Surface acoustic wave techniques in laser pump-probe systems and its applications in studying mechanical properties of materials

Ultrasonic technologies in kHz and MHz frequency regime have been well developed and widely applied in the past half century, thanks to the utilization of piezoelectric transducer. Acoustic techniques that can reach GHz level is needed as the scales of materials under study reaches sub-micron and nanometer. In this dissertation, I develop and improve surface acoustic wave (SAW) techniques in optical pump-probe system, and apply them to measure shear elastic constants of thin films and damping of high frequency shear acoustic waves. While longitudinal elastic constants of thin films can be measured conveniently in optical pump-probe system, there is no practical methods to measure shear elastic constants of thin films. I develop a SAW measurement technique using phase-shift mask in optical system. It is convenient and compatible with various kinds of thin films materials. I demonstrate the capability of the technique by measuring shear elastic constants of hard materials, soft materials, and layered materials with thickness as small as 60 nm. Ultra small shear elastic constant is observed in misfit layered compound [SnSe][MoSe2]. The intrinsic attenuation of GHz acoustic wave in dielectric sets the upper bound of the quality factor of mechanical oscillation system operated at GHz frequency regime. There was no reliable experimental results regarding the intrinsic attenuation of GHz shear acoustic in Si. By careful experimental design, I am able to measure the attenuation of 7 GHz SAW on Si, which largely represents the attenuation of shear acoustic wave. The experimental scheme can be readily applied to other dielectric materials. For quantitative analysis, I implement a general calculation scheme which can calculate SAW of layered structure with any number of layers of anisotropic material and SAW of structure with thin grating on surface. It can also be used to calculate various other acoustic modes such as Stoneley wave and Lamb wave. Density functional theory (DFT) calculation is used in the these studies to assist the analysis and understanding of the experiments. I calculate elastic constant of cubic crystals and hexagonal layered materials (graphite, MoS2, and misfit layered compound [SnSe][MoSe2]). I calculate the generalized mode Gruneisen parameter of Si in order to understand the relatively small attenuation of shear acoustic waves comparing with longitudinal acoustic wave in Si.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2017

Author dc:creator

• Li, Dongyao

Contributors dc:contributor

• Cahill, David G.
• Schleife, André
• Sottos, Nancy R.
• Hughes, Taylor L.

• Surface acoustic wave
• Pump-probe

Statement dc:rights

• Copyright 2017 Dongyao Li

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/97665

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/97665