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Utilization of transient reflectivity measurements for nondestructive characterization of material properties are attractive for understanding materials behavior in harsh environments. Two experimental approaches that implement an all optical pump-probe ultrafast spectroscopy will be presented.
In picosecond ultrasonics, the pump beam is used to generate ultrasonic wave modes propagating normal to the surface. Transient reflectivity signal exhibits Brillouin oscillations that originate from the interference between two reflections of the probe beam: one from the surface and another from the wavefront of the acoustic wave. The frequency and amplitude of the oscillations depends on the type and velocity of the excited modes and is determined by the crystalline orientation. These features are used to obtain microstructural information of the material. Application of the method to subsurface grain boundary imaging in transparent metal oxides will be presented.
The transient thermoreflectance method is used to characterize conductive heat transfer in solid materials, particularly, across interface and grain boundaries. In this approach, the pump beam is used as a heater, and the probe beam measures temperature induced reflectivity changes. Measured reflectivity profiles in the time and spatial domain are analyzed to characterize the heat transfer characteristics of the material with a few micrometer spatial resolution. Examples include measurement of radiation damage induced thermal conductivity degradation in ceramic materials such silicon carbide and cerium dioxide and thermal transport across the interfaces in ceramic matrix composite materials.
Presented methods combined with other methods such as Raman spectroscopy offer capability to characterize material properties nondestructively and are attractive methods for in-situ studies of materials behavior in harsh environments.
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