Phonon imaging

This thesis describes the development of the phonon imaging method. This technique permits the direct measurement of the angular and temporal distribution of phonons emitted from a point source of heat in insulators at low temperatures. It is a derivative of the ballistic heat pulse method, but one which uses a pulsed laser to allow continuous two dimensional scanning of the phonon source. In this work we have applied this method to the following topics: 1. Phonon Focusing. In pure defect-free crystaline insulators, heat pulses may propagate macroscopiC distances without scattering. In contrast to diffusive transport of phonons, which is nearly isotropic, the flux of ballistic phonons coming from a heat pulse will display a large anisotropy directly associated with the elastic anisotropy of the crystal. We use phonon imaging to map this anisotropy in Ge, and analyze the data in terms of directions of theoretically singular flux, or singularity lines. 2. Dispersive Phonon Focusing. The shapes of the geometric phonon focusing patterns observed in a ballistic phonon image are independent of phonon frequency in the long wavelength limit. With the aid of a frequency selective detector, which is sensitive only to high frequencies, we observe dispersive shifts in the phonon focusing singularity pattern. The concept of phonon focusing is extended to allow for dispersion, and the results are applied, along with a lattice dynamics model, to predicting the expected singularity shift vs. phonon frequency in Ge. There is good agreement between the images and theory at 800 GHz. 3. Phonon-Dislocation Scattering in LiF. Phonons are known to be strongly scattered by dislocations in LiF and other alkali-halides. We use the phonon imaging method to probe the anisotropy of this scattering cross section in plastically deformed LiF. We measure a strong dependence upon phonon polarization, with a subset of polarizations propagating the length of the sample without scattering. The results fit the coupling anisotropy predicted by the vibrating string model of phonon-dislocation scattering, confirming it as the primary scattering mechanism in this system.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Northrop, Gregory Allen

Contributors dc:contributor

• Wolfe, J.P.

• phonon imaging
• phonon focusing
• phonon-dislocation scattering

Statement dc:rights

• 1982 Gregory Allen Northrop

Language dc:language

en

Identifier

92631

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/25403