Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Margaret M. Murnane

Second Advisor

Henry C. Kapteyn

Third Advisor

Ronggui Yang

Abstract

Phonon transport is essential in both understanding and characterizing materials, particularly in nanoscale systems. In this thesis, I use coherent ultrafast extreme ultraviolet (EUV) beams from high-order harmonic generation, to study the dynamics of photoacoustics and energy-carrying phonons at the nanoscale. I first generate and detect short-wavelength photoacoustic waves by impulsively heating sub-optical phononic crystals with an infrared laser. By monitoring the diffraction dynamics of EUV beams I observe the shortest-wavelength surface acoustic waves to date at 35 nm, corresponding to an interface layer sensitivity of sub-10 nm. I also achieve coherent control of SAW generation and preferentially enhance higher-order SAWs which allows us to reduce the generated SAW wavelength by a factor of two for a defined nanostructure period. I apply this photoacoustic technique to thin film metrology metrology: by generating nanoscale longitudinal and surface acoustic waves simultaneously, I am able to characterize the mechanical properties of ultrathin film samples. Secondly, I study thermal transport dynamics in nano-to-bulk systems where phonons are heat carriers. I first observed quasi-ballistic thermal transport in 1D nano-to-bulk systems, and detect a stronger ballistic effect in 2D nanostructured materials. Temperature- and polarization-dependent experiments are also reported in this thesis. Furthermore, I was able to make a first attempt in dynamic thermal imaging using coherent diffraction of EUV beams.

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