Instrumentation will be developed to permit optical generation and time-resolved measurement of coherent acoustic waves at nearly all wavelengths that propagate through most materials. This extraordinary range will permit tabletop experimental study of structural disorder on the same range of length scales, from nearly 1 millimeter, i.e. clearly macroscopic, to as short as 10 nanometers. It also will provide direct experimental access to dynamical changes in structure that occur over a similarly wide range of time scales, from faster than 1 picosecond to many microseconds, given by the acoustic frequency range of roughly 10 MHz - 500 GHz to which access will be gained. This unique materials research capability will be used for fundamental study of complex liquids, amorphous solids, and partially disordered crystals whose key properties are mediated by structural variation on these length and time scales. The instrumentation also will be used for characterization of advanced structures including thin films and multilayer assemblies of interest in microelectronics and many other applications. The instrumentation will provide access to coherent, narrowband acoustic phonons across most of the Brillouin zone in bulk and thin film materials. Non-Technical Summary
Sound waves with wavelengths of meters or millimeters are commonly used to probe structures of comparable size, such as features within the earth's mantle, two-by-four beams behind drywall, or fingers and toes (and their motions) inside the womb. The same principles of ultrasonic imaging and probing can apply to much smaller length scales as well, and there are plenty of micrometer and nanometer size structures that need characterization. These include multilayer thin films in microelectronics devices; nanospheres, nanorods, and other structures fabricated for nanotechnology; the constituents of heterogeneous materials like alloys, suspensions, and gels; and even transient irregularities that form during natural fluctuations or flow in viscous liquids, polymers, and biological fluids. But generating acoustic waves with such short wavelengths, directing them along or through the material of interest, and then detecting them often present daunting challenges. In recent years, novel methods have been developed through which finely tailored laser pulses may be used to generate and detect acoustic waves with specified wavelengths or frequencies. In some cases, a "comb" of laser light is used to imprint the acoustic wave pattern directly onto the material of interest, just as a real comb that suddenly, gently touches a water surface might generate acoustic waves whose wavelength matches the comb spacing (except that the laser light fringes are only microns apart!). In other situations, a timed sequence of laser pulses is used to launch an acoustic wave into a material, just like sequential taps on the side of an aquarium might send acoustic waves into the water within it (except that the light pulses are only picoseconds, i.e. 10X( -12) seconds, apart!). The acoustic waves are not only generated but also detected optically, so no mechanical contact with the sample is needed. These methods have been used to measure nanometer thicknesses of film layers and lateral dimensions of tiny features, transient evolution of viscoelastic fluctuations that govern polymer processing or biological system responses, and a host of other small structures and their dynamics. In this project, equipment will be developed that will permit optical generation and detection of acoustic waves with essentially all possible wavelengths and frequencies that can propagate within a wide range of materials and structural elements. The equipment will be designed to make these measurements not only possible but robust, such that they can be made by high school students in an outreach lab as well as by Ph.D. science and engineering students. In this manner, a new window into microscale and nanoscale structure and behavior will be made widely available.
