RUI: Acoustic Phonons in Nanostructures: Surface Waves, Thermal Transport, and Imaging

Vassar College, Poughkeepsie NY

Investigators

Abstract

Non-Technical Abstract Can laser generated high frequency ultrasound be used to make images of nanostructures buried inside semiconductor devices? It is taken as a given that progress in nanoscale science and technology relies on continuing progress in our ability to make nanoscale images. This project will employ ultrafast lasers that generate light pulses that are less than a millionth of a millionth of a second in duration in order to generate and detect ultrasound that is over 1000 times higher in frequency than medical or industrial ultrasound. The research will entail the use of new 2-Dimensional materials to enhance our ability to detect these ultrasonic waves, and will also broaden our understanding of how such high frequency waves behave as they travel around inside solid semiconductor materials. The project will involve a wide range of undergraduate students in summer research and independent research courses for credit during the semester. It will also enhance a program for incoming freshmen who are under-represented minorities or first-generation college students. Technical Abstract This project will seek to demonstrate that coherent acoustic phonons can be used to image buried nanostructures. Previous work on this has shown promise, but issues of low signal to noise due to high attenuation of phonons in the 0.1 to 1 THz range have proved daunting. The work will investigate whether 2-D materials such as transition metal dichalcogenides or graphene could prove to be effective transducer layers for a picosecond laser ultrasonic imaging experiment. A second investigation will explore high frequency surface acoustic wave generation and detection by ultrafast optical pump-probe experiments on periodically patterned layered nanostructures. Ultrafast optics has produced the highest surface acoustic wave frequencies ever detected, but a complete understanding of the optical generation and detection is lacking. The work will compare the optical data with computer simulations of the mechanical displacements and the incident/reflected electromagnetic waves. The final component of the project will be a study of phonon attenuation and thermal transport in thin amorphous films and crystals. Time-domain thermoreflectance measurements will be combined with coherent phonon attenuation measurements to form a complete picture of the contribution of long wavelength vibrational modes to thermal transport. Similarly, the measurement of coherent phonon attenuation in crystalline materials, where the thermal conductivity is already well known, will supply important data that may help answer the question of what the behavior of sub-THz phonons says about a material's thermal conductivity.