laser is 10 Hz. On the target the pulses have an energy of less than 100 mL. The energy can be varied by a half-wave plate in conjunction with a polarizing beam-splitter.
561 Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
Excitation Beam
Detection Beam Fig. 1 Experimental schematic for the generation and detection of surface acoustic waves. The beam of the excitation laser is focused with either a spherical or cylindrical lens onto the test specimen. The diameter of the focal spot is approximately 80-ýtm. A He-Ne laser with a continuous, single-mode output of 1.5-mw was spatially filtered using microscope objective with a 25-tm aperture. An inverted telescope located behind the spatial filter produces a collimated beam of 1.5-mm diameter. This beam is reflected off the target surface at a location 5 to 10-mm from the excitation spot. As the surface wave travels by the detection spot, the local surface angle changes, deflecting the beam by twice the change in angle. The reflected light travels for 2.5-m before it encounters a knife edge that intercepts approximately half of the beam. These beam deflections are converted into modulations in the signal power on the photodetector. A lens of 150-mm focal length focuses the light that escapes the knife edge into a fast silicon photodiode. A translation stage was used to facilitate translation and detection of the He-Ne laser spot on the sample in a repetitive and accurate manner. The signal produced by the photodiode is amplified with a high-gain amplifier and recorded by a Tektronix 7094 oscilloscope equipped with a digital camera. The combined frequency bandwidth of the electronics is limited by the frequency response of the oscilloscope to 400 MHz. Results and Discussion The detected waveforms using knife-edge detection is shown in Figure 2 for a gallium arsenide(GaAs) sample. Each of the five traces corresponds to measurements performed at different locations of the He-Ne detection laser from the YAG laser spot. The zero (0) distancespot corresponds to the location at which the measurement was first made. The locations at ±1.72 and ±3.44-mm are relative to the first location along a line connecting the excitation spot and the first measurement location. A straight line was drawn through the mid-points of the peak waves on all five traces. T
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