Energy transmission through and along thin films mediated by surface phonon-polaritons

Abstract

Surface phonon-polaritons are hybrid electromagnetic modes that are the result of photons coupling to transverse optical phonons. Recently, these surface modes have received much renewed interest primarily due to the fact that micro-fabrication techniques can now routinely create structures at the length scales of interest (nanometers to microns). This thesis investigates the transmission of energy mediated by surface phonon-polaritons. First, the heat flux transported along the in-plane direction of a thin film is explored. A kinetic theory-based calculation is performed using a diffusion approximation. These results are further confirmed by simulations using fluctuational electrodynamics. It was found that for amorphous silicon dioxide films tens of nanometers thick, the in-plane heat flux carried by surface phonon-polaritons can exceed the heat flux carried by phonons in the film. The results also show that the effective thermal conductivity due to surface polaritons increases with decreasing film thickness, offering a method to potentially offset the reduction in thermal conductivity due to increased interface scattering of phonons in crystalline thin films. Both calculations point to the propagation length of the surface phonon-polariton as the source for the large heat flux. An experimental measurement of the surface phononpolariton propagation length on amorphous silicon dioxide is performed using attenuated total reflection and is found to agree well with the calculated value. The last part of this thesis examines the energy transmission in the direction normal to the plane of the film. Specifically, the transmission of light through an amorphous silicon dioxide film perforated by sub-wavelength holes is experimentally measured. A five-fold increase through the perforated film versus through a solid film is observed in discrete frequency ranges, which strongly suggests the involvement of surface phonon-polaritons.