Picosecond Ultrasonic Analysis of Surface Acoustic Waves on Patterned Nanostructures
Samuel A. Gartenstein ’17, Anthony L. Bartolotta ’18, Issai A. Torres ’20, and Brian Daly (Physics)
Picosecond ultrasonics is a method of using extremely rapid laser pulses to study very small and short-lived phenomena. By analyzing changes in sample reflectivity due to surface waves over various time delays, many mechanical properties of thin films and substrates can be deduced, helping to understand the possible applications of such materials. We used these techniques to investigate the creation and subsequent evolution of surface acoustic waves on patterned thin film nanostructures by ~ 100 femtosecond titanium sapphire laser pulses. The samples in question consist of silicon wafers coated with ~ 60 nanometers (nm) of silicon oxide with a pattern of 25 nm thick aluminum lines on top, with widths of 100, 300, and 500 nm. The surface of each sample was struck with 800 nm wavelength near-infrared laser pulses, creating an ultrasonic surface wave on the order of several gigahertz. The motion of the wave was then tracked by measuring the reflectivity of the material a number of microns away from the generation spot of the wave. We note the detection of traveling surface waves in the 300 and 500 nm samples of frequency 5 and 7GHz, respectively, with lifetime longer than the ~ 13 ns delay of the laser. We also simulated this procedure with a computer program that calculates the behavior of the ultrasound traveling into the layer in order to predict other effects in the data. We hope to expand on these results with future work on new samples with different geometries and further computer simulations.