Ph.D. Dissertation Defense by Ceji Fu
Thursday, November 9, 2004
(Dr. Zhuomin Zhang, Chair)
"Radiative Properties of Emerging Materials and Radiation Heat Transfer at the Nanoscale"
A negative index material (NIM), which possesses simultaneously a negative permeability and a negative permittivity, is an emerging material that has caught the attention of many scientists and engineers after it was first demonstrated in 2001. It has been shown that NIMs have some remarkable properties such as a negative phase velocity and negative refraction and hold enormous promise for applications such as perfect lens and optical communications. This dissertation is centered on investigating the unique aspects of the radiative properties of negative index materials (NIMs). Photon tunneling, which relies on evanescent waves to transfer radiation energy, has important applications for radiative transfer in thin-film structures, microscale thermophotovoltaic devices, and scanning thermal microscopes. With multilayer thin-film structures, photon tunneling is shown to be greatly enhanced by using NIM layers. The enhancement is attributed to the excitation of surface or bulk polaritons, which depends on the thicknesses of the NIM layers according to the phase matching condition. A new kind of coherent thermal emission source is proposed by pairing a negative permittivity (but positive permeability) layer with a negative permeability (but positive permittivity) layer. The merits of such a coherent thermal emission source are that coherent thermal emission occurs not only for p-polarization, but also for s-polarization, without the use of grating structures. By analyzing the polariton dispersion relation, a method for control of the emission frequency and emission angle is provided. The power reflectance from NIM can be zero for both polarizations, that is, there exist Brewster angles for both polarizations under certain conditions. The criteria for zero reflectance or the Brewster angle are determined analytically and presented in a regime map. The findings on the unique radiative properties of NIMs may help develop advanced energy conversion devices. Motivated by the recent advancement in scanning probe microscopy, the last part of this dissertation focuses on the prediction of the radiation heat transfer between two closely spaced semi-infinite media. The objective is to investigate the dopant concentration of silicon on the near-field radiation heat transfer. It is found that the radiative energy flux can be significantly augmented by using heavily doped silicon for the two media separated at nanometric distances. Large enhancement of radiation heat transfer at the nanoscale may have an impact on the development of near-field thermal probing and nanomanufacturing techniques.